Why Extreme Heat Plus Pollution Is a Deadly Combination

TClimate-related risks to health are increasing at alarming rates. These include record temperatures and increased air pollution, such as car exhaust or wildfire smoke. In addition, they can lead to acute illnesses and increase existing health issues. What happens when these conditions occur together?

This question was recently addressed by researchers from the University of Southern California. The results were based upon mortality data for California from 2014 to 2019, and were published in June by the University of Southern California. American Journal of Respiratory and Critical Care MedicineAccording to the study, the combination of high temperatures and heavy pollution can result in a significantly higher mortality rate than their individual effects.

As the chart below shows, a person’s odds of dying increased 6.1% on extreme temperature days and 5% on extreme pollution days compared with non-extreme days. However, death rates rose by 21% on extreme days.

As vehicle exhausts, wildfires produce PM2.5. It is a fine particle that has a diameter of less than 2.5 micrometers. For comparison, a single hair’s diameter is 30% larger than that of the fine particles. The USC researchers looked at PM2.5 levels and found that pollution from all sources was not the same. They also discovered that high-level pollution days coincided with California wildfires. “When you consider our top 1% of most polluted days, the pollution concentration is really very, very high… four times higher [than normal],” says Md Mostafijur Rahman, a postdoctoral researcher in the Department of Population and Public Health Sciences at USC’s Keck School of Medicine and one of the study’s co-authors. “That is definitely driven by another source. It’s not like the normal source from the traffic.”

Fine particulate matter can penetrate deep into the lungs and enter the bloodstream, says Francesca Dominici, a biostatistics professor at Harvard’s T.H. Chan School of Public Health, who have studied this noxious substance. Although PM2.5 has been shown to be a risk factor for cancer, heart disease, and respiratory issues, there are some types of PM2.5 that can cause more harm than others. “Fine particulate matter during wildfires tends to be even more toxic,” Dominici says. “We have buildings burning, we have cars burning, we have all kinds of stuff that is burning. There is emerging research to show that the chemical composition is even more dangerous.”

What’s more, when those tiny particles react with high temperatures and sunlight, they can worsen ground-level ozone—smog—which can trigger respiratory effects like asthma attacks. One study from Washington State University published earlier this year found that periods of high ​​PM2.5 and ozone have “become significantly more frequent and persistent” across the western U.S. in the last 20 years, due to “simultaneous widespread heat and wildfire activity.” A notable 12-day stretch in the summer of 2020 included one August day where nearly 70% of that region—encompassing 43 million people—was affected by harmful levels of air pollution due to unprecedented wildfire activity around that time.

However, this is not a mistake. It is not just the American West that faces the dual threat of pollution and heat. This summer has seen extreme temperatures in almost every part of the country, with fires raging through forests even as far as Alaska. It is well-known for having hot summers that are dangerous and bushfires. The 2018-2020 season was devastating in Eastern Australia. Russia saw one of the most destructive wildfires in its history last year, which occurred in Siberia during hot and dry conditions. In Europe, infernos ravaged Turkey and Greece last year; this year they’re sweeping through Spain and France, fueled by heat waves that smashed records for both how early in the year they appeared and how high the mercury rose.

This confluence of events during summer months, when temperatures soar to unbearable levels that our bodies cannot handle, are becoming more common: The heat waves make dry regions even drier—and ideal for wildfires which spew smoke plumes far and wide. Erika Garcia, assistant professor in the Department of Population and Public Health Sciences at USC’s Keck School of Medicine who co-authored the study with Rahman, warns that even though wildfires are episodic, their effects can last for weeks.

“With climate change progression, we will continue to experience more frequent, more intense, and longer extreme heat events, and extreme particulate pollution events,” she says. “We really need to have better interventions and adaptation policies so that we can save lives during these extreme heat and pollution days.”

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