TOPSHOT - Flames overtake the intersection of Temescal Canyon and Pacific Coast Highway Fire at the Palisades Fire in Pacific Palisades California on January 7, 2025. A fast-moving wildfire in a Los Angeles suburb burned buildings and sparked panic, with thousands ordered to evacuate (Photo by Robyn Beck / AFP via Getty Images)

AFP via Getty Images

A growing body of research shows that wildfire health risk is now an urban problem

At the global level, August and September represent the most dangerous months of the year for wildfires. So it’s no surprise that our news feeds have been filled with devastating footage of hills ablaze, smoke-filled orange skies, and news of large-scale evacuations, particularly across parts of North America and Southern Europe.

The situation is not expected to improve in the coming years. Climate change is not only making wildfires significantly more likely; it is also greatly increasing their extent. And while wildfires pose the most immediate threat to those living close to the flames, their health implications are felt much further afield.

Over the past while, I’ve come across a lot of research papers that focus on the impact of wildfires on human health. When viewed through a city-lens – as I tend to do in this column – they paint a rather complicated, worrying picture of what the future of these events might mean for urbanites, no matter where they are on the planet.

More Than Smoke

For cities downwind of wildfires, the fine particulate matter (PM2.5) in the smoke is the first cause for concern. A recent study that looked at more than a thousand communities across Australia, Brazil, Canada, Chile, New Zealand, Vietnam, Thailand and Taiwan found that wildfire-specific PM2.5 was associated with elevated risks of asthma hospitalizations compared to exposure to PM2.5 from other sources.

In addition, as smoke travels away from a fire, UV light from the sun causes various chemical reactions, generating a class of chemicals called volatile organic compounds (VOCs). Chronic exposure to some VOCs has been linked “to a range of adverse health outcomes, including respiratory, neurological, cardiovascular damage, and an increased cancer risk.” Sunlit reactions between VOCs and nitrogen oxides (NOx) in the air also form ground-level ozone; a key component of smog, which is known to irritate the lungs even in healthy people. For those living with asthma or other lung disease, it can cause longer-term issues. During one week of the June heatwave that drove wildfires in Europe, ground-level ozone reached an average of 190 µg/m³; almost twice the exposure limit recommended by the World Health Organization.

And the effect of these events is not localized. As Dr Kamal Jyoti Maji from the London School of Hygiene and Tropical Medicine has written, in the right conditions, “the toxic particles caused by wildfire smoke can remain suspended in the atmosphere for weeks and carried thousands of kilometers.”

A paper published in August examined the surface-level atmospheric chemistry in Missoula, Montana; a mid-sized city that absorbs ‘aged’ smoke (3+ days old) transported from wildfires in California and the Pacific Northwest. The researchers found that during smoke events in 2020, levels of carbon monoxide, PM2.5, and total VOCs rose by factors of 3-8 above normal levels. Ozone levels increased under light smoke, but once PM2.5 exceeded ~30-40 μg/m³, they declined, likely because heavy smoke blocks the sunlight needed to drive ozone photochemistry. The authors also showed that a widely-used chemical transport model tended to greatly underestimate the concentration levels of many of these pollutants, which in turn has an impact on predicted health impacts. They estimate that if a 2020-level fire season recurred annually, the lifetime excess cancer risk would reach 100-in-1-million; approximately 7x more than if there was no wildfire smoke.

In a large, separate study from earlier this year, scientists focused on the link between fire smoke and ozone pollution. They found that, after accounting for weather conditions, the presence of smoke caused an increase in ground-level ozone of up to 16% in some parts of the US. This, they said, can lead to more than 2000 excess deaths per year nationally. The effect of the 2023 Canadian wildfire season – still the most destructive ever recorded – could also be seen in the data. They found that rather than the ozone-related deaths representing 15.8% of PM2.5 deaths, that year they represented 61.5%.

They also made a rather surprising finding – smoke-driven ozone enhancement was higher in the eastern US, despite the western US consistently having higher smoke PM2.5. They attribute this to higher air temperatures and UV radiation levels on smoke days, which may lead to a higher concentration of NOx/VOC precursors in the air for smoke to react with. This again suggests that the link between wildfire smoke and ozone may not yet be fully understood.

Health System Pressures

When a wildfire starts near a major city, how does that impact the health system? That was the focus of a study on the 2025 Los Angeles wildfires, which utilized the electronic health care records of 3.7 million Angelenos. The researchers found that in the week following the fire’s ignition, virtual respiratory and cardiovascular visits were significantly higher than normal in all of the groups exposed to the smoke, with those living within 20 km of burn zones most likely to seek care. Daily outpatient acute care utilization was also higher as a result of the fires.

In terms of the response of the healthcare system, the use of mobile healthcare vehicles during the LA fires was shown to be particularly effective, as was “the successful establishment of comprehensive 24 h medical services at the Pasadena Convention Center”. But there were issues with communication, evacuation of medical staff, and the management of infectious diseases. A study from Harvard researchers found that as wildfires become more common and our cities continue to grow, healthcare facilities themselves are increasingly at risk. They write that “wildfires have been getting closer to healthcare facilities by an average of 628 feet per year,” with 53% more inpatient beds now exposed to a wildfire within a five-mile radius. Other studies have shown numerous gaps in how emergency medical services respond to wildfires, including insufficient equipment and inadequate training.

There are many reasons why Dorota Jarosinska from the World Health Organization recently described wildfires as “not merely fire episodes affecting livelihoods and creating this immediate danger…[but] a true health emergency”.

Unequal Impact

In late 2025, a global team of scientists published an important paper – it used population statistics and wildfire data to quantify how many people are exposed to wildfires worldwide. The number they came up with was 440.2 million between 2002 and 2021. The headline stat was counterintuitive –while the total area burned by wildfires decreased (by 26%), the population directly exposed to wildland fires increased by 40% globally, with most of the increase occurring in Africa. They attribute the disproportionate impact in Africa to land fragmentation, with human settlements moving into the “wildland-urban interface”; those areas already prone to wildfires.

Population growth and migration played a major role in the overall increase. The authors calculate that without these changes, cumulative exposure to wildland fire would have been 10 – 28% lower in Africa, Asia, North America, and South America, while in Europe and Oceania, exposure rates would have been ~2 – 5% higher. My takeaway from this? It’s not only the ecosystem that’s negatively impacted by urban sprawl. As new suburbs encroach into areas that were previously wilderness, it’s also a public health issue.

Two other papers, published in the same journal a month apart, together show how inequity is baked into wildfire exposure.

The first mapped how three climate hazards – extreme heat, wildfire burn zones, and wildfire smoke – impacted 11 Western US states over a 15-year period. They used a mix of high-resolution satellite and model data to investigate where and when these hazards co-occurred across an area home to more than 75 million people. They found the most common co-exposure was extreme heat + wildfire smoke, and that the occurrence of this was increasing over time. Co-exposure to all three hazards was rarer, but tended to be concentrated in California, Oregon, and Arizona. Crucially, the American Indian and Alaska Native populations were found to have disproportionate exposure to every hazard combination studied. Older, disabled and poorer residents were more likely to experience co-exposures than other population groups.

The second paper looked at a very different topic – indoor exposure to wildfire-PM2.5. Their simulation showed that wildfire-PM2.5 represents just 1.3% of all indoor PM2.5 globally, but the proportion of people exposed to it varies by regions. South America exhibited the highest proportion, followed by Africa, Oceania, and North America. In total, they found that more than a billion people are “exposed to at least one substantial indoor wildfire-air pollution day per year.”

The researchers also examined the economic cost of improving indoor air quality with purifiers. They estimate that reaching the WHO’s best standards for indoor air (5 μg/m3) would cost $4.24 trillion per year, with the ‘extra’ annual cost due to wildfire-PM2.5 reaching $68.8 billion. The actual cost of mitigation to local resident showed a huge disparity. For example, the authors found that the annual cost of an air purifier was 207% of the average income of people in Niger – an impossible burden. In contrast, in Qatar, the cost of air purifier represented just 0.1% of income. Additionally, DR Congo, Central African Republic, and Guinea ranked high in all three dimensions – smoke exposure, cost, and cost-to-income ratio.

The final paper I want to highlight is arguably the odd one out. Rather than look at the impact of wildfires directly, it instead reviewed the occurrence of studies on human health and wildfires, in order to map what the field had (and hadn’t) studied between 1997 and 2023. The authors found that before 2000 there were virtually no publications on the health impacts of wildfires. By 2002, a few more papers had emerged; by 2006, a few more. But it wasn’t until 2010 that the topic began to become mainstream. From 2019 onward, publication figures surged, reflecting the increasing severity and visibility of wildfires.

In terms of where the research was done, the US dominates, followed by Australia and Canada. While this may reflect the higher incidence of wildfires in these regions, the authors say that access to “more substantial research funding and resources dedicated to understanding and mitigating wildfire impacts” likely plays a larger part. As we’ve seen from previous papers, low-income countries are disproportionally impacted by wildfires, but their scientists are underrepresented in academic research. The authors write, “the global body of literature increasingly reflects not only where wildfires occur, but where research infrastructure exists to analyze them.” In other words, the evidence base on wildfire health is as geographically biased as the exposure rates.

Wildfires may start in the wilderness, but as this group of research papers shows, their consequences are increasingly urban, unequal, and unmeasured. Closing those gaps may be just as important as putting out the fires themselves.