A high pollution advisory issued by environmental regulators says ozone levels in metro Phoenix will exceed federal health standards on Friday.
The Arizona Department of Environmental Quality says ozone can irritate the respiratory system, aggravate asthma and reduce the immune system's ability to fight off respiratory infections.
Ozone-related health problems include shortness of breath, coughing, wheezing, headaches, nausea, plus throat and lung irritation.
The department says people most affected by ozone include children, older people, people with respiratory disease and people who work and exercise outdoors.
The advisory urges residents and businesses to drive as little as possible, refuel vehicles after dark and avoid waiting in long drive-thru lines.