Pima County issued a ground-level ozone advisory until 6:00 pm Thursday, marking the second straight day for such an advisory.
The County Department of Environmental Quality's Natalie Shepp said ozone warnings get more likely on days with a certain mix of intense sunlight and high carbon emissions.
"So when you get emissions from motor vehicles, industry, power plants and things along those lines mixing in the presence of sunlight on these warm summer days, then you're more likely to have ground-level ozone issues," she said.
Shepp added that more afternoons with ozone warnings are likely until conditions change.
A contributing factor is that monsoon cloud cover has not been rolling in until later in the day, allowing intense sunlight to hit the area.
"In this case, we see the higher levels before we get that cloud cover. And if we have less winds, then there's less opportunity to clear it out and it has more opportunity to kind of cook into ozone."
Until that happens, people should avoid outdoor activities that can cause increased breathing between 12:00 and 6:00 pm, particularly if they're sensitive to air quality issues.
The public can help mitigate the problem by minimizing the amount of air pollutants they create, primarily by minimizing how much they drive and their electricity use. They can also avoid filling up a car's gasoline tank during the warmest part of the day.