Heat waves and air pollution caused by wildfire smoke and other sources pose significant health risks, especially for vulnerable groups like the elderly. But what happens when these two issues occur simultaneously? We analyzed over 1.5 million deaths in California—a state prone to summer heat waves and air pollution from wildfires—dating from 2014 to 2020 to find out. The number of deaths increased on both hot days and days with high levels of fine particulate air pollution known as PM2.5. However, when areas experienced both high heat and air pollution, the impact was much more severe than with each factor alone. The risk of death on these extremely hot and polluted days was about three times greater than the effect of just high heat or air pollution individually.
The higher the temperatures and pollution, the higher the risk. On the top 10% of the hottest and most polluted days, the risk of death increased by 4% compared to ordinary days. On the top 1% of these days, this risk increased by 21%. For older adults over 75, the risk of death rose by more than a third on such days. There are multiple ways that the combined exposure to extreme heat and particulate air pollution threatens human health. Oxidative stress is the main biological process associated with both particulate air pollution and heat exposure. It represents an imbalance between the generation of highly reactive molecules known as reactive oxygen species (ROS) and the body’s capacity to eliminate them. It’s connected to lung diseases and other conditions. Antioxidants help neutralize these molecules, but particulate air pollution and heat disrupt this equilibrium by heightening metabolic ROS production and reducing antioxidant activity. Our research also indicated that the effects of particulate air pollution and extreme heat are greater when high nighttime temperatures and pollution overlap. High nighttime temperatures can disrupt normal sleep, potentially leading to chronic health issues like heart disease and obesity, and interfere with the body’s temperature regulation.
Older adults might be especially prone to the effects of extreme heat and air pollution exposure, partly because these stresses add to existing age-related chronic health issues such as heart disease, high blood pressure, diabetes, or chronic lung disease. As people age, their ability to regulate body temperature in response to heat often declines. Furthermore, older adults may have less mobility and, therefore, struggle to reach cooling centers or medical care, or have difficulty affording air conditioning. This issue doesn’t just affect California. Climate change is increasing the likelihood of high heat and air pollution exposure across many areas in the country.
The U.S. average yearly temperatures are already over 1.8 degrees Fahrenheit (1 degree Celsius) warmer than they were at the start of the 1900s. By century’s end, global temperatures are on track to increase by nearly 5 F (2.7 C). Currently rare, dangerous and extreme heat waves will become more frequent. Climate change also influences levels of fine particulate outdoor pollution—through weather changes such as air stagnation, wind and dust storms, and drier, warmer conditions that lead to more frequent and severe wildfires. Further studies are needed to understand these impacts fully, such as the overall effect of wildfire smoke exposure. Yet, current knowledge is sufficient for taking precautionary measures to minimize harm during periods of extreme heat or air pollution. This includes staying hydrated and keeping cool. Shopping malls and other air-conditioned public spaces can offer a refuge from heat.
Having air conditioning at home, especially at night, can reduce mortality. Having a portable air filter in the bedroom can significantly lower particle pollution levels. People who show signs of heat stress—such as headaches, nausea, dizziness, or confusion—especially elderly individuals, should seek medical attention. Many health departments at the county and state levels already issue alerts for extreme heat and air pollution. Introducing a special alert category for simultaneous extremes may benefit public health.