Exposure to wildfire smoke is linked with higher risk of dementia, UW study shows

A new study, led by researchers at the University of Washington, has linked exposure to wildfire smoke with a higher risk of developing dementia. The study followed more than 1.2 million Kaiser Permanente Southern California members from 2008 to 2019. It also found that minority communities and those living in higher-poverty census tracts were more likely to be diagnosed with dementia than white or higher-income patients.

Joan Casey is an associate professor of environmental and occupational health sciences at UW. She joins us with more details on how wildfire smoke can affect our brains and bodies.

Note: The following transcript was transcribed digitally and validated for accuracy, readability and formatting by an OPB volunteer.

Dave Miller: From the Gert Boyle Studio at OPB, this is Think Out Loud. I’m Dave Miller. A new study found that, as exposure to wildfire smoke increases, so does the risk of developing dementia. The study was led by Joan Casey, an associate professor of environmental and occupational health sciences at the University of Washington. She joins us now. It’s great to have you on Think Out Loud.

Joan Casey: Thanks for having me.

Miller: Earlier studies – I saw one from the University of Michigan in 2023 – have shown a link between dementia and fine particulate air pollution. The EPA put out a major summary of research about five years ago, saying that there’s likely a causal relationship between long-term exposure to these fine particles and various neurological problems. What was new about your study?

Casey: Here, we were really focused on a specific source of that fine particulate matter: PM2.5. We wanted to look at wildfires. And that was important because, especially on the West Coast, wildfire smoke is a driving contributor to PM2.5 levels, as folks living here are well aware. So, because those levels are increasing from wildfire smoke, we were really curious: does this relationship we’ve seen with all source PM2.5 extend to wildfire smoke?

Miller: Can you just remind us what PM2.5 means?

Casey: Absolutely. PM2.5 are these very small particles that can float around in the air. They’re smaller than 2.5 micrometers in diameter. That means about 30 of them can fit across a human hair.

Miller: Why are they so dangerous in general? What is it about their size that leads to their potential toxicity?

Casey: The idea is that when these small particles are inhaled, they can move much deeper into the lungs than larger particles that might impact in the back of the throat, for example. These particles, PM2.5, can go all the way down into the lung. They can actually move into our bloodstreams. From there, they can cause a host of problems in the body. They can cause inflammation, oxidative stress, they can damage our DNA – things we don’t want happening in the body. That’s why PM2.5 is related to so many different health outcomes, including neurological diseases.

Miller: Does that continue if you go even smaller than the 2.5 … you said micro? I mean, what happens if they’re even smaller? Are they even more dangerous, potentially?

Casey: We don’t fully understand that yet. For one reason, because the U.S. EPA, the smallest particle they regulate is PM2.5. So that’s the smallest particles we have great measurements on. But you’re exactly right, there are these particles called “ultrafine particles,” 25 times smaller than PM2.5. And what we know about ultrafine particles is they can move directly from the nose into the brain because they’re so tiny. Those particles can actually directly damage brain cells and could potentially lead to things like dementia down the road. Wildfire smoke does contain a fairly high fraction of those ultrafine particles.

Miller: As you said, what you wanted to do with this research was look directly at wildfire smoke, as opposed to this fine particulate air pollution more broadly. You were looking at data from Kaiser Permanente patients in Southern California that went over the course of most of a recent decade. Southern California is a place that has, in the past at least, been famous for air pollution problems for smog. How do you tease out the wildfire particulate pieces of that from other stuff, from cars or any other sources?

Casey: That’s a great question. We were using data on 1.6 million Kaiser members, like you said, from 2006 to 2019. The fact is there is air pollution across this entire region. We had access to daily estimates of wildfire fine particulate matter and non-wildfire fine particulate matter. So in the study, we were able to look at the relationship with both of those types of particulate matter and compare them over a long-term period. We were looking at people’s average exposure over a three-year period and seeing whether that was related to them developing dementia down the road.

Miller: How big a link did you find between an increased risk of dementia and an increase in the density in the air of this particular kind of pollution?

Casey: A surprisingly strong relationship, honestly. We saw, for a 1 microgram per meter cubed increase in a three-year average exposure to wildfire PM2.5, an 18% increase in the odds of developing dementia.

Miller: OK, what you’ve just used … that description is not the way the general public or I have ever been taught to think about air pollution. We’ve often, in recent years, heard about AQI [Air Quality Index], and other folks may have seen the green, the yellow, the red, the terrifying purple on the map. How does AQI correspond to what you’ve just described, where there’s an 18% increase in the odds of a dementia diagnosis for every 1 microgram per cubic meter increase in the three-year average? Can you put that in an AQI version?

Casey: Sure. You’re hitting on what is complex about studying the health impacts of wildfire smoke with this question, so I love it.

It’s true, the way we’re exposed to wildfire smoke, you don’t think about a three-year average exposure. You think about those days when it’s extremely smoky and like you said, the AQI is over 150, and maybe you stay indoors. But you’re not thinking about that spread across a long-term period, which is what we are interested in here. So when we’re talking about a single day of high levels of wildfire PM2.5 exposure, that’s something like an AQI of 151.

You’d need to have, let’s say, 10 days of that level of exposure to get this 1 microgram per meter cubed that I’m talking about over a 3-year period. So it’s not directly translatable, but it’s something that’s important for us to think about. Is it these days that are really, really high in levels of exposure that matter for brain health? Or is it more of an average over a longer term period? And the jury’s still a bit out on that question.

Miller: I should say that, if I understood you correctly, there are plenty of people in the American West, and in recent years … there were people in D.C., in Boston, in New York [who] experienced wildfire smoke for the first time in their lives, leading them to say, maybe we should be worried about this as well. But that’s a separate question about politics, and power, and geography.

What I was gonna say is that plenty of people in the American West in recent years have had more than 10 days of AQI’s over 151. It’s become the norm for “smokey season” for plenty of people.

Casey: Exactly. You’re exactly right, because the study period we’re looking at here that ends in 2019, the wildfires generally weren’t as bad as we are now seeing today. So we have more people falling into this level, where we are seeing this pretty big impact on risk of developing dementia. So you’re exactly right, which is why we’re concerned and doing more work in this area.

Miller: Am I right that we are still, in terms of a careful scientific process, talking here in the results of your study about correlation, as opposed to causation?

Casey: Yes, this is an observational study, albeit a very large one with really quite good health data. This is not a study that was able to assess in a causal inference manner. So, I would call it a correlation, at this point.

Miller: How much does that matter, in terms of the way you think a study like this should impact policy?

Casey: This is always the issue in environmental health because, ideally, we would randomize people to different levels of wildfire PM2.5 exposure, and then follow them over time and see what happens. We can’t ethically do that. I can’t take a certain group of people and expose them more to wildfire smoke. So we have to do these types of observational studies. And when you get to this large of a sample size over this many years, we have over 80,000 people developing dementia. We’re carefully tracking them. We have updated residential addresses and we can follow people over time. We get close to meeting a lot of the criteria for making causal inference, but it’s a fine line and we could have hours of debate about it.

Miller: Or, we could just keep talking about other aspects of what this study found.

You found a much bigger link between the particulate air pollution from wildfire smoke, these tiny particles, and other kinds of tiny particles. There was a very minor increase in the risk of dementia from PM2.5 that did not come from wildfire smoke. How do you explain that?

Casey: I think there are a few things potentially going on here. We’ve already touched on a couple of them. The way people are exposed to wildfire smoke, versus background PM2.5 that’s coming from factories and cars, is quite different. We have a pretty constant level of exposure to non-wildfire PM2.5. Whereas, wildfire PM2.5, we can have days of 300 micrograms per meter cubed – extremely high levels that may overwhelm the body’s defenses. So that’s one thing, very different ways we’re exposed.

The second thing is wildfire smoke is not just PM2.5, it contains other gases, things like volatile organic compounds, that are also health harmful. So it may be we’re picking up a signal from some additional pollutants. It might be those ultrafines that are showing up more consistently in wildfire smoke than from PM2.5 from other sources. But, in general, we don’t know quite yet why we’re seeing such a stronger signal here. And it will be important to look at this in other populations and other time periods.

Miller: We’re talking about wildfire smoke. Is there a difference in that smoke from a wildfire, or just from a wood burning stove where people are doing it on purpose? Is there a difference besides the scale of the smoke?

Casey: Yes, I would say so. People, when they’re burning logs near their home or in their home, they’re much closer to the source of combustion. They’re also only burning wood in that scenario. With wildfires, depending on the type, we can also be burning buildings that can release all sorts of chemicals. There’s also transformation going on in the air, because there’s a pretty long distance being traveled often. For example, you’re talking about folks on the East Coast exposed to West Coast wildfires. The particles can change from the time they’re emitted to exposure on the other side of the country.

So there are absolutely some differences. And the levels will likely be higher if you’re exposed to this wildfire smoke compared to a well-ventilated home if you’re burning wood indoors, for example.

Miller: One of the important pieces of your study is that you found a lot of variation in the increase in dementia risk from wildfire smoke based on race or class. What exactly did you find?

Casey: I think this is another example of climate change hitting the most disadvantaged people first and worst. We do see here a much stronger relationship for racially-minoritized people in the Kaiser population. And we also see a similarly increased risk of developing dementia for people living in communities with higher levels of poverty.

I think this could be happening for a few reasons. Disadvantaged groups might not have access to messaging about poor air quality days, staying indoors when the AQI exceeds a specific threshold. Folks might not have the ability to change their behavior. If you have to go to work or you work outdoors, you can’t shelter inside and run air filtration. We know HEPA air filtration can basically wipe out wildfire smoke exposure if run in a well-sealed room, and some people may not have access to that.

So it’s a pretty concerning finding, because one thing we’re worried about in public health is climate change increasing the health inequities that we see in the United States. And this points to that happening with dementia.

Miller: You’re touching on this a little bit, but I want to just zero in on the preventive question in particular. What can people do – especially people who, because of age or other risk of cognitive decline – to protect themselves from wildfire smoke and other fine particulates, especially as climate change exacerbates wildfire risk?

Casey: I’d say the first step is to know the air quality where you live, in the way that we check our weather app in the morning to see if we should bring a coat – or an umbrella, in my case in Seattle. You can check the air quality, you can go to AirNow.gov, which uses those high quality air monitors and see what the AQI is in your area.

If you’re an older adult, someone with an underlying condition, the magic number is 101. If the AQI is 101 or higher, try to reduce outdoor activities, pick up one of those KN95 or N95 masks that we got used to wearing during COVID. They can really prevent you from inhaling fine particulates. Shut windows and run air filtration if it’s available to you. For everyone else, an AQI of 151 is the level you’d want to stay inside and do some of those behavior change activities.

Probably the most important thing that was alluded to earlier, though, is asking policymakers to pass strong climate legislation. We know climate change is a major driver of these wildfire smoke events, and there are things that can be done to start ratcheting this down.

Miller: What do you see as the most important scientific questions to answer that would have a direct impact on policy?

Casey: I think we want to understand the mechanism at play here more directly. That’s going to involve collaborating with neurologists and looking at specific subtypes of dementia. Here, we looked at this umbrella of all dementias, but there are things like Alzheimer’s Disease that people are familiar with, or Vascular Dementia. Some of these types of dementia might be more strongly linked to PM2.5 exposure. Understanding that would help us understand the underlying mechanisms.

The other thing that I think we really need to understand, in terms of resource allocation, is what happens when it’s not just a wildfire smoke day but also it’s really hot outside, or also, people experience a power outage? It’s climate change driving these co-occurring hazards that can really lead to risky health situations. For example, the power is out and no one can run HEPA air filtration. So, getting to the bottom of some of these questions will help us decide where to go next and who to send resources to first.

Miller: Joan Casey, thanks very much.

Casey: Thank you.

Miller: Joan Casey is an associate professor of environmental and occupational health sciences at the University of Washington.

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