Skip to main content Skip to secondary navigation
Main content start

Health Impacts of Wildfire Smoke

Recent wildfires have blackened over 5 million acres, taken at least 35 lives and destroyed over 6,000 buildings explained Chris Field, Director of the Stanford Woods Institute for the Environment, at a recent event hosted by the Stanford Woods Institute for the Environment to discuss wildfire smoke exposure, health impacts, and how to keep communities safe. 


Recent wildfires have blackened over 5 million acres, taken at least 35 lives and destroyed over 6,000 buildings explained Chris Field, Director of the Stanford Woods Institute for the Environment, at a recent event hosted by the Stanford Woods Institute for the Environment to discuss wildfire smoke exposure, health impacts, and how to keep communities safe. Research shows the number of days with extreme wildfire in California has more than doubled in the past 4 years due to warming temperatures and vegetation drying. Three of the four largest fires in California history started this year. The largest, the August Complex Fire, has become a ‘gigafire’ — a term for a wildfire burning over 1 million acres — and is still only at 70% containment after burning for 2 months. Smoke from the fires led California to have 30 consecutive spare-the-air days, more than doubling the previous record set in 2018.

Air Quality and Health Impacts

U.S. air quality had improved substantially over time through policy interventions, such as the Clean Air Act, but in recent years, those improvements have slowed and reversed, in large part due to wildfires, said Marshall Burke, Associate Professor of Earth System Science at Stanford. Fifteen to 20 years ago, about 5% to 10% of PM2.5 was from wildfire smoke. In recent years, it has become 25% nationally and, in the West, it is 50% or more in some years. 

There is no safe aspect of wildfire smoke, explained Kari Nadeau, Naddisy Foundation Professor of Pediatric Food Allergy, Immunology and Asthma at Stanford. Exposure to wildfire smoke over 5 to 7 days can cause damage to the lungs, blood, and heart and cause strokes. There is no safe distance from smoke. Disadvantaged and marginalized communities are disproportionately affected because they oftentimes cannot access a clean room. Infants, young children, and the elderly are more affected by wildfire smoke. Children exposed to 5 days of wildfire smoke have 2-fold increase(link is external) in the rate of asthma. Elderly people over 65 could potentially face a 40% increase in the rate of strokes and an increased rate of heart attacks. Pregnant women are also at greater risk of premature birth and decreased birth weight after wildfire smoke exposure. So far, researchers looking at connections between wildfire smoke and COVID-19 think negative health impacts for COVID infected patients are more severe with wildfire smoke exposure and that due to lowered immune system function in the lungs, people exposed to wildfire smoke may be more likely to get COVID because they can’t fight off the infection as well. Researchers are now studying fire fighters and long-term chronic exposure to see how smoke damages the immune system and parts of the DNA over time. 

Exposure Inequalities

Older homes tend to have more leakage of outdoor air into the indoor environment than newer or more recently weatherized homes, said Lynn Hildemann, Professor of Civil and Environmental Engineering at Stanford. Thus, for older homes, the indoors is not giving people much protection against the highly polluted outdoor air and even indoor air filters are less effective because of the leakage. 

There’s been research for decades showing inequalities in terms of exposure to key pollutants and minority and low-income communities are more exposed overall to PM2.5, said Burke. With wildfires, wealthy and middle communities tend to be just as if not more exposed to PM2.5 as lower income communities. The same trend applies to racial disparities in exposure where white communities are less exposed overall but more exposed to wildfire smoke PM2.5 specifically. 

An AQI measurement of 20 is equivalent to smoking one cigarette a day, explained Nadeau. Communities exposed to wildfire smoke causing AQI of 150 for several days is equivalent to about seven cigarettes a day if someone were outside the whole time. Even if you’re indoors, you could be breathing in this poor air quality due to leakage. Cigarette smoke is the best analogy researchers have right now but more research is being done to account for toxins in the air caused by wildfires that may go beyond the dangers of cigarettes. 

After they are emitted into the atmosphere, smoke particles undergo further chemical reactions which may change the toxicity, making it easier for the body to uptake some of the organic coatings on smoke particles, said Hildemann. Thus, breathing smoke that is a couple of days old could have a different level of health effect than breathing fresh smoke particles and an AQI of wildfire smoke versus typical air pollution may be more negatively impacting on health.

Health impacts from acute exposure to wildfire smoke over a short period of time have been found to be similar to those from chronic exposure to low levels of air pollution over a long period of time, said Nadeau. Acute exposure could include irreversible effects and it is an active area of research. Fire fighters with repeated acute exposure to fire smoke have a decreased life expectancy by about 10 years. 

Prevention Options

An N95 mask, which can be difficult to find given the pandemic, will decrease breathing in particles but does not completely protect wearers from all of the toxicants during wildfire smoke, said Nadeau. However, only if you wear the mask correctly and not too loosely, said Hildemann. N95 masks are an option for those who have to work outside but for recreational exercise, people should hold off until AQI is at least in the green zone (below 50). Rather than wearing an N95 mask indoors, using an electric HEPA air filter in only one room at a time that is sealed is an effective option. This is also recommended for babies’ rooms as infants are at high risk from exposure. 

Your Questions Answered

The researchers answered individual questions to help the public prepare and protect themselves from the negative health impacts of wildfire smoke.

What websites/tools are the best option for accurate and real-time air quality information. 

The Purple Air maps have especially good spatial and time resolution for PM2.5. The accuracy will vary with the main type of particles in the air but will tend to be in the right ballpark. 

Are electrostatic filters on central systems effective? 

Electrostatic filters tend to be more efficient at collecting particles than the average furnace filter. 

What are the indices used to report air quality and what do the safety categories mean?

What you hear about most often is the AQI (Air Quality Index). In general, an AQI of 100 is the dividing line between “moderate” and “unhealthy for sensitive individuals” and corresponds to the EPA’s standard for that pollutant. However, the AQI doesn’t vary linearly with concentration – for smoke, when the AQI is 200 (the dividing line between “unhealthy” and “very unhealthy”), the concentration of PM2.5 is about 4 times as high. 

A way to think of the safety categories is whether you should be exercising, heavily (e.g., running) or lightly (e.g., walking). If you are a healthy person, you can go running if the AQI is below 150 but should limit yourself to walking for an AQI of 150-200. For an AQI above 200, the only thing you should be doing is sitting quietly indoors. If you have some health issues, you should subtract 50 to 100 for each of the above recommendations, depending on the severity of your health issues. 

What CADR range do you need for an air filter for a bedroom?

Clean Air Delivery Rate (CADR), often reported in cubic feet per minute (CFM), represents how many CFM of clean air the room filter delivers. Since this depends on the efficiency of the filter, as well as the air flow rate of the device, CADR can depend greatly on what sizes of particles/smoke you need to filter. Filters tend to be least efficient at capturing tiny smoke particles, so you should focus on the smoke CADR, which will tend to be the lowest.

A rule of thumb is that the CADR should be able to fill the entire room with clean air in 12 minutes or less – this ensures that it will be generating clean air for the room much more quickly than the outdoor air is entering the room. A medium-sized bedroom, 10 x 12 x 8 ft high, has a volume of 960 cubic feet, so you would want a filter with a smoke CADR of at least 960 cubic feet / 12 mins = 80 CFM. 

For homes, what age of home is considered older or newer? Is the sealing level a gradual change with time or were there important transitions due to technology or law in California?

It can be tough to generalize how well a home is sealed, based just on the age of the home. Instead, to assess how well-sealed your home is, request an energy audit, which is free in many areas (including Santa Clara County). This will tell you how leaky your home is, and the person doing the test may be able to offer recommendations on what changes would lead to the most improvement.

Should there be an effort to regulate indoor air purifiers with respect to their efficiency at removing Volatile Organic Compounds (VOCs) and separately PM? CADR doesn't set any standard for how clean the air in the outflow be.

CADR is aimed specifically at PM removal – it tells you what volume of particle-free air per unit time your filtration unit is delivering to the room. CADR is calculated as the volumetric flowrate of the filter multiplied by what fraction of particles are captured by the filter. The two things to be careful about in considering CADR, are: (1) CADR involves a test using a specific type (size) of particle – for wildfire smoke you would only want to consider the smoke CADR; and (2) the method of determining CADR only considers performance of the filtration unit early on. For many devices, as the filter fills up with collected particles, the flowrate through the filter will slow down. 

To remove VOCs, a special type of filtration device is needed, which can be expensive. Some types of VOC removal devices, such as activated carbon, are quite efficient at removing some VOC compounds, and much less efficient at removing others, so it would be much more challenging to come up with a single CADR-like value for a VOC filter. 

Does closing windows indoors effect other indoor exposures (e.g., from cooking, cleaning)?

Ventilation rates can be reduced by 2-10X, when a house that normally has lots of windows open is completely closed. This means it will take 2-10X as long for indoor emissions from cooking and cleaning to leave your house. Using a hood vent that vents outdoors can greatly reduce indoor pollutants from cooking. 

Does Lysol spray hurt indoor air quality?

Rather than Lysol spray for COVID sanitation, hand sanitizer (with 70% or more alcohol) and alcohol wipes for computer keyboards, phones and keys are enough to use. COVID is aerosolized and so HEPA filters are very useful as well as a mask and staying 6 ft away from others.

What do you think of vinegar and baking soda as cleaners of surfaces?

That is ok but for COVID, you need more (see above answer).

What thoughts do you have on workers from the underserved populations doing professional disinfections on a regular basis, during the wildfires, with EPA & CDC approved chemicals.

They should be very careful, wear N95 masks, and only use disinfectants that work and that they don't have to breathe in.

What is the equivalency between smoking a cigarette and an AQI number?

An AQI for PM2.5 of about 22 outside for 8 hours is about the same as smoking one cigarette.

Are long term effects of wildfire smoke exposure due to residual particulate matter in the lungs, or due to damage to organs?


Is the concept of limited exposure the only viable option we have to protect our pets?

We believe so.

Do you think it is okay to walk while AQI scores are in the yellow or even orange range?

If you have to walk outside in yellow or orange, please wear an N95 mask. Please don't exercise even with a mask in those color AQI.

Are there any long-term epidemiological studies of the effects of smoke on firefighters' health?

Here are a recent review and study concerning the risk of firefighters’ exposures to smoke from wildfires over the long course of their careers. One of the authors of these articles says it best: ‘long-term outcomes is another key gap in the current understanding of how wildfires affect those who respond to them’. There are studies that have shown that life expectancy is shortened by 10 years as a firefighter and that the most frequent causes of death are typically cardiovascular disease. More studies are needed but there do seem to be links between exposures and health conditions since wearing a respirator versus wearing a bandana can decrease the rate of lung disease symptoms in firefighters.

Can one recover from smoke damage to lungs, organs, and the blood stream or is damage permanent? 

It is hard to say where the point of reversibility is. We know that retired firefighters (who have been fighting fires for more than 15-20 years) have a life expectancy 10 years less than average adult most because of damage to the lungs.

EPA’s Air Now app rates any AQI up to 50 as “good”. So, what does “good” mean in this context?

It means that now, with diesel exhaust, etc. we are readjusting what we consider good. Over the centuries, our air has gotten dirtier. Good is relative at this point.

Is chronic exposure to prescribed burns better for your health compared to wildfires?

Prescribed burns are better for the health of the forest and can be used to thin out already dead trees or underbrush so that when a wildfire does occur, it is less likely to be as intense, toxic and fast. Prescribed burns can occur safely in communities with trained firefighters and plenty of warning so they are away from the fire. The health risks of a wildfire are severe and there are little to no health risks with a proper prescribed burn.

As 'controlled burns' (also known as ‘prescribed burns’) play a larger role in our forest management policies, do you have any recommendations for the protection of public health?

Prescribed burns are better for the health of the forest and can be used to thin out already dead trees or underbrush so that when a wildfire does occur, it is less likely to be as intense, toxic and fast. Prescribed burns can occur safely in communities with trained firefighters and plenty of warning so they are away from the fire. The health risks of a wildfire are severe and there are little to no health risks with a proper prescribed burn. At this point, prescribed burns are such a small part of the fire-risk-reduction portfolio that the smoke from them is not much of an issue. 

In planning for prescribed burns, air quality impacts are a key consideration, and getting permission from the air quality management district is sometimes an issue. At this point, it doesn’t seem that permission from AQMD is the dominant limiting factor, but it could become limiting if the area increases dramatically. I see two main elements of the path to addressing that concern. One is that we need to think about framing the smoke and loss of carbon stocks from prescribed burns in the context of the alternative, which is the risk of massive smoke from catastrophic wildfires. An increase in prescribed burns looks a lot better, if it is clear that the effect of burning on reducing the wildfire risk leads to an overall decrease in smoke exposure. The second is that, in many places, it may be better to reduce fuel accumulations with thinning or harvesting than with prescribed burns. Especially in areas with large accumulations of ladder fuels and areas near communities, mechanical fuel reduction offers a better balance of cost and safety than prescribed burns. In some places, a combined strategy, with some thinning and some burning might make the most sense. In any case, we should be managing forests more actively and recognizing that this more active management involves costs. Of course, in California, most forests are federally owned, and changes in management will need to be worked out with federal agencies.

How does the size of the smoke particles compare to the size of the COVID virus?

Smoke particles are 2.5 micrometers and the COVID virus is 0.1 micrometers.

How can DNA damage happen to children because of breathing in wildfire smoke?

There are epigenetic changes that can occur with smoke exposure on DNA and that may be passed to the next generation.

What are the mechanisms by which smoke exacerbates asthma and risk of stroke or heart attack, e.g., what happens in the lung to increase asthma? Can the effects be reversed by withdrawal from the smoke?

We don't know how reversible these effects are. Since typically smoke has about 200 toxins in it to the human body, there are many different mechanisms. The main one is that the chemicals (PM2.5, VOCs like phenanthrene, nitrogen species, heavy metals, carbon monoxide) can be breathed in and our immune system is activated since it sees these particles as foreign and that causes inflammation and could lead to asthma and activation of clotting cascades that could lead to heart attacks or strokes. In addition, it causes direct mucus production and some of the chemicals themselves can directly kill lung cells they come in contact with. Lastly, carbon monoxide can go directly into the blood stream, bind hemoglobin in our red blood cells and compete for oxygen which can lead to death.

So if the AQI is 80-90, we still shouldn’t spend time/exercise outside even for a short period of time?


Apart from the long-term risk, are acute exposures more likely to induce acute events like myocardial infarction (MI) or stroke?

Yes, acute exposures even for as little as 3-5 days can increase the likelihood of MI or stroke, especially in those older than 65 years.

Are the health impacts of smoke dependent on what is burning? (Example: forest material burning versus homes and structures burning.)

We believe so, but this still needs to be studied. For example, many fires include residential buildings and commercial buildings so microplastics, paint, detergents, etc. can be in the smoke and can cause worse toxic health effects added to forest material burning.

Recent studies have shown that wildland fire smoke also aerosolizes and transports high concentrations of bacteria and fungi- and that the majority of these cells are intact (likely viable). What are your thoughts about this expansion of our understanding of the diversity of particles aerosolized via biomass combustion, and that these include common allergens and pathogens?

Yes, this is something that needs to be studied further and allergens, which include molds, can be highly inflammatory to the lungs, nose, eyes, skin. We should try to control wildfires better. In biomass combustion, filters are often placed (compared to wildfires) so there is much less dispersal.

Are there masks available that could replace N95? 

I am not aware of any replacements. I would always advise people to wear an N95 mask outside during moderate to high air pollution days (yellow and above) and the only exception is if people feel they cannot breathe well with the N95. If possible, stay inside instead in a place with a filter and where you can get the AQI lower to green. 

For how long the N95 can be used in average before it is not effective anymore?

I believe you can continue to wear it at least 10 times but I am not sure if anyone has really studied the N95 use in different wildfire smoke conditions.

Should people be advised to wear a regular surgical mask (or equivalent non-medical mask) outdoors to get at least some level of protection? What if you wear two surgical masks, one over the other?

It is not protective against toxins or against the bad PM2.5. Wearing two surgical masks might make you feel better and smell less smoke, but it won't help prevent the toxins and pm2.5 from being breathed into your lungs.

How do you approach your medical practitioners for help with health issues like low oxygen, pain in your chest, cognitive issues? 

Please go see your primary care doctor and if you are not satisfied, please try to see another doctor.

Is there any link between wildfire smoke and coronavirus?

Yes, there could be. If you have a lot of exposure to wildfire smoke, it could make already existing COVID infection worse or make you more apt to be infected.

I recall that Marshall Burke did a really interesting study during the initial COVID outbreak that found that the lives saved from the reduction of industrial pollution in China outweighed the number of COVID deaths. Has he done any similar comparisons for US wildfires vs COVID deaths?

Not yet – although we are working on the wildfire/COVID link. Stay tuned!

How does a couple weeks exposure to heavy smoke in the Bay Area compare to people in the developing nations who cook constantly over open fires, sometimes in an enclosed space?

This is a very good question. In much of the developing world, overall exposure to pollutants is way higher than in the Bay Area, even after we’ve experienced a couple weeks exposure to heavy smoke. For a week or so, cities on the West Coast broke into the top-10 most polluted cities on earth, but then dropped off that list. And those lists just focus on ambient air quality – they won’t pick up the important differences in indoor air quality mentioned in the question. So overall, exposures to polluted air – both indoors and outdoors – are substantially lower in the U.S. than in most developing countries, even with a week of very heavy smoke factored in. 

Can we identify wildfire regions and use spatial data analysis methods, to predict when we might possibly have a wildfire in that region, and take measures beforehand to abate the damages caused by the wildfires?

Also, a great question. I think the answer is “sort of”. There are great new datasets being developed by folks at Stanford (e.g. Alex Konings and her group) that give very localized, precise information on how dry fuels are. Similarly, we have good forecasts of strong wind events and where they’re likely to happen. So, these give some sense of where fire risk is particularly high. But actual ignition events are somewhat random – a confluence of available dry fuel, meteorological conditions, and difficult-to-predict human or natural ignition sources. It remains very difficult to predict precisely where a fire might break out. 

But even having a general sense can help target efforts in the medium term – e.g. where to focus prescribed burn effort. And having very specific information on where small ignitions have occurred in the past – e.g. where grasslands or forests are along busy roadways – might allow prophylactic use of fire retardants in a way that limits fire risk. Eric Appel at Stanford is doing some really great work on this. 



Explore More