Wearing face masks has become second nature to most of us during the coronavirus pandemic. Masks are known to mitigate the risk of airborne infectious disease transmission, and studies of their efficacy have become increasingly common. But researchers still lack a rigorous framework for estimating the potential health benefits of mask wearing more generally—as a means of reducing our exposure to dust, fossil-fuel combustion products, or wildfire smoke.

Colorado State University’s John Kodros and colleagues have now done a study that helps to fill that gap. They measured the filtration efficiency of a wide range of face coverings—cotton masks, synthetic-fiber masks, surgical masks, and N95 respirators—over a wide range of particle sizes. (N95s differ from other mask types by meeting national standards set for breathability and filtration efficiency.) Their focus was on wildfire pollution, whose health hazard comes from the high concentrations of fine particles that the fires produce. Prolonged exposure to particles smaller than 2.5 µm in diameter, the size of a bacterium, carries the risks of asthma, respiratory infections, and chronic obstructive pulmonary disease.

During experiments, Kodros and colleagues placed different masks over a 90-mm-wide cylinder, meant to replicate the area that air flows through a mask when worn. A constant rate of particles then flowed through that cylinder. The extent to which each mask reduces the intake of pollutants is quantified by its so-called protection factor, which describes the ratio of particles upstream and downstream of the mask. In each case, the researchers measured how well the different masks filtered out the different sizes of particles, as shown in the figure.

Not surprisingly, the N95 offers the highest degree of protection at all particle sizes, largely because it combines mechanical and electrostatic filtration, by which stationary electric charges in the mask’s polypropylene microfibers attract and ensnare submicron particles. (Cotton and uncharged synthetic fibers filter only by mechanically blocking particles.) The researchers’ modeling shows that an N95 reduces a person’s exposure to wildfire smoke by a factor of 14. Synthetic-fiber and cotton masks reduce that exposure by much smaller factors, of up to 4.4 and 1.9, respectively. Surgical masks turn out to be effective filters, but they also allow up to 50% of the air to leak around the mask. That limitation makes them about as effective, in general, as synthetic- and cotton-fiber masks.

To assess the ability of face masks to provide population-level protection against wildfire smoke, the researchers also performed a case study for the 2012 Washington state fire season. Accounting for the percentage of people likely to wear masks, that study found that N95s would have prevented as many as 22–39% of smoke-related hospitalizations. (J. K. Kodros et al., GeoHealth 5, e2021GH000482, 2021.)

Thumbnail image credit- Friends of the Earth Scotland, CC BY 2.0