N95s, but not cloth masks, offer protection against wildfire smoke

wildfire smoke

The Kruger Rock Fire in Estes Park has been a stark reminder that wildfires are a normal part of life in the West, and fire seasons are stretching longer as a result of climate change. Air pollution researchers at Colorado State University conducted a study earlier this year, published in GeoHealth, examining whether objects made familiar by the coronavirus pandemic – N95 masks – are effective in preventing health effects of breathing wildfire smoke. Lead researcher Jack Kodros, a research scientist in the Center for Energy Development and Health, answered some questions about the study and why it matters.

Why did you decide to do this study?

We had the idea for this project last winter when mask mandates were in full effect. We were testing a lot of cloth masks for their performance in protecting against particle sizes relevant for the coronavirus, and I started getting a few questions from friends asking if these same cloth masks would be useful in protecting against wildfire smoke in the summer. I realized there weren’t many studies offering quantitative guidelines about what masks would or would not be useful in protecting against wildfire smoke.

What are your study’s main scientific takeaways?

We found that N95 respirators can offer substantial protection against wildfire smoke, as well as protection against other sources of particulate air pollution. The limiting factor is how often the mask is worn, and if it is worn tightly around the face. In contrast, cloth masks and surgical masks likely don’t offer much protection. Cloth masks don’t filter wildfire smoke-sized particles very well, while surgical masks tend to let air bypass the filtering part of the mask.

Describe your methodology and how you came to your results.

We wanted our results to offer guidance to both individuals and policymakers. We started by expanding our laboratory setup for measuring mask filtration efficiency for particle sizes relevant to the coronavirus pandemic to a range of particle sizes relevant for air pollution. This allowed us to estimate the degree of protection each mask or respirator would offer to different sources of air pollution, which is informative to individuals. If you wear your mask correctly when exposed to wildfire smoke, this estimate represents the reduction of particle concentration you would be breathing in. Next, we looked at a case study during the 2012 fire season in Washington state. We calculated hypothetical percent reductions in hospital admissions due to exposure to wildfire smoke when different fractions of the population wore masks or respirators. These results can offer guidance to policymakers or others thinking about public health.

jack kodros
Jack Kodros, research scientist, Center for Energy Development and Health

Why are cloth masks somewhat useless in a wildfire smoke situation, but effective against the spread of SARS-CoV-2?

There are two main differences between looking at masks to protect against wildfire smoke and from the coronavirus. The first is that for the coronavirus we use masks to reduce our own emission of respiratory droplets, in addition to limiting the droplets we breathe in. In the case of wildfire smoke, we are only wearing a mask to limit what we breathe in. The mechanics of limiting the emission of droplets is different from inhaling droplets.

Second, the size range of particles relevant for wildfire smoke is different from the respiratory droplets that contain the coronavirus. Particle sizes relevant for the coronavirus are larger (diameters of 0.5-100 microns) than for wildfire smoke (diameters 0.05-0.5 microns). Cloth masks are able to filter nearly all particles with diameters upwards of about 10 microns (relevant for the coronavirus) but are actually least able to filter particle sizes around 0.3 microns (relevant for wildfire smoke particles).

What were the roles of your collaborators, and how did they contribute?

We had a great team working on this project. John Volckens (a professor in mechanical engineering and biomedical engineering) and I thought through the structure and direction of the project. (Assistant Research Professor) Christian L’Orange and I collaborated in the laboratory to measure the filtration efficiency of the masks and N95 respirators. (Graduate researcher) Kate O’Dell, (Professor) Jeff Pierce and I worked through estimating the potential health benefits of wearing a mask in the case study in Washington. Finally, Jon Samet (dean of the Colorado School of Public Health) offered guidance on the public health aspects of the project.

Why do you think your study has received so much attention and interest?

I think our experience with the pandemic has highlighted how little control we have over the air we breathe. When we consider common risks to our health, we usually have some choice built in. We can choose to eat healthy, not smoke cigarettes, get exercise. However, if there is something in the air – a virus, or toxic particles from smoke – there’s little we can do about it. Our study offers one more option for a personal intervention to limit exposure to an environmental pollutant that has a large impact on our health.

Do you personally wear N95s during smoky days after what you’ve learned?

I do keep some N95s handy for smokey days because they are a relatively low-cost way to limit my exposure. However, I can consider them one option of a few possibilities to limit exposure to wildfire smoke and air pollution in general. For instance, during the wildfire season, I check the EPA’s website to see what the air quality looks like where I am and try to limit my time outdoors on days where smoke is substantially impacting our air quality.

Anything else you want to add?

Every summer (and now we even have a fire in Estes Park in mid-November), we have smoke-impacted days. The discussion we are having, such as wearing N95s to limit smoke exposure, should be a warning sign about the state of our environment. If we don’t address the underlying cause of the increase in frequency and severity of fires, which is climate change, we are only going to have more and more of these discussions about trying to adapt to living with a haze of smoke during our summers.