The ongoing COVID-19 pandemic and the potential for co-occurring wildfires pose health threats to people around the globe. Along with the direct impacts of wildfires, exposure to fine particulate matter (PM 2.5)—pollution composed of small inhalable particles with diameters of 2.5 micrometers or smaller—from wildfire smoke is a growing public health issue with potentially serious short-term and long-term consequences [1, 2]. In the United States, models suggest that by the end of the century, fire-related pollution could account for more than 50% of the annual average PM 2.5 concentration, and deaths attributable to fire-related PM 2.5 exposure could reach 44 000 per year [3]. Recent research indicates that increased long-term exposure to PM 2.5 may be linked to increases in the COVID-19 case fatality rate [4, 5]. Peak wildfire season is underway in the western U.S. [6], where in August 2020, nearly 12 000 lightning strikes hit the state of California, starting hundreds of fires and blanketing the region in thick smoke [7]. The simultaneous crises of smoke exposure and COVID-19 pose grave challenges for those already vulnerable to COVID-19 [4, 5], thus demanding action from agencies and experts charged with providing public health guidance.

According to public health experts, wearing a face mask can effectively mitigate wildfire smoke and COVID-19 exposure [8–10]. However, not all face masks are created equal. Only certain masks are effective during wildfires, while a range of face coverings may help prevent coronavirus transmission, although experts have called for additional research into mask effectiveness [8]. Currently the Environmental Protection Agency (EPA) and the U.S. Centers for Disease Control and Prevention (CDC) provide different face mask recommendations for protection against wildfire smoke and the coronavirus, respectively. The EPA recommends that adults spending time outdoors during a wildfire wear a N95 or P100 particulate respirator, which have been shown to effectively filter the PM 2.5 from wildfire smoke, while surgical masks and alternative face coverings have not [10, 11]. To our knowledge, the EPA has yet to address the complexity of mask use decisions during wildfire season and the COVID-19 pandemic. In contrast, based on evidence that cloth face coverings provide a barrier to COVID-19 transmission [12–14], the CDC currently advises that people wear cloth face coverings in public and reserve scarce N95 masks for medical workers [9]. Additionally, the CDC does not recommend N95 masks with respirators, or exhalation valves, because although they may protect the wearer, expelled respiratory droplets may still transmit the coronavirus [15]. When addressing the overlap of wildfire season and COVID-19, the CDC notes that cloth face coverings offer limited protection against wildfire smoke, N95 masks may be scarce, and N95 masks without exhalation valves are preferred when wildfire smoke and COVID-19 are simultaneous threats [16, 17]. The CDC also acknowledges local resource constraints and suggests that public health officials should determine mask recommendations based on the local supply of N95 respirators, the severity of the wildfire smoke, and COVID-19 community transmission levels [16]. However, this guidance aimed at multiple contexts and localities may be confusing to the public and does not reconcile with EPA's longstanding recommendations on mask use during wildfire smoke events [10]. In light of these conflicting and changing recommendations from government agencies, and as wildfire season progresses, we expect that public confusion around face mask use will increase.

Government agencies and public health researchers should respond to the overlap of wildfire season and COVID-19. Drawing on validated models of health behaviors [18–20] and our research investigating mask behaviors during wildfires, we show how social norms are one possible pathway influencing protective behaviors in response to respiratory health threats (see figure 1). We recommend that 1) agencies and officials reconcile the current inconsistencies in mask use recommendations, 2) behavioral researchers study decision-making processes for overlapping health threats, and 3) public health communications leverage social norms, along with other behavioral science-based approaches, in mask-use messaging and guidance.