After months of devastating fires, Canada鈥檚 worst wildfire season worsened even further in September. In just seven days, blazes across the country burned through as much land as is usually scorched in an entire fire season. The smoke stretched over Greenland and northern Europe, bringing with it toxic particles that can lead to everything from asthma to heart attacks.
This story is increasingly common. Canadian wildfires earlier in the year sent smoke south that blanketed much of聽the聽eastern and central US, choking the sky and prompting warnings of dangerous air quality. Meanwhile, an extreme heatwave in July and August fuelled flames along the Mediterranean, including the largest blaze ever recorded in the European Union.
The increase in both the frequency and severity of wildfires in recent years, and the resulting smoke plumes, have undone years of聽progress. 鈥淔or a long time, wildfires have been a pretty small component of overall air quality,鈥� says at Stanford University in California. 鈥淭hat has聽started to change.鈥�
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For human health, pollution particles less than 2.5 micrometres in diameter 鈥� known as PM2.5 鈥� are most concerning. These particles constitute up to 90 per cent of wildfire smoke. 鈥淲hen you inhale PM2.5, the particles are so small that they can actually cross over at the base of the lungs into the bloodstream,鈥� says at Harvard University. She says this leads to respiratory and neurological issues.
Over the past two decades, countries across the globe have worked to reduce levels of PM2.5, mainly from industrial sources. The US began regulating the pollutant under the Clean Air Act in 1997. Canada set its first targets for PM2.5 in 2000, and the European Union did so in 2005. As聽a result of these efforts, global PM2.5 exposure fell by nearly 11聽per cent between 2011 and 2019.
Wildfire smoke now jeopardises these improvements. Using ground and satellite-based air聽pollution data, Burke and his聽colleagues between聽2000 and 2022. They found that average annual PM2.5 concentrations declined in nearly every state from 2000 to 2016. Then, the downward trend started to significantly slow or begin ticking upwards. 鈥淲e find that, absent wildfires, concentrations of聽PM2.5 would have continued to聽decline around the country. Instead, we see those declines flatten and, on the west coast, start聽to reverse,鈥� he says.
Wildfire smoke influenced PM2.5 levels in nearly three-quarters of states in the contiguous US, eroding about 25 per cent of the progress on air quality made since 2000. In western states, wildfires negated more than half of such progress.
Wildfires are common in the western US and have only grown fiercer over the past decade. Some of the region鈥檚 most destructive fires occurred between 2015 and 2020, and megafires in 2020 resulted in the country鈥檚 worst fire聽season on record.
The team estimates that, without interventions, wildfires鈥� contribution to air quality trends in the US will continue to grow as the climate warms. Projections show that in the western US, an average annual 1掳C increase in temperature could increase the amount of land wildfires burn by聽as much as 600 per cent.
A global phenomenon
The US isn鈥檛 alone in this trend. Beginning in 2017, PM2.5 levels during the summer in British Columbia skyrocketed. For instance, peak concentrations in聽2020 were nearly three times as聽high as those between 2010 and 2016, and the province saw annual fine particulate levels rise by about 50 per cent between 2005 and 2018. In south-east Australia, wildfires accounted for between December 2019 and January 2020, when unprecedented bushfires scorched the country. The fires increased annual exposure to PM2.5 in most of the nation during聽those years, reversing improvements in 2017 and 2018.
鈥淚t is going to get worse because this isn鈥檛 just one place. It鈥檚 a global phenomenon,鈥� says at the University of British聽Columbia in Canada.
Inhaling all this particulate matter is hard on the body. In a separate study, Burke and his team estimated that wildfire smoke in California led to an per year between 2006 and 2017. They also found that in each week following a day of extreme smoke, visits for asthma, cough and chronic obstructive pulmonary disease (COPD) increased by 30 to 110 per cent.
A large body of evidence indicates that even a few hours of聽exposure to elevated PM2.5 can trigger heart attacks and death. An from exposure to wildfire-related PM2.5 worldwide. During Australia鈥檚 devastating bushfire season in October 2019 to January 2020, deaths from short-term compared with a scenario where no fires occurred.
Exposure to wildfire smoke also聽impairs the immune system, leaving people vulnerable to pathogens, including covid-19, says Johnson. This could be due to聽smoke depleting immune cells critical for fighting off disease, she says. For example, attributed 41 per cent of covid-19 deaths and 17 per cent of covid-19 cases in 2020 in Butte county, California 鈥� a region hard hit by fire that year 鈥� to wildfire exposure.
Pregnant people and children appear to be most at risk. Studies have shown that consistent, long-term PM2.5 exposure during pregnancy raises the likelihood of聽preterm birth and fetal death, as聽well as neurological and cognitive problems in children. Long-term exposure during childhood has also been linked to聽impairments in lung function and higher rates of asthma, chronic bronchitis and obesity.
But PM2.5 exposure from wildfire smoke is unique, and few聽studies have investigated the effects of short, repeated bouts of high PM2.5 from wildfires. 鈥淲e definitely know there are health complications with chronic exposure, but we don鈥檛 have a good grasp on [what happens to] healthy people if they are exposed to one wildfire, say, every three years,鈥� says Johnson.
鈥淲e didn鈥檛 have to ask this question before, but now [people] are likely to get hit again and again聽by smoke,鈥� says Brauer. 鈥淲hat does that do for your overall lifespan?鈥�
Fight fire with fire
Humans have suppressed fires for聽centuries, which leaves fuel just waiting to burn, says Burke. Prescribed burns 鈥� small, low-intensity fires purposefully ignited to clear underbrush 鈥� can聽reduce the frequency and intensity of wildfires, says at the US Government Accountability Office (GAO) in Washington DC. 鈥淚f you do that, you don鈥檛 generate that much smoke and you reduce the likelihood of extreme fires in聽the聽future,鈥� says Burke.
However, in the US, smoke from聽prescribed burns is regulated under the Clean Air Act, and counties are fined if these burns elevate PM2.5. But the air pollution from wildfires doesn鈥檛 trigger the same fines. G贸mez says this disincentivises proper land management techniques. Earlier this year, he and his colleagues at the GAO recommended the US government change air pollution laws to exempt prescribed burns.
This may have to be combined with efforts to protect indoor air quality, as people spend more time inside during the worst air quality days. 鈥淲e鈥檝e looked at indoor air quality during wildfire events, and it is really bad,鈥� says Burke. Even shutting windows and聽doors doesn鈥檛 keep out wildfire pollutants, he says.
One solution is air filters, which聽remove particulate matter. Providing at-home filtration devices for free or at a reduced cost聽would be an effective solution, says Brauer.聽 Law-makers could also amend air pollution laws to include indoor air quality, which would incentivise schools, businesses, workplaces and landlords to install air filtration devices, says Burke.
鈥淲e need to think more about protection. How do we, basically, live with it? That鈥檚 a really different approach than we鈥檝e previously had for air quality,鈥� says Brauer.
What is in wildfire smoke?
In addition to fine particulate matter, wildfire smoke contains many pollutants that can also harm health.
Heavy metals in soil and infrastructure, such as lead, aluminium and copper, are picked up and dispersed by fire. When inhaled, these can cause lasting damage to the lungs and nervous system.
Carbon monoxide can linger in聽the atmosphere for a month and travel great distances. It prevents oxygen from reaching organs, leading to headaches, nausea, vision problems and, in聽rare instances, coma or even聽death.
Ozone is a potent greenhouse gas that can also cause chest pain,聽coughing, wheezing and聽shortness of breath. This聽is聽especially dangerous for聽people聽with asthma or other聽lung聽conditions.
Nitrogen dioxide can harm the聽lungs and heart, leading to聽coughing, wheezing and increased inflammation of airways. It may also raise the聽risk聽of asthma in children.
