Ask a Scientist

Why is smog worse in the summer?

The chemical reactions that form ozone, a principal component of smog, happen more quickly at higher temperatures. So ozone levels are typically higher in the summer and higher during the day compared to nighttime. Ozone is formed by precursor gases (volatile organic compounds and nitrogen oxides) in the presence of heat and sunlight. Ozone levels are also tied to wind patterns and emissions: Traffic and industry are primary manmade sources of ozone precursors. Ozone has been linked to increased risk of respiratory symptoms like wheezing and respiratory infections that can result in hospitalizations and mortality. More than 100 million Americans live in areas that exceed current health-based standards for ozone. It’s especially problematic in the eastern US, California, and Texas urban centers. And it’s a growing problem for countries with expanding transportation networks.

Climate change might cause worse smog. In one study, we found climate change could raise ozone levels even higher, further harming human health and increasing mortality.        

Issue 25

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