Wildfire smoke is a new national crisis for the United States. Under the influence of climate change, fires are growing and turning into monsters, consuming vast landscapes and entire cities. Mounting evidence shows that these fires kill many more people than previously thought as smoke travels hundreds or even thousands of miles, exacerbating conditions such as asthma and heart disease. One study estimates that the Los Angeles inferno did not kill 30 people as official estimates last January. but 440 or more considering the smoke. Another recent study found that haze from wildfires already kills 40,000 Americans a yearwhich could increase to 71,000 by 2050.
Two additional studies published last month paint an even bleaker picture of the crisis in the United States and other countries. first detects that emissions greenhouse gases and the amount of airborne particles from wildfires worldwide could be 70 percent higher than previously thought. second found that wildfires in Canada in 2023 significantly worsened childhood asthma across the border in Vermont. Taken together, they illustrate the desperate need to protect public health from the growing threat of wildfire smoke such as better air quality monitoring with sensor networks.
The emissions study is not an indictment of previous estimates, but a revision of them based on new data. Satellites have been monitoring wildfires for decades, although in a somewhat limited way – they break up the landscape into squares measuring 500 by 500 meters, or about 1,600 by 1,600 feet. If the wildfire does not completely fill this space, it does not count. This new study increases that resolution to 20 by 20 meters (about 66 by 66 feet) in several key fire regions, meaning it can capture many small fires.
Individually, smaller fires do not produce as much smoke as larger fires. urban alignment in the American West. But “they add up, and they take a long time,” said Guido van der Werf, a wildfire researcher at Wageningen University and Research in the Netherlands and lead author of the paper. “They basically double the amount of burned area that we have around the world.”
According to 500-meter satellite data, the previous estimate was about 400 million hectares charred annually. If you add small fires to this, this figure increases to 800 million hectares, roughly the size of Australia. In some parts of the world, such as Europe and Southeast Asia, this improved resolution can triple or even quadruple the size of fires. Scientists previously thought annual emissions from wildfires amounted to about 2 gigatons of carbon, or about a fifth of what humanity produces by burning fossil fuels, but the new estimate now puts it more like 3.4 gigatons.
The type of fire also has a huge impact on emissions. A forest fire must burn a large amount of biomass – bushes, grasses, trees, sometimes even part of the soil – and turn it into carbon dioxide and methane and particulate matter, but in a grass fire on the prairie there is much less of it. The fires also burn at vastly different speeds: Flames can spread quickly through a forest, but the carbon-rich soil known as peat can smolder for days or weeks. Peat fires are so persistent that when they ignite in the Arctic, they can remain hidden under falling snow and then pops up again when the temperature rises and everything melts. Scientists call them zombie fires. “Where you burn is very important and also how intense the fire can become,” van der Werf said.
But why can the fire remain small when in recent years we have seen how large and destructive these fires can become? This is partly due to the fragmentation of the landscape: roads can prevent them from spreading, and firefighters prevent them from reaching cities. And in general, the long history of firefighting means that they are often extinguished quickly. (Ironically, this also helped create some monsters, because vegetation grows all over the landscape, ready to burn. This disrupts the natural order of things, in which low-intensity fires from lightning strikes clear dead bush, restoring the ecosystem for new growth – which is why the indigenous tribes prescribed burns have been done for a long time.) Farmers also burn waste biomass and apparently keep the fire from getting out of control.
While in remote areas such as the boreal forests of the far north, lightning strikes typically start fires, the study found that in densely populated regions, many small fires occur. In general, the more people dotting the landscape, the more ignition sources there are: cigarette butts, spark-producing electrical equipment, even chains trailing from trucks.
Yes, these smaller fires are less destructive than giant fires, but they can still have catastrophic consequences in a more indirect way by sending smoke into populated areas. “These small fires are not the cause of most problems,” van der Werf said. “But of course they happen more often, closer to where people live, and that also has health implications.”
That's why the second asthma study is so alarming. The researchers compared Vermont's extremely smoky 2023 to 2022 and 2024, when skies were clearer. They were interested in PM 2.5, or particulate matter less than 2.5 millionths of a meter in size, coming from wildfire smoke coming out of Quebec, Canada. “It can be particularly difficult to clear from the lungs and particularly irritating to the airways,” said Anna Massel, a graduate student at the University of Vermont and lead author of the study. “There are studies that show that exposure to bushfire smoke can have much longer-term effects, including the development of asthma, especially in early childhood.”
However, this study examined exacerbation of asthma symptoms in children already living with the condition. While children with asthma typically have fewer attacks in the summer because they are out of school and constantly exposed to respiratory viruses and other pathogens indoors, the data showed that their condition was much less well controlled in the summer of 2023, when huge wildfires were burning. (Clinically, “asthma control” refers to milder symptoms, such as cough and shortness of breath, as well as more serious problems, such as attacks. Thus, during that summer, pediatric patients reported more symptoms.) At the same time, climate change is extending growing seasons. this means plants produce more pollenwhich also aggravates this chronic disease. “All of these factors really complicate what health care professionals previously thought was a safe time of year for children with asthma,” Massel said.
The researchers also discovered that smoke is transformed as it travels through the atmosphere. For example, it tends to produce ozone, which irritates the lungs and causes asthma. “There's also the potential for increased production of substances like formaldehyde, which is also harmful to human health. It's a dangerous air pollutant,” said Rebecca Hornbrook, who studies wildfire smoke at the National Center for Atmospheric Research (NCAR) but was not involved in either study, although a colleague was involved in the emissions study. (Last month, the Trump administration announced plans to eliminate NCAR. which experts say could have catastrophic consequences.)
As wildfires intensify, so does the public health problem of smoke, even in places that have never had to deal with haze before. Governments now have to work harder to protect their people, for example by improving access to air purifiers, especially in schools. “This is no longer an isolated or geographically limited problem,” Massel said. “It’s really spreading all over the world and to places it’s never been seen before.”
