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The heatwave that brought 100-degree temperatures to the Philadelphia region in June also brought warnings that the air quality was unhealthy for sensitive groups.
This was not a coincidence.
Heat and sunlight are key ingredients in the reaction that creates ground-level ozone, a common air pollutant that can cause coughing, wheezing and more frequent asthma attacks.
Ozone is what’s known as a secondary pollutant. It forms from other pollutants produced by sources such as cars, trucks and power plants.
In an urban area like Philadelphia, the air already contains plenty of these precursor pollutants, said Kabindra Shakya, a professor at Villanova University who studies air quality.
“They also need one other ingredient, which is the sunlight,” Shakya said. “When we see the high heat and the sun, then that actually drives more formation of the ozone.”