The Phoenix metro area is seeing a sharp rise this year in the number of days it has exceeded the federal health standard for the acceptable level of ozone in the air.
Maricopa County has surpassed the standard 18 days so far this year, compared with seven in 2015 and 11 in 2014.
With three months left in the county's ozone season, the increase raises the possibility that it could break its record in 2012 of having 29 days in which it exceeded the standard.
The county attributes the increase to the federal government's decision to toughen its ozone standard to 70 parts per billion, rather than the previous measure of 75 parts per billion.
The change took effect last year and applies to counties across the country.