Extreme heat and pollution are harmful in and of themselves. Together they are even more dangerous.

(ORDO NEWS) — You see on the morning news that the weather forecast is for extreme heat, and at the end of the week an “excessive heat watch” has been announced.

You hoped the weather would be cooler, but another heat wave threatens people’s health and increases the likelihood of forest fires. In addition to these warm days and nights, air quality data reveals levels of pollution that are dangerous to health.

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

Extreme heat and air pollution are human health concerns, especially for vulnerable populations such as the elderly. But what happens when they come at the same time?

We looked at the more than 1.5 million deaths reported between 2014 and 2020 in California, a state prone to summer heatwaves and air pollution from wildfires, to find out.

Mortality increases when both risks are high

The number of deaths increased both on hot days and on days with high levels of air pollution from fine particles known as PM2.5. But on days when the area was hit by the double whammy of extreme heat and high air pollution, the effect was far greater than either of these conditions alone.

The risk of death on these ultra-hot and polluted days was about three times higher than in extreme heat or high air pollution.

The more extreme the temperature and pollution were, the greater the risk.

On the 10 percent of the hottest and most polluted days, the risk of death increased by 4 percent compared to days without extreme temperatures. In the first 1% of days, the risk of death increased by 21%; and among older people over 75 years of age, the risk of dying on such days increased by more than a third.

Why risks are higher when both factors work simultaneously

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

Oxidative stress is the most common biological pathway associated with particulate air pollution and heat exposure.

Oxidative stress is an imbalance between the production of highly reactive molecules known as reactive oxygen species, or ROS, and the body’s ability to remove them. It is associated with lung diseases as well as other diseases.

Antioxidants help clear these molecules, but particulate air pollution and heat upset this balance due to excessive metabolic production of ROS and reduced antioxidant activity.

Our studies also showed that the effects of particulate air pollution and extreme heat were stronger when nighttime heat and air pollution occurred together.

High night temperatures can interfere with normal sleep and potentially contribute to the development of chronic diseases such as heart disease and obesity, as well as disrupt the regulation of body temperature.

Older adults may be more susceptible to the effects of extreme heat and air pollution, in part because this stress is superimposed on age-related chronic diseases such as heart disease, high blood pressure, diabetes, or chronic lung disease.

Dysregulation of body temperature in response to heat can also occur with age. In addition, older people may be less mobile and therefore have less access to cooling centers or medical care, and may not be able to afford air conditioning.

The future of high temperatures and air pollution

This is not just a California problem. Climate change will increase the impact of heat waves and air pollution in many parts of the country.

Average annual temperatures in the US are already more than 1.8 degrees Fahrenheit (1 degree Celsius) warmer than they were in the early 1900s.

By the end of this century, global temperatures will warmer by nearly 5 degrees Fahrenheit (2.7 degrees Celsius). Dangerous extreme heatwaves, now rare, will become more common.

Climate change is also affecting levels of outdoor fine air pollution – for example, through weather changes such as air stagnation, winds and dust storms, as well as drier and warmer conditions that contribute to more frequent and intense wildfires.

What to do to stay safe

Further research is needed to better understand these effects, such as the overall impact of wildfire smoke.

However, it is already well known that people should take steps to reduce their risk of harm during periods of extreme heat or air pollution.

This means drinking plenty of water and staying cool. Shopping malls and other air-conditioned public places can provide shelter from the heat. Home air conditioning, especially at night, can reduce mortality. A portable bedroom air filter can significantly reduce particulate air pollution.

People with symptoms of heat stress such as headache, nausea, dizziness or confusion, especially the elderly, should seek medical attention.

Many county and state health departments are already providing warnings for extreme heat and extreme air pollution. The development of a special category of alerts for simultaneous occurrence of extreme situations can be useful for public health.

Governments also need to take action now to avoid the worst-case climate change scenarios in the future. Some best practices for cities include creating cooling shade covers and green spaces that will also reduce particulate air pollution.

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