The ozone health watch that began Tuesday in the Valley has been extended through Thursday by the Arizona Department of Environmental Quality.
Ozone concentrations on Tuesday reached health watch criteria at a half-dozen monitoring sites, with levels nearly above national standards at Cave Creek and north Phoenix.
With a rapid increase in afternoon high temperatures and relatively light winds expected Thursday, the health watch has been extended.
Ozone is an invisible gas created when pollutants in the air are heated by the sun. It can aggravate existing health conditions such as asthma.
People with respiratory disease, or those at risk because of a compromised immune system, should limit time outdoors Wednesday, especially in the early evening when ozone production is at its highest.
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