Assessing effectiveness of air purifiers (HEPA) for controlling indoor particulate pollution
The present study deals with an evaluation of the air purifier's effectiveness in reducing the concentration of different sized particulate matter (PM) and ions in the real-world indoor environment. Two types of air purifiers (API and APII) mainly equipped with High-Efficiency Particulate Air (HEPA) filters that differed in other specifications were employed in general indoor air and the presence of an external source (candles and incense). The gravimetric sampling of PM was carried out by SKC Cascade Impactor and further samples were analyzed for determining ions' concentration while real-time monitoring of different sized PM was done through Grimm Aerosol Spectrometer (1.109). The result showed that API reduced PM levels of different sizes ranged from 12-52% and 29–53% in general indoor air and presence of external source respectively. Concerning the APII, a higher decrease percent in PM level was explored in presence of an external source (52–68%) as compared to scenarios of general indoor air (37–64%). The concentrations of the ions were noticed to be decreased in all three size fractions but surprisingly some ions' (not specific) concentrations increased on the operation of both types of air purifiers. Overall, the study recommends the use of air purifiers with mechanical filters (HEPA) instead of those which release ions for air purification.
The foremost objective of the study is to gain in-depth knowledge of the usage of air purifiers in general indoor air and with an external source (candles and incense smoke) and to assess the efficiency of the air purifiers in terms of reducing PMs (or particles), cations and anions’. The specific objectives are (i) to provide quantified information on the concentration levels of particulate matter during filtering and non-filtering period in a room chamber. (ii) to assess the ions concentration in different sizes of PMs collected in filtering and non-filtering periods.
Concerning the fact of expenditure of 80–90% of people's time in the indoor environment ( Nazaroff and Goldstein, 2015 ), the problem of Indoor Air Pollution (IAP) depends on multiple factors (viz. indoor emission sources, outdoor concentration, airflow and other) has gain enormous expansion of research in past years. In indoors, people get exposed to number of indoor and outdoor pollutants which ultimately prompt different acute and severe diseases ( Maji et al., 2017 ). According to State of Global Air (2019) report, about 846 million people in India (60% of the population) were exposed to IAP and the pollution has contributed to 1.6 million deaths worldwide in 2017 ( SOGA, 2019 ). It has been mentioned in World Health Organization ( WHO, 2018 ) report that IAP may results in ∼3.8 million premature deaths annually which include diseases like chronic obstructive pulmonary diseases, lung cancer, ischemic heart diseases, and stroke. As per State of Global Air ( SOGA, 2019 ) report, India recorded the second-highest number of deaths of children (below 5 years) due to the exposure of indoor air pollution in 2016, and 98% of them were exposed to PM 2.5 . Owing to its very small size, chemical composition, and a significant risk factor for adverse human health outcomes; Particulate Matter (PM) (a key indicator of air pollution and also a major determinant of indoor air quality) has gathered attention among various pollutants. Based on their size (diameter of PM), the U.S. Environmental Protection Agency (USEPA) has differentiated PM into three categories i.e. coarse particles, PM 10 (PM<10 μm in diameter); fine particles, PM 2.5 (PM<2.5 μm in diameter) and ultrafine particles, PM 1.0 (PM<0.1μm in diameter). The size of PM is directly proportional to penetration power into the lungs' bloodstreams and leads to cardiovascular and respiratory diseases ( Karimi and Samadi, 2019 ). Apart from size, chemicals bound to PM greatly determine the toxic and carcinogenetic character of PM. Along with carbonaceous fraction, inorganic components especially water-soluble ions (i.e. cations e.g. NH 4 + , Na + , K + , Ca 2+ , Mg 2+ , and anions e.g. NO 3 – , SO 4 2– , Cl – , F – , NO 2 – , Br − ) bound to airborne PM ( Xiang et al., 2017 ; Yang et al., 2018 ) play a significant role in controlling the mass concentration of PM and constitute 35–60% of PM mass ( Saxena et al., 2017 ). Moreover, the extent of acidity and toxicity of PM is governed by secondary inorganic ions (as SO 4 2- , NO 3 - and NH 4 + ) which may lead to adverse health outcomes ( Cao et al., 2017 ).
ELCR is defined as the incremental probability of an individual to develop cancer over a lifetime as a result of exposure to a potential carcinogen. Its reference value is 3.14×10 3 , which pertains to one cancer incidence for every one million people. The equations for the calculation of ELCR are narrated in the supplementary file. The health risk (carcinogenic and non-carcinogenic) imposed by PM when exposed to such concentration was carried out for both adults and children by using exposure factors mentioned in USEPA, 2014 .
where REL (reference exposure level) refers to the dose at which significant detrimental health effects will occur in the exposed group. In this study, REL for PM 10 was adopted from guidelines provided by CPCB according to which the mass concentration of PM 10 must be 100 μg/m 3 for an exposure time of 24 h.
The health risk posed by PM (via inhalation) before and after deployment of air purifier in general indoor air as well as candles and incense smoke was quantified. Non-carcinogenic risk (posed by PM 10 ) estimated by Hazard Quotient (HQ) and carcinogenic risk (posed by PM 2.5 ) by Excess Lifetime Cancer Risk (ELCR) was performed similarly to Morakinyo et al. (2017) and Kim et al. (2018) respectively.
Statistical Analysis was performed using MS Excel 2010 for Windows. Paired t-test was carried out to observe the difference in mass concentration of different sized PMs after the operation of the air purifier during each sampling scenario. Statistical significance was a 5% level (p < 0.05). The increase and decrease percentage in mass concentration of ions and decrease percentage in case of PM have been calculated by dividing the increased and decreased concentration by their initial concentration and further multiplying the resultant with 100.
The water extraction method was used for the determination of the concentration of ions using Ultrasonicator and the procedures followed for this were adopted from Satsangi et al. (2016) . The exposed filter papers were cut into strips followed by digestion in a 50 ml prewashed Borosil beaker using double distilled water for 2 h. The solution was then filtered using prewashed Whatman filter paper followed by washing of beakers two to three times and then the solution was makeup with 25ml of double-distilled water. Finally, sample solutions were stored in polypropylene sample bottles in a refrigerator under 4°C until got analyzed with Ion chromatography (Dionex 1100). To maintain quality control in the work, precautions’ regarding sample storage and glassware cleaning procedures were done according to Rohra et al. (2018) .
The purifier was placed at a height of 1.5m from the ground (average inhalation height), while candles and incense were placed at a distance of 1m from the air purifier in both cases. The overall methodology adopted in the study is depicted in .
In the present study, two types of air purifiers with different CADR and other specifications were employed. The first air purifier (API) comprised of an anti-dust filter, activated carbon filter, active HEPA filter, electrostatic filter, vita ions, cold catalyst filter with programmable control panel, sleep mode, timer function, and independent air ducts, while the second air purifier (APII) was equipped with six sense technology, humidifier, filter replacement indicator along with filters viz. pre-dust filter, activated carbon filter, HEPA filter, nanocaptur filter; UV lamps and also ionizer function. This was done to measure the effectiveness of air purifiers that are commonly used. Along with the different types of filters used in air purifiers, they also have different specifications the details of which have been provided below in .
Gravimetric PM sampling in three size fractions i.e. 2.5–1.0 μm, 1.0–0.5 μm, and 0.5–0.25μm was done using Leland Legacy pump (SKC Inc. Eighty-Four PA USA) in combination with a five-stage, Sioutas Cascade Impactor and the instrument's calibration was performed using a Drycal DC-2 calibrator (Bios International Corporation, NJ USA). In this study, PM samples were collected on 25 mm PTFE (Poly Tetra Fluor Ethylene) filters with pore size 0.5 μm with a pump operation rate of 9 l/min. The PM samples collected through SKC Cascade Impactor were then analyzed for the determination of ions' concentration. Real-time monitoring of PM per minute was done by Grimm Portable Aerosol Spectrometer (1.109), a portable environment dust monitor (which measure PM with a diameter range from 0.22 to 32 μm) in general indoor air and during (candles and incense smoke) events, with and without operation of air purifier. As per the constraint of working time of the Leland Legacy pump, sampling was carried out for 6 h in each sampling scenario. Candles and incense (Dhoopbatti) opted as sources as they are most commonly used in buildings (homes and worship places) and are also one of the prominent sources of indoor pollutants. Burning of candles produce PM 2.5 (1,200 μg/m 3 ), PM 10 (200 μg/m 3 ) ( Chuang et al., 2012 ) with emission factor (5–56 mg/g) for PM 2.5 ( Jetter et al., 2002 ) and trace amounts of organic chemicals (C 2 H 4 O, CH 2 O, C 3 H 4 O, and C 10 H 8 ) ( Lau et al., 1997 ), while incense burning generates large quantities of PM (0.24 < median diameter <0.40 μm) ( Mannix et al., 1996 ). Chuang et al. (2012) reported the mass concentration of PM 10 and PM 2.5 as 91.6μg/m 3 and 38.9 μg/m 3 respectively, when the burning of candles and incense was impaired.
Sampling was undergone in a room chamber (length = 6.3m, breadth = 3.2m, and height = 3.5m with effective volume) located in the Department of Chemistry at Khandari campus, Agra during May and June 2018. To maintain the thermal comfort of residents in summers (due to the flow of summer winds locally called loo), the window was kept closed and ventilation was through the door during sampling to observe the effectiveness of air purifiers in real-world indoor environments.
Agra (27˚10′ and 78˚2′ E) is the city of imitable Taj founded by Sikandar Lodi (ruler of Lodi Dynasty) is located in the north-central part of India and is situated on the bank of the river Yamuna. It is 200 km south of the national capital New Delhi and 363 km west of the state capital Lucknow. It is one of the prominent destinations on the world tourism map with three heritage monuments- Taj Mahal, Red Fort, and Fatehpur Sikri. Along with industrialization and urbanization, Agra also faces a high transportation load (due to major tourist spots in India and 3 major National Highways) which leads to deterioration of outdoor air quality and in turn affects the air quality of indoor spaces.
The value of ELCR for both child and adult was also reduced significantly (Table S1) and almost similar to that of HQ after the application of air purifier in general indoor air as well as candles and incense smoke.
In general indoor air, all the values of HQ were found to be less than 1.0 which indicate the negligible risk posed by PM 10 , while in presence of candles and incense smoke it was found to be greater than 1.0 (pose threat to human health) which was significantly reduced to a large extent by both air purifiers. The HQ (risk posed to adults and children) in case of acute and chronic exposure was reduced by 2.07 times and 1.41 times when air purifier I was operated in general indoor air and presence of external source (incense and candles smoke) respectively, while air purifier II reduced the risk by 2.16 times and 2.94 times in a similar sampling case.
In the case of adults, the health risk posed in terms of acute and chronic exposure significantly reduces with the employment of air purifiers. However, no health risk due to PM 10 (HQ ≤ 1.0) was observed in general indoor air as well as in presence of candles and incense smoke .
Also, both the air purifiers reduced ions concentration significantly while the concentration of some of the ions increased after the application of the air purifier. AP I reduced the ions concentration (in three different size fractions of PM) such that cations were reduced by 21–56% and 82–97% while anions by 30–93% and 48–97% in general indoor air and presence of source (candles and incense smoke) respectively. The reduction in the mass concentration of ions was also significant in the case of AP II i.e. for cations the decrease percentage (%) lies in the range 20–99% while it was 14–40% for anions in general indoor air. In presence of candles and incense smoke, the reduction percentage in cations concentrations ranged from 81-149%, and in the case of anions, it was 33–76%. It was observed that a higher reduction in the mass concentration of ions takes place after deployment of air purifier in presence of sources like candles and incense in comparison to its absence.
The result infers that the effectiveness of both air purifiers (in terms of reduction of PM levels) was enhanced in case of external source event (29–53% (API) and 52–68% (APII)) than general indoor air (12–52% (API) and 37–73% (APII)). Also, the PM reduction percentage did not follow any fixed trend in terms of size for the APII operational scenario for both sampling events whereas the AP I operational phase depicted enhanced reduction for PMs with larger diameters in general indoor air events with an inverse trend for external source event.
Both air purifiers with different specifications and Clean Air Delivery Rate (CADR) employed in the present study showed distinct efficacy in terms of decrease in the concentration of different sized PM and ions. The varied reduction percentages in the mass concentration of different sized PM and ions are summarized in . The range of reduction percentage reported in is based on the lowest to highest reduction percentage in the case of cations and anions. In some sampling cases, a similar reduction percentage was observed which is presented as a single reduction percentage while all the increased values have not been included.
In the case of AP II, the mass concentration of F − ion was significantly reduced (77%) in PM 2.5-1.0 , while it was highly increased viz. 429% and 596% in PM 1.0-0.5 and PM 0.5-0.25 respectively ( b). Contrary to a slight increment (2%) in the mass concentration of NO 3 - in PM 2.5-1.0 , a moderate reduction in rest size fractions (PM 1.0-0.5 and PM 0.5-0.25 ) was observed. The percent decrease in mass concentration of cations was found in the order as Na + > K + > Mg 2+ for PM 2.5-1.0 with an exception for Mg 2+ and Ca 2+ (highly increased) that depicted a similar trend in PM 1.0-0.5 . In case of PM 0.5-0.25 , after the purification of incense and candle smoke by AP II, an increase in the mass concentration of cations were found in the order (Mg 2+ >K + ); Mg 2+ was highly increased (454%) followed by K + which was increased moderately (27%).
In candles and incense smoke event, the efficacy of AP I revealed that anions mass concentration was reduced in trend as F - ∼ NO 3 - > Cl - , Cl - > F - > NO 3 - and NO 3 - > F - in PM 2.5-1.0 , PM 1.0-0.5 and PM 0.5-0.25 respectively. Except for NO 3 - which was moderately reduced (48%) in PM 1.0-0.5 , all other anions showed a significant decrease (66–92%) in their mass concentration. In the case of cations, the trend in reduction percentage was found as K + > Na + > Ca 2+ > Mg 2+ in PM 2.5-1.0 . In PM 1.0-0.5 , except for Na + and K + (Na + > K + ) decrease in the mass concentration of cations followed a similar trend as in PM 2.5-1.0 , while in PM 0.5-0.25 the trend obtained was as Mg 2+ > Ca 2+ > K + . In all size fractions, cations described above significantly reduced (83–99%) and ions showed no increase in their mass concentration after turning on the air purifier depicted in a.
The effectiveness of AP II on anions concentration was observed in the manner that NO 3 - showed more reduction in its mass concentration followed by F − ion (NO 3 - > F − ) and both ions reduced moderately in PM 2.5-1.0 and PM 1.0-0.5 . Whereas in PM 0.5-0.25 , surprisingly both ions concentration was found to be increased in the same trend (same to reduction %) as NO 3 - was highly (110%) and F − was significantly increased (97%). Moreover, in case of cations, both Na + and K + were significantly reduced (Na + > K + ) in PM 2.5-1.0 while K + was moderately reduced in the rest two size fractions. Na + concentration was most reduced among all cations in all size fractions ( b).
The efficacy of AP I in terms of anions followed the trend as Cl − > F − in PM 2.5-1.0 and reverse for PM 1.0-0.5 . In terms of anions, the mass concentration of F − significantly reduced in all three size fractions ( a). Cl − ion was significantly (82%) and moderately (31%) decreased in PM 2.5-1.0 and PM 1.0-0.5 respectively except for PM 0.5-0.25 for which the mass concentration increased significantly (52%) after air filtration. A similar trend of increased mass concentration of NO 3 - was observed in all three size fractions in such a way that mass concentration significantly increased in PM 2.5-1.0 , PM 1.0-0.5 , and highly increased in PM 0.5-0.25 . No effect of air purifier on mass concentration of cations in PM 2.5-1.0 was observed, while in PM 1.0-0.5 reduction in the mass concentrations of cations followed the order: K + >Ca 2+ >Na + >Mg 2+ in such a way that K + was reduced significantly (56%) and Mg 2+ was moderately decreased (44%). In PM 0.5-0.25 , Mg 2+ showed a moderate decrease (22%) in its mass concentration whereas the concentration of Na + was highly increased (349%) followed by K + (significant increase i.e. 68%) on the operation of air purifier.
Ions were grouped into six classes viz. moderate decrease (reduction<50%), significant decrease (reduction 50–100%), high decrease (reduction >100%), moderate increase (increase<50%), significant increase (increase 50–100%) and high increase (increase >100%) based upon ions efficacy of purifiers’ (% decrease as well as an increase of ions concentration) in general indoor air as well as candles and incense smoke event. In some sampling scenarios, values that define the mass concentration of ions were found below the detectable limit (shown by zero in graphs), hence increase and decrease percentages are excluded in that case.
An ample of studies had been conducted to evaluate the effectiveness of air purifiers in terms of PM and showed significant and varied reduction percentage in the mass concentration of different sized PM. gives a global scenario in terms of reduction in PM level by HEPA filters employed in current and previous studies.
In contrast with the findings of AP I, APII depicted the following trend (PM 2.5 > PM 1.0 > PM 5.0 >PM 10 > PM 0.5 >PM 0.25 ) after the removal of particles associated with candles and incense burning. The mass concentration of PM 2.5 was reduced the most (68%) while PM 0.25 was reduced the least (52%) after air filtration. During HEPA OFF and HEPA ON periods, a non-significant difference (p > 0.05) in the mass concentration of PM 1.0 , PM 2.5 , and PM 5.0 was observed. On the other hand, a significant difference (p < 0.05) in case of PM 0.25 and PM 0.5 while borderline significant difference (p = 0.05) in case of PM 10 was observed under HEPA and non-HEPA conditions. The efficiency of both air purifiers in candles and incense smoke events is given in .
In the presence of external source (candles and incense smoke event), AP I showed the highest effectiveness on PM 0.25 (53%) and least on PM 10 (29%) and also a reverse trend (PM 0.25 > PM 0.5 > PM 1.0 > PM 2.5 > PM 5.0 > PM 10 ) in mean reduction percentage was observed as compared to general indoor air. During HEPA OFF and HEPA ON periods, for PM 0.25 and PM 0.5 a significant difference (p < 0.05) in mass concentration was noticed whereas a non-significant (p > 0.05) difference was observed for rest particle sizes. The mass concentration of small-sized PM showed a maximal reduction with the deployment of AP I while the large-sized PM was reduced the least in terms of concentration.
In contrast to the above findings no specific trend (in reduction %) (PM 0.5 > PM 0.25 > PM 1.0 > PM 10 > PM 5.0 > PM 2.5 ) was obtained after the deployment of AP II. The effectiveness of AP II in general indoor air was found maximal (73%) for PM 0.5 and minimal (37%) for PM 2.5 . All PMs showed non-significant (p>0.05) difference except PM 5.0 which showed a significant difference (p<0.05) in its mass concentration under HEPA and non-HEPA periods. depicts the filtration efficacy of AP I and AP II in general indoor air.
In the case of AP I, both PM 10 and PM 5.0 showed a maximal decrease (52%) whilst decrease in mass concentration of PM 0.5 was least (12%) during the filtering vs non-filtering period. Significant (p < 0.05) and borderline significant (p = 0.05) difference in particle mass concentration between HEPA and non-HEPA conditions was observed at PM 1.0 and PM 2.5 respectively while the non-significant difference was observed at rest particle size. Except for PM 0.5 , the trend (PM 10 ≈ PM 5.0 > PM 2.5 > PM 1.0 > PM 0.25 > PM 0.5 ) attained in reduction percentages (in mass concentrations of different sized PM) revealed that the HEPA filter was more effective in the case of larger particles (as PM 10 and PM 5.0 ) as compared to PM with small size.
The overall scenario of the effectiveness of air purifier in terms of particulate exposure is discussed foremost followed by its efficacy on PM bounded ionic exposure. This is further presented in a way to portray particle size dynamics in conjugation with the presence and absence of an external source. The study attempts to make a comparison of PM levels with guidelines proposed by different national and international organizations in different sampling scenarios. At last, the upgrading in IAQ as exhibited by quantitative health risk after deployment of air purifiers is discussed.
4. Discussion
The indoor level of different sized PM characteristics and ions (associated with PMs) under HEPA and non HEPA conditions in two different sampling scenarios viz. general indoor air and external source (candles and incense smoke) was assessed. After the API employment in general indoor air, the reduction of larger PMs was more obvious than the reduction of the smaller ones, which is in conjugation with a former study of the Department of Energy, USA (DOE, 2005). The obtained trend can be attributed to the fact that larger particles (more inertia) are found in higher concentration in general indoor air (as a result of mechanical (human) activities as walking, sweeping and vacuuming) as compared to smaller ones that travel in airstream direction to get through cross-hatching of fiber and are intercepted by fiber (Wallace, 2008). Shiue et al. (2011) had reported a similar reduction percentage in the mass concentration of PM0.5 and PM0.25, whereas reduction percentage in the mass concentration of PM2.5 resembles with results in studies by Scheepers et al. (2015), Cheng et al. (2016), and Park et al. (2017).
After deployment of AP II in the same sampling conditions, no such similar trend in reduction percentage (in the mass concentration of PMs) was noticed in the case of API . AP II showed the lowest efficacy on PM2.5 as its mass concentration was only reduced by 37% (from 40.42 μg/m3 to 25.27 μg/m3). A similar mean reduction percentage in the mass concentration of PM2.5 was reported by Cheng et al. (2016), Scheepers et al. (2015), and Chuang et al. (2017), while Brauner et al. (2008) mentioned a remarkable reduction of 63% in PM2.5 after the installation of HEPA air purifier. In the case of PM10, the obtained reduction percentage (54%) in mass concentration was comparable to the estimate provided by Brauner et al. (2008) in which air purifier was operated in homes located proximity to roads, while Xu et al. (2010) has reported most notable decrease percentage (72%).
The efficacy of AP I in presence of candles and incense smoke was found as: small-sized PMs reduced more as compared to large-sized PM. This can be attributed to the fact that small-sized particles travel farther and faster due to less inertia and are more likely to be hit and trapped by fiber on the filter (Wallace, 2008). The reduction percentage in the mass concentration of PM10 associated with incense and candle smoke after the deployment of AP I was 29.14. This is incomparable to the study by Butz et al. (2011) in which HEPA air purifier was operated in presence of ETS. No as such trend in reduction percentage (in mass concentrations of PMs) after deployment of AP II in candles and incense smoke was observed as obtained in AP I. Mean mass concentration of PM2.5 was reduced by 69% (from 605.77 μg/m3 to 191.60 μg/m3) which was highest among other sized PMs. The acquired reduction percentage in the mean concentration of PM2.5 is consistent with the findings reported by Henderson et al. (2005) and Barn et al. (2008) that evaluated HEPA filter effectiveness in the appearance of wildfires and prescribed burns and residential wood smoke respectively. This is analogous to the studies by Allen et al. (2011) and Butz et al. (2011) when the same type of air purifier was operated in presence of wood smoke and ETS respectively.
The comparison of PM concentration in different sampling scenarios with guidelines proposed by national and international organizations depicted that in the case of both air purifiers, the concentration of PM10 and PM2.5 was very high as compared to the prescribed limit for 24-hour exposure concentration (μg/m3) by WHO, USEPA, and Central Pollution Control Board (CPCB) and remained very high even after the filtration of candles and incense smoke. In the case of general indoor air, both air purifiers reduced the concentration of PM10 in manner that the resultant concentration lied under the prescribed limit of USEPA and National Ambient Air Quality Standards (NAAQS) but remained higher than the limits prescribed by WHO. Moreover, the concentration of PM2.5 in general indoor air which was lower than the prescribed limit of USEPA and NAAQS and higher than that of WHO before filtration reduced to the prescribed limit of WHO and lower than that of NAAQS and USEPA limits after purification of air by both air purifiers is depicted by .
Along with a significant decrease, results from the study also revealed an increase in the mass concentration of ions (not specific) after the operation of air purifiers. Studies like Nishikawa and Nojima (2001), Nojima and Nishikawa (2002), Nishikawa (2006), Kawamoto et al. (2006) confirm the release of both negative and positive ions from air purifiers (by the electric discharge) into the air to make it free from bacteria, mold and other allergens by deteriorating and making them inactive. The increase in the mass concentration of ions after the application of the air purifier may be due to the reason that air purifiers release ions continuously to purify the air and the release of ions continued even after the purification of air which results in increased concentration of ions. However, there was no such specific ion whose mass concentration was increased in presence of air purifiers and neither has it been provided in the literature.
4.1. Limitations and future studies
Along with important findings, the study has few limitations such that the effectiveness of air purifiers was observed for a short period and in the case of PM and ions only. As the toxicity of other chemical constituents of PM (such as metals) and other toxic gaseous pollutants are well known which is not encompasses in the study. The study also lacks in terms of observation of the effectiveness of air purifiers in different seasons.
The study that dealt with an observation of the effectiveness of air purifiers on different chemical species associated with PM in different seasons and microenvironments can be carried out in the future which provides a clear picture regarding improvement of indoor air quality on the application of different types of purifiers. The dispersion and decay rate of PM in different indoor spaces can also be carried out.
A type of simple, DIY air filter can be an effective way to filter out indoor air pollutants
PROVIDENCE, R.I. [Brown University] — A team of researchers from Brown University's School of Public Health, Brown’s School of Engineering and Silent Spring Institute found that simple air filtration devices called Corsi-Rosenthal boxes are effective at reducing indoor air pollutants.
The study, which analyzed the effectiveness of Corsi-Rosenthal boxes installed at the School of Public Health to help prevent the spread of COVID-19, is the first peer-reviewed study of the efficacy of the boxes on indoor pollutants, according to the authors.
Lowering indoor air concentrations of commonly-found chemicals known to pose a risk to human health is a way to improve occupant health, according to lead author Joseph Braun, an associate professor of epidemiology at Brown.
“The findings show that an inexpensive, easy-to-construct air filter can protect against illness caused not only by viruses but also by chemical pollutants,” Braun said. “This type of highly-accessible public health intervention can empower community groups to take steps to improve their air quality and therefore, their health.”
Corsi-Rosenthal boxes, or cubes, can be constructed from materials found at hardware stores: four MERV-13 filters, duct tape, a 20-inch box fan and a cardboard box. As part of a school-wide project, boxes were assembled by students and campus community members and installed in the School of Public Health as well as other buildings on the Brown University campus.
To assess the cubes’ efficacy at removing chemicals from the air, Braun and his team compared a room’s concentrations of semi-volatile organic compounds before and during the box’s operation.
The results, published in Environmental Science & Technology, showed that Corsi-Rosenthal boxes significantly decreased the concentrations of several PFAS and phthalates in 17 rooms at the School of Public Health during the period they were used (February to March 2022). PFAS, a type of synthetic chemical found in a range of products including cleaners, textiles and wire insulation, decreased by 40% to 60%; phthalates, commonly found in building materials and personal care products, were reduced by 30% to 60%.
Development of Cellulose Air Filters for Capturing Fine and Ultrafine Particles through the Valorization of Banana Cultivation Biomass Waste
This study aims to accomplish two primary objectives: firstly, to enhance the value of agricultural residue by manufacturing air filters with cellulose extracted from banana pseudostems; and secondly, to assess various manufacturing methods to identify a simplified approach for filter production. The selected methodology should meet two critical criteria: ensuring high efficiency in capturing fine and ultrafine particles, while simultaneously minimizing pressure drop.
Cellulose is the most widely used biopolymer due to its abundance in nature. Agricultural waste is one of the raw materials used to obtain it. In the case of banana crops, the cellulose is extracted in larger quantities from banana pseudostem [ 46 52 ]. In the Canary Islands (Spain), where the present study was carried out, banana plantations are widespread and, with a production of 347,378 ton·yearin 2022, represented 84% of total agricultural production (Ministry of Agriculture, Fisheries and Food of the Government of Spain, 2022). This results in a large amount of banana pseudostem waste as the banana tree only bears fruit once [ 51 ], with a production of around 17 million tons per year in the Canary Islands. This biomass has been subject to analysis in various applications, such as the use of its fibers as reinforcement in polymeric materials [ 53 55 ], the extraction of subcomponents [ 56 57 ], the textile industry [ 58 59 ], or the generation of biochar for industrial applications [ 60 62 ], among others. Another potential application could be found in the extraction of its cellulose for use in filtration purposes, allowing for the substitution of conventional raw materials based on non-renewable materials, such as those derived from fossil fuels.
There are many technologies that have been developed for PM removal, including scrubbers [ 31 ], electrostatic precipitators [ 32 ], and air filters [ 33 36 ]. The filtration method consists of the separation of particles by their retention in a porous surface when the gas stream passes through it, via chemisorption and physisorption processes [ 37 ]. Fibrous filters are the most used [ 38 39 ], and the particle capture efficiency and pressure drop depend on the filtration material (fiber size, surface area, etc.) [ 36 38 ]. In recent decades, thermoplastic polymers made from fossil fuels (e.g., polyethylene) or glass fiber [ 37 42 ] have been used as materials for the manufacture of air filters. From such materials, and depending on the manufacturing process, it is possible to achieve particle capture efficiency ranging 50–90% (ePM1, ePM2.5, and ePM10 in industrial filters according to ISO 16890 standards) [ 43 ]. Moreover, these materials can be used to produce filters with very high particle retention efficiency for particles larger than 0.3 μm, exceeding 99% in the case of HEPA-type filters, with pressure drops below 300 Pa [ 44 ]. However, these filters have significant disadvantages due to their high cost and non-biodegradable nature [ 37 41 ]. As an alternative, biopolymer-based air filters are now being manufactured as they are biodegradable, easily available, and recyclable. In addition, they have more functional groups that increase the particle capture efficiency [ 45 ]. Keratin, chitin, and cellulose are examples of biopolymers used in air filter manufacturing [ 36 47 ]. In this context of biomass utilization, various studies have focused on developing filters using wood biomass, achieving particle retention efficiencies ranging from 70% to 97% [ 48 ] by incorporating corn proteins and nanocellulose. Other biomass combinations derived from Konjac glucomannan, gelatin, starch, and wheat straw have been successfully tested to produce filtering aerogels with efficiencies exceeding 99% in enclosure filtration studies [ 49 ]. Lignin aerogels have also been developed to achieve high particle filtration efficiencies (exceeding 99%) with low pressure loss, albeit through complex manufacturing processes [ 50 ].
The sources of PM are highly varied, which explains its complex chemical composition, as mentioned above. Natural sources include mineral dust from arid areas, such as the Saharan desert [ 19 ] and volcanic eruptions [ 20 ]. Maritime and vehicular traffic [ 21 22 ], industrial activities [ 23 24 ], or agricultural activities [ 25 ] are examples of anthropogenic sources. In addition, indoor activities are a very important source of PM [ 26 27 ]. Sources of indoor air pollution include, among many others, gas or kerosene cooking (especially in developing countries), combustion activities (use of fireplaces, kerosene heater, etc.), cleaning activities (use of air fresheners or cleaning solutions), the infiltration of outdoor pollutants, the use of printers, permanent-marker, and photographic solutions, and even hobbies like carpentry or metal working [ 27 30 ]. The implementation of air purification measures, both indoors and outdoors, will very often be required to stay below the limits established by the World Health Organization (WHO). For PM10, the annual and daily limits are 15 µg·mand of 45 µg·m, respectively, and for PM2.5, 5 µg·mand 15 µg·m, respectively. Currently, more than 99% of the world’s population is exposed to concentrations above these limits, according to the latest WHO report [ 1 ].
Air pollution is a major problem that has been increasing due to industrial and technological development and is the fourth leading cause of death worldwide, with 6.7 million deaths each year [ 1 ]. Atmospheric particulate matter (PM) is a complex composite of heavy metals, inorganic compounds, water soluble secondary inorganic compounds (sulphates, nitrates and ammonia), organic compounds (HAPs and COVs), and elemental carbon [ 2 8 ]. PM is one of the most dangerous atmospheric pollutants, with very harmful effects on human health. It causes cardiovascular [ 6 9 ], respiratory [ 10 12 ] and brain diseases [ 9 14 ], sleep disorders [ 8 ], dermal cancer [ 15 ], and bone lesions. In addition, PM affects the lymphatic system, causing damage to the spleen [ 14 16 ], as well as the digestive system (leading to damage in the liver, pancreas, and gastrointestinal tract) [ 13 14 ] and reproductive systems [ 17 ]. The extent to which PM is a risk to human health depends not only on its chemical composition, but also the particle size: the finer the particle, the more easily it is able to penetrate into the human body. In the study of air quality, three ranges of particles size are identified: (a) coarse particle, which includes particles with an aerodynamic diameter equal to or less than 10 µm (PM10), with special attention paid to particles with diameters between 10 and 2.5 µm; (b) fine particle, which comprises particles with an aerodynamic diameter equal to or less than 2.5 µm (PM2.5); and (c) ultrafine particle, which comprises particles with aerodynamic diameters of submicron scale (PM1). Particles in the first classification are known as thoracic particles and are retained between the larynx and bronchial tree. The other two inhalable particles lodge in the alveolar region, with ultrafine particles passing directly in the bloodstream [ 6 18 ].
Although the applications and types of air particle filters are extensive, in order to establish a comparative framework with the developed filters and analyze their potential practical application, two commercial filtering elements made from non-biobased materials have been tested. The first filtering element corresponds to a certified VP1 IIR surgical mask according to UNE-EN 14683:2019 (grammage 77 g·m −2 ), manufactured using different layers of polypropylene. The second one is a high-efficiency multipurpose glass microfiber filter Whatman GF/A CAT 1820-061 (Whatman PCL, Kent, UK) with a grammage of 105 g·m −2 .
Capture efficiency of particles with an aerodynamics diameter between 0.3 and 1 µm (corresponding to the smallest particles, the most hazardous, and referred to in this work as PM1).
The di-ethyl-hexyl-sebacat (DESH) aerosol was chosen to study the capture efficiency, generated using an ATM 221 (TOPAS GmbH, Dresden, Germany) aerosol generator with an inlet air pressure of 0.5 bar. Upstream and downstream particle counting was carried out, alternatively, with an optical particle sizer (OPS), model 3330 (TSI). Both particle concentration and particle size distribution were measured over a range of 0.3 to 10 µm, with a size resolution < 5%. The pressure drop was measured with a Belimo 22ADP-184 differential pressure sensor (BELIMO Ibérica de Servomotores S.A., Madrid, Spain). The passage diameter of the filter holder was 31.8 mm, with an aerosol flow rate of 4.5 L·min −1 (regulated with a flow meter and rotameters). The duration of each particle count was 10 min at both inlet and outlet, carried out in triplicate for each filter.
The manufactured filters were tested using the test bench depicted in Figure 2 , which was developed based on the following standards: UNE-EN 149:2001+A1:2010, UNE-EN 13274-7:2020 and NIOSH TEB-APR-STP-0003/0007/0059 [ 64 66 ]. However, due to the low flow rate required in this case, a similar configuration developed by the Massachusetts Institute of Technology (MIT) was selected.
While the filter grammage has been established as the basic variable for comparison, the average thicknesses of each filter type were measured using a Mitutoyo digital micrometer, with 5 measurements taken for each of the 3 replicas. For the first generation of filters, with a grammage of 100 g·m −2 , mean thicknesses of 0.22, 0.39, and 4.77 mm were recorded for pressed (8 t), filtered, and freeze-dried filters, respectively. As anticipated, the lyophilization process resulted in porous filters with greater thickness. The reduction in weight (70 g·m −2 ) led to a significant decrease in thickness for all filters, reaching thicknesses of 0.17 mm for pressed and filtered filters, and 4.03 mm for freeze-dried filters. Conversely, increasing the weight to 160 g·m −2 only affected the thickness of pressed filters (0.32, 0.34, 0.42, and 0.45 mm for pressing pressures of 8, 6, 4, and 2 t, respectively), while lyophilized filters remained within the same thickness range.
A second generation of filters was developed based on the first-generation filters with the best results, varying filter grammage for a more in-depth analysis. Finally, with the best filter obtained (in terms of particle capture efficiency results), a third and final generation was developed by modifying the main variable of its manufacturing process to optimize the filtration results.
A first generation of filters with a grammage of 100 g·m(typical filter grammage) were manufactured using both alkali-treated pulp and bleached pulp with four manufacturing process variants ( Figure 1 ): crushing, vacuum filtration, pressing, and freeze-drying. For the first variant, the crushed pulp (as described in Section 2.1 ) was molded into a circular shape using a Petri plate, after being previously moistened to facilitate its handling. In the second variant, the pulp was homogenized in 20 mL of deionized water under magnetic stirring for five days and filtered under vacuum with a Buchner funnel at a vacuum pressure of 8 mbar and using a filter with Whatman filter paper grade 1. After filtration, the filters were dried in a stove at about 30 °C for 24 h. In the third variant, the pulp was placed in a Petri place to acquire the circular shape and pressed using a hydraulic press, applying a pressure of 8 t. As in the case of crushed filters, the pulp was pre-wetted. Finally, in the fourth variant, the freeze-drying technique was carried out, which consists of freezing the sample and removing the water by sublimation under vacuum conditions (0.3 mbar). The pulp was homogenized following the same procedure described for the filtered filters. The resulting mass was poured into a Petri place and was placed in the freeze-dryer, with a freeze temperature of −57.2 °C. Figure 1 shows the filters resulting from each process, manufactured in triplicate.
The physical characterization of both the alkali-treated and bleached pulp was carried out using the following three techniques. Scanning electron microscope (SEM) images were taken with a Hitachi TM3030 scanning electron microscope (Hitachi High-Tech Corporation, Tokyo, Japan) operating at 15 kV. Samples were Au/Pd coated using a Quorum SC7620 mini sputter coater (Quorum Technologies Ltd., Laughton, UK). Fourier transmission infrared spectroscopy (FTIR) was applied using a Thermo Scientific infrared spectrophotometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) in the range of 400 to 4000 cm −1 and 32 scans. The disk was prepared with KBr at a pressure of approximately 2 t. Finally, thermogravimetric analysis (TGA) was performed using a NETZSCH STA 449 F3 Jupiter (NETZSCH-Gerätebau GmbH, Selb, Germany). The samples were weighed in alumina crucibles and heated from 40 °C to 900 °C, with a heating rate of 40 °C/min and in an inert atmosphere, with a nitrogen flow rate of 20 mL·min −1 .
As a secondary treatment, and in a second variety of filters, a bleaching process was applied to the alkali-treated pulp to remove as much of the remaining lignin and hemicellulose as possible and to analyze potential filtration improvements. The procedure is as follows: (1) 160 mL of 0.3 M NaClOwas added to 1 g of pulp, as well as 2 mL of glacial acetic acid to adjust the pH to approximately 4.5. The bleaching reaction was carried at 70 °C for an hour and with slight magnetic stirring; (2) the bleached pulp was filtered under vacuum using a Buchner funnel and Whatman filter paper grade 1. It was then washed with abundant water until a neutral pH was achieved; and (3) the bleached pulp was dried at 35–40 °C in a stove and also crushed as described in Section 2.1
The banana fibers were obtained from banana pseudostem (Dwarf Cavendish variety) using a slitting machine designed and patented by the Integrated and Advanced Manufacturing Research Group [ 63 ], where the pseudostem fibers are mechanically separated by different and automated steps. Extraction of the cellulose pulp from fibers was carried out using a delignification method with soda-anthraquinone (alkali treatment). A large amount of lignin was removed during this process (about 70%), enabling purer cellulose fiber to be obtained. For this, NaOH, deionized water, and anthraquinone, which acts as a catalyst, were added to the fibers, for a reaction time of 2.5 h at 160 °C. At the end of this time, the alkali-treated pulp obtained was washed with water, and then crushed using a ZM 200 ultra-centrifugal mill (Retsch GmbH, Haan, Germany) with a built-in sieve with a 0.5 mm light passage. For the air filter manufacturing, only the centrifuged pulp was used (homogeneous and fine pulp).
Comparing the performance of these hybrid filters to the tested commercial filters ( Table 9 ), the efficiencies are very close to those of surgical respiratory filters but still fall short of high-efficiency fiberglass filters, albeit with lower pressure drops than the latter. Observing the efficiency trend of hybrid filters, increasing pressures could potentially further enhance filtration efficiencies, although at the expense of generating higher pressure loads. In any case, the results of these hybrid filters reveal their potential application in industrial filtration processes by combining efficiencies and pressure drops that are acceptable for a wide spectrum of airborne particulate filtration processes.
The increase in pressing pressures led to a gradual improvement in filtration efficiencies across all particle size ranges, although it also resulted in a progressive increase in pressure drop, albeit significantly lower than pressed filters. Particle retention efficiencies close to 90% for PM10 and PM2.5 sizes were achieved. The combination of these two processes demonstrates how it is possible to achieve higher efficiencies with lower pressing pressures (2 t) compared to the best pressed filters (8 t), while maintaining reasonable pressure drop levels.
Due to the superior filtration efficiencies of pressed filters and the low pressure drop of freeze-dried filters, a final variation of filters has been generated by combining freeze-dried processes, to achieve a better fibrillar structure and pressing. From freeze-dried filters with a grammage of 160 g·mand bleaching treatment, different replicas of hybrid filters have been obtained with various pressing pressures, corresponding to 0.25, 0.5, 1, and 2 t. Table 8 presents the particle capture efficiency for each of the manufactured hybrid filters, along with their corresponding pressure drop values.
As for the pressed filters, significant differences were observed between unbleached and bleached types. In the former, there are no increase in capture efficiencies in any of the grammages studied. In the latter, an increase was observed in the grammage of 160 g·m, reaching very high efficiency values of close to 90% in PM10 and PM2.5 (an increase of 10% and 9%, respectively) and 83% (a 12% increase) in the PM1 range. This may indicate that the bleaching process improves the contact surface, since pulp components such as the lignin are removed, as previously mentioned. The change in grammage from 100 g·mto 160 g·malso caused an increase in the pressed filter pressure drops ( Table 6 ). This may reveal a greater disposition of the filters with a grammage of 160 g·mto clogging, decreasing their useful life. However, in the freeze-dried filter with a grammage of 160 g·m, the pressure drop decreased only in the alkali-treated sample. This could be attributed to the increased need for water and agitation during the manufacturing of the filter, given its higher mass, resulting in a more compact material.
The decrease in capture efficiency observed in the filters with a grammage of 70 g·mwas more significant in the freeze-dried filters ( Figure 10 ). Less homogeneity was observed in these filters due to the lower mass of the pulp used. In addition, in the freeze-dried filters with bleached pulp, it was observed that the number of particles at the outlet was higher than at the inlet in the case of sizes larger than eight microns, which may be due to the dragging of the filter material by the aerosol stream. For this reason, these filters were discarded from the analysis. In contrast, in this type of filters (with bleached), the highest increase in capture efficiency was experienced in the case of the grammage of 160 g·m, with a 67% increase in PM10 and PM2.5 and a 58% increase in PM1. However, the obtained efficiency values were low, falling below 50%.
With respect to the filtered filters with a grammage of 70 g·m −2 , the particle capture efficiencies of the filters manufactured with bleached pulp decreased, especially in the range PM1. In contrast, the particle retention efficiencies of the alkali-treated filters with the same grammage increased. This may also be related to the problem of grammage control as mentioned above, requiring further in-depth analysis.
Considering the higher particle retention efficiencies observed in the filtered filters with a grammage of 100 g·m, it was initially also decided to test those with a grammage of 160 g·m, despite the high pressure drops. At the end of the drying stage at room temperature in the manufacturing process, filter diameters decreased about 7 mm, which meant a drastic change in the final grammage, passing from 160 g·mto 200 g·m. For this reason, these filters were dried using an absorbent material. On this occasion, the grammage was maintained, but anomalous particle capture efficiency and pressure drop values were obtained. A SEM analysis ( Figure 9 ) was carried out, and it was observed that the surface in the filtered filters of 160 g·mwas less uniform than in the case of those of 100 g·m, which might allow better flow of the aerosol. In view of this and the fact that it was not possible to control the grammage with the vacuum pressure applied in this study, the filtered filters of 160 g·mwere not considered in this secondary study. We intend for them to be the subject of future research.
In view of the results obtained for filters with a grammage of 100 g·m −2 , filters with lower and higher grammage (70 g·m −2 and 160 g·m −2 , respectively) were tested in order to evaluate the effect of the grammage on capture efficiency and pressure drop. In addition, as the pressed filters with bleached pulp had high efficiencies but also high pressure drops, the influence of pressing pressure was also analyzed by testing filters pressed at 2, 4, and 6 t. The results obtained are described in the following subsections.
In addition to filter efficiency, the pressure drop across each filter was studied. These values are shown in Figure 7 in red. The lowest pressure drop of 17 Pa was recorded with the freeze-dried filters manufactured with bleached pulp, which implies a reduction greater than 90% compared to the filter manufactured with alkali-treated pulp. As mentioned above, the bleached pulp fibers were more separated and cleaner in the SEM images ( Figure 8 ). The lignin in the pulp filters, which acts as a binder, provides greater resistance to the aerosol pass as well as to reducing the filter sponging. In contrast, in the pressed filters, the pressure drop was around twice as high in the bleached pulp filters (813 Pa). A possible reason for this may be the greater effect of the press in the bleached pulp because the fibers were freer, which may allow for greater fiber coupling and bundling. Finally, the pressure drop was also higher in the bleached pulp filtered filters than in the alkali-treated type, with values above 2500 Pa in the former, possibly due to the same reason as in the case of pressed filters. In summary, considering the above, the filters that were subjected to a compression force, as in the case of filtered and pressed filters, had higher pressure drops, creating very little space between fibers and, thus, significantly reducing the air flow, as observed in the SEM images ( Figure 8 ).
With respect to data reproducibility, the filter type that showed the highest variation was the freeze-dried pulp filter, with 10, 14, and 11 times higher standard variation for PM10, PM2.5, and PM1, respectively, compared to the freeze-dried bleached pulp filter. This was also observed with the filtered filter, with a standard deviation twice as high as in the case of the pulp filters. In addition to the effect of the bleaching process, for the manufacturing of both freeze-dried and filtered filters, the pulp (alkali-treated and bleached) was subjected to magnetic stirring for five days. This homogenization process may be affected by external variables such as the stirring power or the structural characteristics of the type of treated pulp used in each case. In contrast, pressed filters’ variability was very low for both alkali-treated and bleached pulp types, with standard deviation values below 3% for all particle size ranges. In this manufacturing process, the pulp used was only wetted and, finally, pressed at the same pressure, reducing the effect of external factors.
Average differences between the processes tested for the same treatment were studied by applying the Kruskal–Wallis test ( Table 5 ) due to the lack of normality of the data set and the small sample size. The results confirmed there were no significant differences between the efficiencies of each process (freeze-drying, filtration, and pressing) for alkali-treated pulp filters. In the case of bleached pulp filters, no significant differences were observed between the filtered and pressed types.
Efficiencies were very similar between filtered and pressed filters. Efficiency values higher than 80% were achieved for PM10 and PM2.5 in the case of filters with bleached pulp and higher than 70% in filters with alkali-treated pulp. The capture efficiencies for the smallest particles were lower, 80% and 56% in the filtered filters manufactured with bleached pulp and with alkali-treated pulp, respectively. To analyze the effect of pretreatment (alkaline and bleaching), the mean differences between both processes were examined, regardless of the manufacturing process, through a two-samples Hotelling’s Ttest. The results are shown in Table 4 . F-values greater than 1 indicate significant differences between the average efficiencies of the bleached pulp filters and alkali-treated pulp filters in the vacuum filtration and pressing processes, with the differences being slightly higher in the first case. This is confirmed with-values below the significance level (= 0.05), discharging the H(equal means).
A semi-quantitative analysis of these granulates was performed, and metals such as manganese, silicon, and calcium were detected in the case of the alkali-treated pulp ( Figure 4 A). The same analysis only revealed the presence of carbon and oxygen in the case of bleached pulp ( Figure 4 B), with the metals possibly removed during the bleaching process. Fiber size was between 15 and 20 microns, with no changes observed after the bleaching and crushing processes.
4. Conclusions
In this study, air filters for capturing fine and ultrafine particles were manufactured with cellulose pulp extracted from banana pseudostem fibers using both alkali-treated pulp and bleached pulp. Four manufacturing processes (crushing, freeze-drying, pressing, and vacuum filtration) were tested, analyzing the capture efficiencies of different particles size ranges: PM10, PM2.5, and PM1. The other parameter studied was the pressure drop of the filters.
Initially, filters with a grammage of 100 g·m−2 were manufactured. Given the results obtained, the crushing process was discarded because the particle retention efficiencies were very low. In contrast to crushed filters, the filtered and pressed filters manufactured with bleached pulp had high efficiencies, reaching as high as 70% of efficiency in the smallest particles retention. However, these last two types of filter also showed high pressure drops, especially in the filtered type, with values above 2500 Pa. In the case of freeze-dried filters, the highest efficiency values were obtained with alkali-treated pulp as opposed to bleached pulp. Nevertheless, these efficiency values remain very low compared to pressed and filtered filters, despite having a favorable pressure drop.
Regarding the analysis of grammage variation (70 g·m−2 and 160 g·m−2), an increase in grammage generally results in an improvement in filtration efficiency. Specifically, the pressed filter type with bleached pulp and a grammage of 160 g·m−2 showed the best results, with efficiencies exceeding 83% in all particle size ranges. However, this type of filter exhibits a relatively high pressure drop, exceeding 2000 Pa. Therefore, after analyzing the influence of pressing pressure on filtration results (tests with filters at 2, 4, 6, and 8 t), it can be concluded that filters pressed at 4 t, featuring a grammage of 160 g·m−2, and manufactured with bleached pulp, represent one of the best solutions. These filters exhibit good capture efficiencies (80.5% for the smallest particles and 88% for others), very close to the maximums observed in filters pressed at 8 t, but with a pressure drop below 1000 Pa.
Considering the combination of filter manufacturing processes and in the final experimental phase, employing a combination of freeze-dried and 2-ton pressing processes yields enhanced results (83% for the smallest particles and 89% for others) with approximately half the pressure drop (558 Pa), establishing it as a superior overall solution. This hybridization of processes allowed the integration, in a single filter, of the advantages that each forming process contributes to them, paving the way for potential future research directions.
Finally, the results of this exploratory study demonstrate the potential use of agricultural waste materials from banana cultivation to develop environmentally friendly filtering materials for airborne particles, through simple manufacturing methods, providing an alternative to fuel-derived polymers. Indeed, considering that conventional shaping processes and basic pulp treatments have been investigated, there are still numerous opportunities for improvement to explore, such as the incorporation or combination of other filter shaping technologies, additional physicochemical treatments, defibrillation of cellulose, as well as the multilayer structuring of filters, among many other possibilities. These alternatives could significantly enhance the results obtained in future research.
