What is worse than extreme heat? Extreme heat plus air pollution Premium

Scientists examined 1.5 million deaths from 2014 to 2020 registered in California – a state prone to summer heat waves and air pollution – to find out which is worse

Heat waves and air pollution from wildfire smoke and other sources are each problematic for human health, particularly for vulnerable populations such as older adults. But what happens when they hit at the same time?

We examined over 1.5 million deaths from 2014 to 2020 registered in California – a state prone to summer heat waves and air pollution from wildfires – to find out.

The number of deaths rose both on hot days and on days with high levels of fine particulate air pollution, known as PM2.5. But on days when an area was hit with a double whammy of both high heat and high air pollution, the effects were much higher than for each condition alone.

The risk of death on those extra-hot and polluted days was about three times greater than the effect of either high heat or high air pollution alone.

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The more extreme the temperatures and pollution, the higher the risk. During the top 10% of hottest and most polluted days, the risk of death increased by 4% compared to days without extremes. During the top 1%, it increased by 21%; and among older adults over age 75, the risk of death increased by more than a third on those days.

There are several ways the combined exposure to extreme heat and particulate air pollution can harm human health.