A high pollution advisory for ozone has been issued for metro Phoenix.
Environmental regulators issued the advisory for Friday because the area's ozone levels are expected to exceed the federal health standard for air quality.
Ozone forms when vehicle exhaust and chemical solvents combine with heat and sunlight.
People with respiratory problems are advised to limit their outdoor activities on days of elevated ozone levels.
Official encourage people to drive only when necessary, use carpools or public transportation and avoid using gas-powered lawn equipment.