By Stephen Beech
Heatwaves can cause more potentially deadly polluted air, according to new research.
It is not just high temperatures that make heatwaves unsafe but also the heat-related increase in airborne pollutants, say scientists.
And they warned that prolonged periods of hot weather are especially dangerous in already hot places.
For example, more than 300 people died in Texas in 2023 from heat, according to local authorities, the most since the state began tracking such deaths in 1989.
Because heatwaves are predicted to worsen in the next few years, researchers from Texas A&M University's Atmospheric Chemistry and the Environment (CACE) team wanted to determine how extreme heat impacts atmospheric chemistry and air quality.
They designed and conducted a pilot study of atmospheric chemistry during a heatwave in Texas last August.
The team collected air samples throughout the day and night from Aug. 5 to Sept. 3, 2024, on the university campus where temperatures ranged from 90 to 106 degrees Fahrenheit (32 to 41 degrees Celsius).
Samples were taken when there were no wildfires nearby, isolating the effects of the heatwave itself without the influence of wildfire smoke on air quality.
The research team analyzed air samples for pollutants of public health concern - including nitrogen oxides, ozone, volatile organic compounds (VOCs) and nanoparticles.
They used a range of sensitive instruments to detect trace gases and measure aerosol properties, including a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-4000).
Team member Bianca Pamela Aridjis-Olivos said: "You can think of it like a super sensitive nose."
She explained that the PTR-ToF-4000 "sniffs" for VOCs then "gently marks them so they could be identified and weighed in real time."
Results showed "concerning" levels of ozone, oxygenated VOCs and acid-rich nanoparticles that increased in concentration with outdoor temperatures.
The researchers also observed increased levels of airborne pollutants created by sunlight-fuelled chemical reactions in the air.
They also found that, during heatwaves, trees release more natural VOC emissions, including isoprene - a precursor to ozone - that could be harmful in heavily wooded places.
Aridjis-Olivos, a graduate student in aerosol and atmospheric chemistry, said: "It really was surprising, how these emissions from trees increase during heatwaves and interact with air pollution."
She added: "On their own, the trees' emissions aren't dangerous.
"It's when they react with other emissions under strong solar radiation that we get elevated ozone and secondary organic aerosols that are dangerous for the public health."
The researchers are currently analyzing additional data from their August 2024 fieldwork, but have compiled a series of recommendations for keeping safe during heatwaves.
These include staying indoors during peak sunlight hours, usually noon to 4pm, when temperatures and ozone levels are highest.
They say people should also avoid exercising or commuting near major roads or urban hot spots during extreme heat as well as keeping windows closed to limit exposure to outdoor air pollutants.
The team advises people to keep an eye on the local air quality index and adjust outdoor plans accordingly, when possible.
While the tips can help in the short-term, the researchers say "real progress" depends on understanding how climate change affects atmospheric chemistry and air quality so scientists can better predict pollutant formation and protect public health.
The findings are due to be presented at the fall meeting of the American Chemical Society (ACS) in Washington, DC.