The Arizona Department of Environmental Quality has issued an ozone health watch for Thursday. People with asthma and other respiratory disease may want to limit time outdoors.
On Wednesday, the Valley was under a high pollution advisory.
Ozone is an invisible pollutant that can aggravate existing respiratory issues. It is created when other pollutants – such as exhaust from vehicles – are heated. A health watch is issued when ozone levels may approach federal standards.
On Wednesday morning, all but two local ozone monitoring stations had readings above the previous 24 hours.
Motorists are asked to carpool, take public transit or telecommute to help cut down ozone production.