On the morning news, you see the weather forecast is for high heat, and there is an “excessive heat watch” for later in the week. You were hoping the weather would cool down, but yet another heat wave is threatening human health and increasing the chance of wildfires. On top of these warm days and nights, air quality data has been showing unhealthy levels of pollution.

Sound familiar? This scenario is increasingly the new normal in many parts of the world.

High heat and air pollution 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.

Deaths spike when both risks are high

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.

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