Figure 1 . Demonstration of test apparatus . Masks and target paper were secured using the cardboard test box , as illustrated in panel A . Target paper was sprayed twice for each mask , then viewed and imaged under UV light using a commercially available handheld light source . Results are shown in panel B .
variable construction : 1 ) A single-layer cotton neck gaiter , 2 ) a single layer cotton mask , 3 ) a two-layer cotton mask , and 4 ) a three-layer surgical style mask with filter . The results are presented in Fig . 1B . It is clear from the images that the neck gaiter was least effective in blocking the hairspray droplets . The single layer cotton mask was more effective than the neck gaiter , while results for the two-layer cotton mask and the surgical-style mask were similar . Importantly , these results are consistent with controversial preliminary data indicating that neck gaiters are poor droplet barriers ( Fischer et al ., 2020 ) and , combined with the data showing greatest droplet reduction with the positive control surgical-style mask , help to validate the test method .
Discussion
Although we believe this test method may be useful , it is important to understand that any convenient at-home test of mask function is likely to be crude given the limited scientific sophistication that is possible for the average consumer . Ours is no exception . For one thing , the physicochemical properties and aerosol behavior of propellant-driven hairspray droplets probably differ significantly from those of respiratory droplets produced by speaking , coughing , or sneezing . Second , the power of the light source , the properties of the fluorophore , and the dynamic range of the human eye probably limit the analytical sensitivity of the test . Finally , it is widely thought that the respiratory droplets most likely to spread infection through aerosols have diameters ≤5 μm 3 and median droplet diameter for propellant-driven hairsprays like ours is approximately 25 μm . 5 Thus , a mask could theoretically prevent almost the entire fluorescent signal in this test method but still allow infectious virus particles to pass through . So , it must be stressed that this is not a test of the ability of a mask to prevent virus infection or transmission . Nevertheless , around 5-10 % of hairspray droplets are in the 1-10 μm range 7 so the test could have some value to detect even those small droplets . Furthermore , the utility of the 5 μm diameter cutoff is somewhat controversial in the first place . For example , a droplet that starts at 10- 100 μm quickly becomes a 1-5 μm particle ( or even simply the remnant nucleus ) within milliseconds to seconds through evaporation of these very small liquid volumes . 5 Since liquid droplets in this range can remain in the air for seconds to minutes , that is more than enough time for them to evaporate and become cause for concern . 5 Moreover , larger droplets deposited on surfaces that we touch or in the upper airways may still be trapped or absorbed and lead to infection . Finally , the widely used 5 μm cutoff seems to be based in part on toxicology studies of particulate matter ( PM ) inhalation ( for example , Brown et al ., 1950 ; Morawska et al ., 1999 ), but solid PM is obviously not subject to evaporation in the air or absorption in mucosa in the same way as respiratory droplets and their content . Therefore , it is likely that the method presented here provides at least some indication of how well a mask works to reduce aerosol and droplet spread , though it should not be considered a test of infectiousness . It is also inexpensive , fast , and can be assembled using widely available resources . Overall , we conclude that this is a rapid and convenient test that the average consumer may use to compare the barrier function of their masks to determine which mask in their wardrobe likely offers the greatest protection .
References
1 . Brown JH , Cook KM , Ney FG , Hatch T . Influence of particle size upon the retention of particulate matter in the human lung . Am J Public Health Nations Health . 40 , 450-458 , 1950 .
2 . CDC COVID-19 Response Team ; Jorden MA , Rudman SL , Villarino E , Hoferka S , Patel MT , Bemis K , Simmons CR , Jespersen M , Johnson JI , Mytty E , Arends KD , Henderson JJ , Mathes RW , Weng CX , Duchin J , Lenahan J , Close N , Bedford T , Boeckh M , Chu HY , Englund JA , Famulare M , Nickerson DA , Rieder MJ , Shendure J , Starita LM . Evidence for Limited Early Spread of COVID-19 Within the United States , January-February 2020 . MMWR Morb Mortal Wkly Rep . 69 , 680-684 , 2020 .
3 . Fennelly KP . Particle sizes of infections aerosols : Implications for infection control . Lancet Respir Med . 8 , 914-924 , 2020 .
4 . Fischer EP , Fischer MC , Grass D , Henrion I , Warren WS , Westman E . Low-cost measurement of face mask efficacy for filtering expelled droplets during speech . Sci Adv . 6 , eabd3083 , 2020
For a complete list of references , email ams @ arkmed . org .
Volume 117 • Number 12 june 2021 • 283