Wildfire Smoke Exposure: What to Know

Gwendolyn Rak

July 26, 2023

This summer has brought another severe season of wildfires in Canada. More than 100,000 square kilometers have already burned in British Columbia, and smoke from Canadian fires has affected air quality across North America. In addition to physical destruction, the fires pose dangerous health risks to those exposed to the smoke, from respiratory and cardiovascular symptoms to mental health effects.

Dr Sarah Henderson

Sarah Henderson, PhD, serves as the scientific director of Environmental Health Services at the British Columbia Centre for Disease Control and the National Collaborating Centre for Environmental Health in Vancouver. She is also an associate professor at the University of British Columbia's School of Population and Public Health. As a leading environmental health scientist, Henderson's areas of study include extreme weather events and air pollutants, such as wildfire smoke.

Medscape Medical News invited Henderson to discuss a range of health conditions affected by wildfire smoke and what patients can do to protect themselves. This interview has been edited.

Medscape: When should a clinician suspect that a patient has had exposure to wildfire smoke?

Henderson: I am an epidemiologist, not a clinician, but if there are individuals with chronic respiratory conditions who are having a difficult time managing their condition and it is smoky, I would absolutely suggest that the wildfire smoke is a contributing factor. There are other symptoms that are associated with smoke exposure that may present and appear to be other illnesses. Runny eyes, a cough, a sore throat, an ongoing headache: if it's smoky outside, all those types of things could potentially be a result of smoke exposure and not other factors — or a result of smoke exposure combined with other factors. Abnormal arrhythmias and any kind of cardiovascular symptoms that aren't normally present or are being exacerbated to a degree that is unusual for the individual also are potentially related to smoke exposure.

Medscape: What respiratory effects does exposure to wildfire smoke have?

Henderson: Wildfire smoke is a really complex form of air pollution made up of gases and small particles. Both the gases and the particles can irritate the lungs when you inhale them. The particles can also travel deep into the lungs right down to the alveolar oxygen exchange because they're so small. When they're in the deep lung, the body is going to respond to them as a foreign entity like a bacterium or a virus and mount an immunological response against them. Because they can't be biologically damaged the way a virus or a bacterium can be, the immunological response is sustained, so they also cause systemic inflammation in the lungs. That inflammation can spill over into the rest of the body as well.

For healthy people, we do see that wildfire smoke can cause respiratory symptoms, a cough, maybe uncomfortable breathing. For people who have obstructive respiratory conditions where their airways are prone to closing up when irritated, it can definitely lead to shortness of breath and severe exacerbations of conditions such as asthma, COPD, or emphysema. Unfortunately, in British Columbia, we had a child who had quite severe asthma pass away last week. The wildfire smoke likely contributed to a fatal asthma attack.

Medscape: How does exposure to wildfire smoke affect the heart?

Henderson: When wildfire smoke is interacting in the deep alveolar regions of the lungs and causing some inflammation, that can spill over into other organ systems. In addition, some of these particles are so small that they're going to translocate across the lung and into the bloodstream, meaning that they have the ability to circulate to all organ systems in the body. Inflammation and oxidative stress caused by the particles are likely the mechanisms by which we see the cardiovascular impacts, and there is pretty conclusive evidence that wildfire smoke can lead to severe, acute cardiovascular events, such as myocardial infarction or out-of-hospital cardiac arrest.

Now, there's one important distinction I think we need to make here. When we talk about the respiratory impacts of the fine particulate matter in wildfire smoke, we see that they are generally larger than the impacts of air pollution from other sources. So, wildfire smoke elicits a stronger effect in those people with asthma and COPD than air pollution from other sources does. When we're talking about the cardiovascular impacts of wildfire smoke, we see that they are quite consistent with the cardiovascular impact of air pollution from other sources.

Medscape: What do we know about the effects of prolonged exposure to wildfire smoke on mental health?

Henderson: At this point, the acute effects of short-term exposure to wildfire smoke are becoming quite well understood. In general, they're consistent with the effects of air pollution from other sources. When we're talking about the effects of long-term exposure on more chronic health outcomes, the literature is not nearly so well developed.

It's important to remember that these exposures are very different from the exposures we have to other forms of air pollution. Air pollution from traffic or industry is pretty steady in space and time, with some short-term fluctuations. Wildfire smoke is highly episodic, and the magnitude of the exposures is just so much larger than we would see from any other source in most high-income countries, so we don't know that the longer-term impacts of wildfire smoke exposures are going to be consistent with the longer-term impacts of air pollution from other sources.

Having said all of that, there's two ways that smoke exposure likely affects mental health. When we're talking about short-term exposures, again, we have these very small particles that can translocate across the lung and into the bloodstream, and also cross the blood-brain barrier. So, there may be these direct physiological impacts on the brain. For longer-duration exposures, we also have emotional effects. It's quite oppressive to be living in smoky conditions, to feel like you can't go outside or enjoy summer. In the longer term, both of those things might affect mental health. We also have to consider the bigger picture of eco-anxiety and the impacts that I think climate change is starting to have on mental health.

Medscape: Could exposure to wildfire smoke affect pregnant women or newborns?

Henderson: This is a huge area of research, and one of the places where I believe smoke exposures are very likely to affect the short- and long-term health of the population. Currently, all the evidence suggests that people who are pregnant should be taking precautions to protect themselves from smoke exposures. We do see that these exposures are associated with risk of preterm birth and very preterm birth. We also see that they're associated with decrements to birth weight, and we know that being low birth weight has lifelong health consequences. There's also evidence from the 1997/1998 haze event in Southeast Asia, where they looked at the demographic profile of the infants born after that event, compared with years before, and they saw that fewer babies were born. So, it's possible, and in some ways probable, that smoke exposures, especially in early pregnancy, lead to pregnancy loss.

There is not too much human evidence on the question of neonates and exposures very early in life right now, but I think it's coming. There's a really fascinating study out of the University of California, Davis. They have a cohort of monkeys every year for research purposes, and these monkeys keep getting exposed to smoke because they're housed outdoors. Obviously, rhesus monkeys are not human babies, but there's enough similarities that we should be looking at the research carefully. They have found that the lungs of smoke-exposed infants never grew as big, and their immune function was dysregulated throughout their lives, compared with the nonexposed infants. Now they're starting to be able to study the offspring of smoke-exposed infants, and they're finding that the immune function of the offspring is also dysregulated, even though those offspring were never exposed to smoke in their lives.

We're also starting to look at prenatal exposures and postnatal outcomes. That evidence is suggesting that health in early life can be affected by these prenatal exposures. Evidence also suggests that smoke exposure can lead to higher risk of things like gestational diabetes and preeclampsia during pregnancy.

Medscape: Does exposure to wildfire smoke affect patients' risk for infectious diseases?

Henderson: This is something that I wrote about back in 2020, very early in the pandemic, to say, "I think there's potential here for these exposures to affect COVID-related outcomes in places where they are co-occurring." There's been quite a lot of research on this, and most of the research is supportive of an association. When we have smoke deep in the lungs, and it is causing an immunological response, your immune system is going to be kind of distracted. So, its ability to tackle another foreign invader in the form of SARS-COV-2 may be compromised. What we're seeing in the COVID and smoke literature is that when populations were co-exposed, there was more COVID and, in many cases, more severe COVID. We're looking at 5% to 15% increases in COVID-related outcomes when there's co-exposure to wildfire smoke. That's not perfectly consistent across the literature, but many studies are falling in that range.

Medscape: How can patients protect themselves from exposure?

Henderson: The first line of defense, especially in high-income countries, is indoor air. Most of us spend the vast majority of our time inside, and if we can protect the indoor environment from becoming smoky, we are protected 80% to 90% of the time, and that's fantastic. The reality of the situation is that smoke penetrates very easily into the indoor environment. You'll often hear the advice to keep the doors and windows closed. Well, that's not terrible advice, but it's not going to keep your indoor environment pristine. In general, indoor concentrations are going to be at least 50% of outdoor concentration. So, we need to be a little bit more proactive about keeping the indoor environment as smoke-free as possible, and that means air cleaning.

Portable air cleaners are very effective at reducing those smoke particles indoors. They are relatively affordable technology, especially if you are choosing just to treat one room. During the pandemic, we also learned that improvised devices using a box fan and furnace filters work really well for people who maybe can't afford or can't find a commercially available device. However, at this point in the conversation around smoke, we really do need to be talking about active cleaning of the indoor air as a way to protect ourselves from these exposures.

For people who are outdoors and worried about their exposures, masks can be quite protective. They need to be well fitted so the air goes through the mask, not around it. What's best is a respirator, an N-95 or a KN-95. But there's also good evidence that a well fitted, multilayer cloth mask can also do a pretty good job of reducing the particles that you breathe. I don't think masking outdoors is necessary for everyone, but you can get some added protection.

The other critical thing is taking it easy. If you're at rest, the average adult is breathing maybe 6 L of air per minute. If you are out for a run, that's going to be more like 60 L of air per minute inhaled deeper into your lungs. So, respiration rate matters. Your respiration rate defines how much smoke you're going to be exposed to, and taking it easy outdoors is one way to reduce your exposure.

Gwendolyn Rak is a health reporter for Medscape and Univadis based in Brooklyn, New York.

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