Millions of people in Arizona are being advised to limit activity that contributes to air pollution, after an air quality alert was posted by the National Weather Service (NWS) in conjunction with local authorities.Why It MattersThe Environmental Protection Agency (EPA) states that ozone exposure can irritate the respiratory system, triggering coughing, throat irritation, and pain while breathing deeply. It may inflame and harm the airways, reduce resistance to infections, and aggravate preexisting lung conditions like asthma, emphysema, and chronic bronchitis.While healthy individuals can experience symptoms, those with respiratory diseases face a higher risk of more serious health effects, the EPA said.What To KnowAn ozone high pollution advisory was issued for the Phoenix metro area—home to nearly 5 million people—by the Arizona Department of Environmental Quality on Thursday. The warning is in effect throughout Friday, it said.”This means that forecast weather conditions combined with existing ozone levels are expected to result in local maximum 8-hour ozone concentrations that pose a health risk. Adverse health effects increase as air quality deteriorates,” read the notice, which was published by the NWS as an air quality alert.”Ozone is an air contaminant which can cause breathing difficulties for children, older adults, as well as persons with respiratory problems,” it cautioned. “A decrease in physical activity is recommended.”Residents in the affected area were also advised to carpool, telecommute or use public transport. Gasoline-powered equipment should be used less or used later in the day, officials said.What People Are Saying Brandon Buckingham, a meteorologist with AccuWeather told Newsweek previously: “Ozone is a secondary pollutant, meaning it’s not emitted directly from sources, but is formed through chemical reactions. These reactions require sunlight and higher temperatures, making warmer months more prone to ozone formation. When air is stagnant, pollutants don’t get dispersed, allowing ozone to build up to unhealthy levels.”Ozone pollution is often more severe in urban areas where there are higher concentrations of pollutants from sources like cars, power plants, and industrial activities.”Jonathan Grigg, professor of pediatric respiratory and environmental medicine at Queen Mary, University of London, previously told Newsweek that there are “very clear links” between inhaling particles and earlier death from both respiratory and cardiovascular diseases.He added: “There are vulnerable groups and classically they are children because they’ve got an extra issue to do with their lungs developing, whereas our lungs are not developing as adults.”What Happens NextAt the time of writing, the air quality alert for the Phoenix metro area was in effect through Friday.The NWS issues regular forecast updates on its website.