key: cord-0760214-va0oe7xb
authors: Ren, Jianlin; Tang, Mengjia; Novoselac, Atila
title: Experimental study to quantify airborne particle deposition onto and resuspension from clothing using a fluorescent-tracking method
date: 2021-11-26
journal: Build Environ
DOI: 10.1016/j.buildenv.2021.108580
sha: 734e780e838f34ef41ca510f6327f2ed73916754
doc_id: 760214
cord_uid: va0oe7xb

The rapid spread and high level of morbidity of the SARS-CoV-2 virus during the COVID-19 pandemic has attracted considerable attention worldwide. Recent studies have shown that clothing is one of the vectors for the transport of airborne particles, including bioaerosols. This study developed a method that can both quantify the deposition of particles onto clothing and the resuspension of particles from clothing using a fluorescent-tracking technology and found that electrical tape can be used as a fluorescent particle collector on irregular clothing surfaces. Results show that 0.07%–6.61% of the fluorescent particles (FPs) previously loaded on the room flooring surfaces moved to the occupant's clothing during the 20-min sampling periods; the percentage depended on the type of activity and the range is for: office work, walking, and vacuuming. Furthermore, both the flooring type (carpet or vinyl composition tile) and flooring condition (clean or dirty) had significant effects on particle resuspension and transport to the occupant's clothing. The average particle deposition factor for carpet flooring was 2.7 (±1.4) times that for vinyl composition tile flooring, while the average particle deposition factor for dirty flooring was 2.4 (±1.6) times that for clean flooring. A multiple regression analysis shows that the activity type had the largest effect on the particle transport among all experimental variables. An additional experiment performed in a full-scale house shows that 46.8% of FPs formerly seeded on clothing resuspended from clothing and dispersed around the house during the 1-hour period of light walking at a speed of 60 steps/min.

infections and illnesses in healthcare workers, patients, and the community. These 1 studies prove that clothing is a vector for particle transport from outdoor to indoor 2 environments, as well as transport within indoor environments. However, evidence that 3 quantifies the importance of this process with regard to particle size, particle 4 aerodynamics, and quantitative characterization remains limited [16] . In addition, 5 differences in particle deposition onto different parts of clothing or the human body 6 have rarely been reported in the published literature, which is important for exposure 7 calculations [17] . 8 Clothing is an airborne particulate source as the initially deposited particles can be 9 resuspended by human movement and become re-airborne; this is a potential source for 10 secondary exposure [16] . Some previous studies have regarded clothing as a particle 11 source by increasing the detachment of skin flakes via friction [18] . However, recent 12 studies have demonstrated that resuspension of outdoor-derived bioaerosols from 13 occupant clothing and indoor surfaces was a stronger source than direct shedding from 14 human bodies [9] . Tian et al. [19] studied the effect of clothing coverage, clothing color 15 and clothing condition on bioaerosol shedding and resuspension in an environmental 16 chamber, and the particle emission rate while walking with a 90 step/min frequency 17 was calculated using a mass-balance model. Licina and Nazaroff [6] found that 0.3%-18 3% of deposited particles with a size of 0.5-10 μm were released on average with the 19 movement of the fabric. McDonagh and Byrne [20, 21] found that physical activity 20 resulted in up to 67% of the deposited particles being resuspended into the air. These 21 quantitative studies were all conducted in well-controlled experimental chambers, and 22 the accurate data about the dispersion of particulate matter resuspended from clothing 23 in real buildings are still scarce. 24 While previous research has discovered the role of clothing in transporting 25 particles, some key information is still lacking. First, quantitative analysis of particle 26 deposition onto clothing and resuspension from clothing is scarce. Different from the 27 biological particle tracking methods used in previous studies, this study used a 5 The first scenario was conducted in a well-controlled stainless-steel environmental 6 chamber with a size of 3×3×3 m. The temperature and humidity in the chamber were 7 kept the same and recorded by HOBO data loggers (UX100-003, Onset, Inc., Bourne, 8 MA) (Table 1) . A male volunteer (who is also the first author of the paper) with a height 9 of 1.75 m and weight of 70 kg remained in the chamber to simulate three common 10 activities typical for an office or a home environment; these are office work (reading a 11 book while occasionally putting down the book and walking for 30 s every 5 min), gloves (Model G10, VWR Corp., Radnor, PA) and glasses. It should be noted that the 17 protective apparel was made of high-density polyethylene fibers randomly laid and 18 J o u r n a l P r e -p r o o f compressed, whose material and weave pattern are different from ordinary clothing. 1 The protective apparel was chosen for its non-linting and anti-static properties [21] .

2 Figure 1 shows the locations of the samples and instruments in the first part of the 3 experiments. There were nine sampling points, referred to as "on-body samples" on the 4 apparel: three on the chest (C1 -C3), three on the arms (A1 -A3) and three on the legs 5 (L1 -L3). Another nine samples, referred to as "static samples", were set in groups of 6 three at heights of 1.5 m, 1.0 m and 0.5 m on a support in the center of the chamber, 7 corresponding to the on-body samples on the chest, arms and legs (denoted by SC1 -8 SC3, SA1 -SA3 and SL1 -SL3, respectively). In addition, three particle counters 9 (Model 9306-V2, TSI, Inc., St. Paul., MN) were placed on a support at the same three 10 heights as the static samples in the corner of this chamber to measure the real-time 11 airborne particle concentrations. [25] . The "dirty" flooring condition was generated by uniformly releasing 1 ultrafine test dust (A1 dust, Power Technology, Inc., Arden Hills, MN) into the chamber 2 prior to the seeding of FPs. The A1 dust-loading density was 14.1 g/m 2 (shown in Table   3 1), which was large enough to form multilayer particle-to-particle deposits [26] . In real 4 situations, the particle-loading density has been measured to be 6.2-20.3 g/m 2 [26, 27] . 5 In addition to the flooring types and dirty/clean conditions, the other settings and 6 procedures for the four cases were similar. The 1 st step was A1 dust generation and 7 seeding (not applied to cases 3 and 4). A total of 130 grams of A1 dust was injected 8 into the chamber with a homemade generator for 5 min and left to settle for two hours. Four mixing fans in the corners were used to uniformly distribute the particles. The 2 nd 10 step was FP generation and seeding. Fifteen glass slides (Premiere 9101-E) were 11 uniformly placed on the floor prior to this step. FPs were injected into the chamber for 12 six hours (details provided in Section 2.2). The mixing fans remained on during the 13 injection. The chamber was left unoccupied for approximately 16 hours to provide 14 sufficient time for the deposition of FPs. Then, the glass slides were collected to count 15 the FP-loading density on the flooring. As summarized in Table 1 , the initial FP-loading 16 density was very similar in all cases (with the largest relative difference <15%). The 3 rd 17 step was office work activity. The following day, one hour of office work was 18 conducted in the chamber by the volunteer. Three groups of on-body samples were 19 collected, each sampled for 20 min. At 20 min and 40 min, the on-body samples were 20 carefully collected by another researcher and replaced with a new set of samples.

samples were collected in this scenario. The FP-density on each sample was then 1 counted and computed (details provided in Section 2.2).

2 Different from previous studies [24], electrical tape was used to collect particles 3 instead of glass slides (shown in Figure S1 ). In the first scenario, the majority of 4 samples were on-body samples. Tape was more adaptable to different body parts and 5 could not slip or fall during activities. Preliminary experiments were conducted to prove 6 the feasibility of this method. First, the sampling accuracy of the tape and glass slide 7 was compared. Three commonly used kinds of tapes were compared: electrical tape, 8 medical tape and black duct tape ( Figure S1 ). Five of each sampling medium were 9 placed randomly in a 1×1×1 m stainless-steel chamber ( Figure S2 ). FPs were injected 10 into the chamber for 10 min, and two small mixing fans were used to mix the generated 11 particles well. After generation, the FPs were left to deposit for 24 hours. Then, the 12 samples were collected and the number of particles on these samples were counted. 13 This experiment was repeated three times. The results are summarized in Figure S3 . FPs were seeded on ten tape samples. For each sample, we taped the electrical tape on 21 clothing and then removed it. This process was repeated three times. The initial number 22 of particles and the numbers after being removed for the first, second and third times 23 were counted. As shown in Figure S4 , 97.5% of particles remained on the tapes after 24 the first removal, and 92.1% of particles remained after the third removal. These 25 preliminary experiments proved that electrical tape performed as well as glass slides in 26 collecting particles, and this tape is more suitable for use on irregular surfaces, e.g., the 27 human clothing. The second scenario was conducted in an unoccupied one-living room/three-2 bedroom/two-bath test house [24] in Austin, TX, with a floor area of 111 m 2 . A detailed 3 description of the test house can be found in [28] . There was minimal furniture inside 4 the house, i.e., a few tables and chairs. Although there were two heating, ventilation, 5 and air conditioning (HVAC) systems, the house air-handling units were turned off 6 during experiments to prevent particle deposition in HVAC components and avoid its 7 impact on particle transport. The average temperature and humidity during experiments 8 were 18.4 °C and 51% RH (Table 1) . During experiments, all exterior doors and 9 windows were closed while all interior doors were open except the doors of closets, 10 which allowed for temperature difference and buoyancy driven air circulation between 11 rooms. Thirty-nine sampling points were set on the floor, and each point had three glass 12 slides; therefore, 117 samples were collected from the house floor. The layout diagram 13 of the test house and samples is shown in Figure S5 . function and they were concatenated into one photo, as shown in Figure S6 . In this study, we define "the particle deposition factor" for the first scenario and 2 "the particle resuspension factor" for the second scenario. The particle deposition factor 3 for the i th sample ( ) can be defined by the following equation:

where Ni is the number of FPs on sample i (particles/cm 2 ) and Nfloor is the number of 6 seeded FPs on the flooring (particles/cm 2 ).

Ni was positively correlated with the particle mass flux due to the same sampling 8 period in different cases [29]:

(2) 10 The particle mass flux ( PM ) can be calculated by the following equation [29]:

where is the fluorescent particle concentration (μg/m 3 ) and is the 13 particle deposition velocity (m/h).

14 Because was approximately the same among all cases in the first scenario 15 (Table 1) Although a quantitative formula is not given due to the complexity of particle 19 deposition onto the human body, the qualitative relationship derived in Equation 4 can 20 still be used in the discussion of the results in this study.

The particle resuspension factor ( ) was defined for the second scenario:

where is the number of FPs on the i th sample on the floor in the second scenario 24 (particles/cm 2 ) and ℎ is the number of seeded FPs on clothing (particles/cm 2 ). 25 The clothing release fraction (CRF) is defined as the ratio of released to deposited Apart from the tracking of FPs on tape samples in the first scenario, three particle 10 counters (Model 9306-V2, TSI, Inc., St. Paul, MN) were used to measure the airborne 11 particle concentrations every 30 s. Because the FPs have a diameter of 3.2 μm, the real-12 time particle number concentration in the size bin of 2.5-5 μm was used to calculate 13 the particle resuspension rate by the following equation:

where r is the resuspension rate (min -1 ); V is the chamber volume (m 3 ), 27 m 3 in this 16 study; Ar is the resuspension area (m 2 ), 9 m 2 in this study; L(t) is particle loading in the 17 size range of interest (particles/m 2 ); Ci is the particle number concentration in the size 18 range of interest (particles/m 3 ); and kn is the deposition loss rate (min -1 ), which is 0.015

In this study, the uncertainty of the results was not caused by the uncertainties 2 associated with the detection and quantification of FPs on a given sample but rather the the corresponding standard deviations were calculated. In addition, all three particle 7 counters used to measure airborne particle concentrations were calibrated with the more experiments (data not reported here) were conducted for both the chamber and test 10 house scenarios when developing the experimental methodology to ensure adequate 11 repeatability of the particle seeding and tracking methods. 

The results from the experiments in the environmental chamber (cases 1 to 4 from 15 the first scenario) characterize the particle deposition factor while results from the 16 experiment in the full-scale house (the second scenario) define the particle resuspension and show the dependency of particle deposition factors on the activity level and position on clothing. During the 20-min periods of the three activities, the average particle 1 deposition factor ranged from 0.07% to 6.61%. A large variation in the deposition factor 2 exists amongst different activity types within the same case (p<0.05). Specifically, results in Figure 3 show that the deposition factor is larger for 10 walking; the average particle deposition factor for office work and vacuuming in each 11 case is 16.3%-55.6% and 16.4%-41.9% of the average deposition factor for walking.

It is not surprising to find that the particle deposition factor during office work was 13 smaller than that during walking due to the relatively lighter activity strength of office and sitting on furniture almost tripled that of one person performing the same activity. 2 Qian and Ferro [32] observed that a heavy and fast walking style was associated with 3 higher resuspension than a less active walking style. In the current study, walking 4 resuspended more particles compared to office work, which caused at least 1.8 times 5 more particles from the floor to deposit onto clothing.

The finding that vacuuming was associated with smaller particle deposition factors respectively). Also, the airborne particles measured by particle counters included FPs

(cases 1 -4) and A1 dust particles (cases 1 and 2). The gray area represents the three 20 20-min activity periods for each activity type in each case. Between every two activity 21 periods, particle concentrations decayed to some extent; however, the gap time was not 22 long enough for the airborne particle concentrations to decay to the background level.

Additional results on effect of flooring on airborne particle concentration can be found 24 in Figure S7 . Coinciding with the first 20-min activity event during walking, activities, walking introduced at least twice more particles by resuspending them from the dirty floor (cases 1 and 2), which explains the higher deposition factor for walking 1 in these two cases (Figure 3 ). In contrast, in cases 3 and 4 where the floor was not pre-2 loaded with dust, the number concentrations of resuspended particles during the three 3 activities are similar (Figure 4) . The higher deposition factor for walking in these two 4 cases shown in Figure 3 can be attributed to a higher particle deposition velocity to 5 clothing during walking. As shown in Equation 4, the particle deposition factor is 6 positively correlated to vd (particle deposition velocity to the clothing) in addition to in case 1 with dirty carpet is the highest, followed by case 2 with dirty VCT or case 3 5 with clean carpet, and lastly case 4 with clean VCT. Especially for the walking activity, 6 the average particle deposition factor in case 1 (5.79 ± 0.59%) was 2.6, 2.8 and 5.6 7 times the deposition factors in case 2 (2.20 ± 0.74%), case 3 (2.04 ± 0.37%) and case 4 8 (1.04 ± 0.33%), respectively. Therefore, both the flooring type (carpet/VCT) and 9 condition (clean/dirty) have significant effects on particle deposition factors, and the 10 deposition factor for walking is the most sensitive to these two factors. Table 2 shows the resuspension rates 2 of FPs and A1 dust in cases 1 and 2 which were calculated by using the airborne particle 3 number concentration reported in Figure 4 and Equation 7. The particle resuspension 4 rates for carpet were 3.05, 2.75 and 2.85 times those for VCT under office work, 5 walking and vacuuming, respectively. These results agree with those from previous 6 studies related to activity-induced particle resuspension conducted in chambers and real The ratio of the deposition factor on dirty flooring over clean flooring for the same 22 flooring type (carpet/VCT) is shown in Figure S9 . Resuspension of FPs which were on the top of multilayer particle structure was 2 considerably greater than FPs which were on the clean floor as monolayer deposits; this 3 was likely due to reduced particle-to-particle adhesion forces, resuspension in the form loading also has a significant effect on particle resuspension [36] . 11 For the chamber experiments conducted for this study, the particle deposition 12 factor ratios for the sampling periods can be defined by the following equations: These two particle deposition factor ratios are presented in Figure 6 . Results show that 16 the sampling period has no significant effect on the measured particle deposition factors 17 (p>0.05). For all cases, the average (± standard deviation) _ is 1.09 ± 0.49, and 18 the average _ is 1.09 ± 0.67. However, for vacuuming activity, the particle 19 deposition factors during different sampling periods are different: _ < _ 20 < 1. One possible reason for this is the difference in the particle concentration in the air 21 (Equation 4). The particle deposition velocity should be the same for the three 20-min 22 periods during the same activity, it is possible that the difference in the particle 23 deposition factor for different sampling periods was caused by the variation of the 24 particle concentration in the air. The increasing or constant concentration of 25 resuspended particles in walking or office work activities (shown in Figure S7 ) can 26 explain why ratios _ and _ are larger than or close to one. However,

vacuuming activity has a similar trend (increasing or constant concentration as shown 28 in Figure S7 ), but it has ratios mostly less than one. One possible reason for this discrepancy is the fact that in this specific experiment, the majority of FPs could be 1 collected in the vacuum bag during vacuuming activity; however, this discrepancy does 2 not change the fact that the sampling period has no significant effect on the measured 3 particle deposition factors. The average _ for office work, walking, vacuuming and the sum of all 16 activity types was 1.11 ± 0.32, 1.37 ± 0.37, 1.28 ± 0.68 and 1.34 ± 0.75, respectively. In this study, the particle deposition factor was affected by many experimental variables. Therefore, a multiple regression analysis [44] was conducted to achieve the 1 best-fit models for the particle deposition factor and other experimental variables. Five 2 characteristics discussed before were considered in the modeling: activity type (office Table   7 S2, and the modeling results are presented in Table 3 . According to the statistical results, 8 the relationship between the particle deposition factor and three characteristics was is analyzed individually. The ratios of the particle deposition factors amongst different 12 sampling periods and sampling locations are close to one, which makes these two 13 factors unlikely to be important in the multiple linear regression. In addition to 14 excluding these two factors, Table 3 shows that the activity type has the largest absolute 15 value of the standardized coefficient (| |), which indicates that the activity type has the 16 largest effect on the particle deposition factor in this study. Unlike the flooring type and 17 flooring condition which affected the particle deposition factor solely by the 18 concentration of resuspended particles, the activity type had an extra impact on the 19 deposition velocity onto clothing.

The results of particle resuspension from clothing in the test house in the second 3 scenario are presented in Figure 8 . The color code (from bright to dark) shows the 4 distribution of resuspended FPs from clothing in different sections of the test house. 5 Results show a widespread distribution of particles. The particle resuspension factor 6 calculated by Equation 5 in the test house experiment ranges from 0.08% to 1.85% at to the heights of on-body samples. Figure 9 shows the comparison between the on-body way of walking, shoe type, etc., are likely to affect on-flooring particle resuspension [27, 46] , and they may also affect the rate of particle resuspension and deposition on 1 clothing. Temperature and humidity can also affect particle resuspension and deposition following experiment. 10 It would be of value to continue studying the effect of ordinary clothing other than 11 protective apparel on particle deposition and resuspension. It is also worthwhile to 12 further study the effect of other factors on particle deposition onto and resuspension 13 from clothing, such as temperature/humidity, static electricity on clothing, different 14 kinds of vacuum cleaners, different vacuuming behaviors, and so on. 

In this study, we developed a novel and accurate sampling method to measure the 18 deposition of particles onto clothing and the resuspension of particles from clothing 19 with fluorescent-tracking technology. Electrical tape was proven to perform as well as 20 glass slides in particle collection and is more suitable for use on irregular surfaces. (carpet/VCT) and flooring condition (clean/dirty) had significant effects on particle 28 deposition onto clothing. The average particle deposition factor for carpet flooring was 29 2.71±1.40 times that for VCT flooring, which was mainly caused by the difference in the particle resuspension rate. The average particle deposition factor for dirty flooring 1 was 2.43±1.57 times that for clean flooring. Sampling periods had no significant effect 2 on the measured particle deposition factors. The number of FPs on arm samples was 3 11% -37% times larger than that on chest and leg samples. A multiple regression shows 4 that the relationship between the particle deposition factor and three characteristics 5 (flooring type, flooring condition and activity) was significant (p<0.05). The activity 6 type had the largest effect on the particle deposition factor in this study. 

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