Are facemasks effective against particulate matter pollution? Evidence from the field

The use of anti-pollution facemasks (APFs) in defense against particulate matter (PM) pollution is subject to debate as air pollution and wildfire events intensify. Inward leakage due to imperfect fitting and the Peltzman effect of people spending more time outdoors when wearing masks have led to mixed evidence regarding the effectiveness of APFs, which in turn has contributed to conflicting public messages about APFs with potentially large public health costs. We conduct a large-scale randomized field study on individuals' daily outdoor time and mask wearing behaviors and the associated health outcomes during an entire winter heating season in a provincial capital city in Northeastern China. We find that APFs used in everyday life achieved an overall efficiency of 80% in reducing respiratory or cardiovascular disease related doctor visits. Mask wearing, due to its discomfort, reduced outdoor time. However, the added protection provided by masks against PM led respondents to spend more time outdoors on smog days, and this relative Peltzman effect wiped out about 12% of APFs' health benefits. Taken together, APFs’ health benefits far exceed their financial costs. These findings call for affirmatory but careful messaging to the public about using APFs as personal protection against PM pollution.