The Phoenix metropolitan area will be under a pollution advisory for ozone for the second consecutive day.
Environmental regulators issued the advisory for Tuesday because the area's ozone levels 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.
People are encouraged to drive only when necessary, use carpools or public transportation and avoid using gas-powered lawn equipment and drive-thru lines.
Ozone forms when vehicle exhaust and chemical solvents combine with heat and sunlight.