For the fourth consecutive day, environmental regulators have issued a high-pollution advisory for ozone in metropolitan Phoenix.
The advisory was issued because ozone levels on Thursday are expected to exceed the federal health standard for air quality.
People with respiratory problems are advised to limit their outdoor activities on days of elevated ozone levels.
Regulators say people also are encouraged to drive only when necessary, use carpools or public transportation, and avoid gas-powered lawn equipment and drive-thru lines.
Ozone forms when vehicle exhaust and chemical solvents combine with heat and sunlight.