Air quality in the Phoenix metro area has improved from previous years, but the region still suffers from high ozone levels and particle pollution, according to the American Lung Association.

While it did have fewer unhealthy days of high ozone in this year’s State of the Air report, Maricopa County still received F's for high levels of ozone and short-term particle pollution, which consists of soot or tiny particles from sources like coal-fired power plants, diesel engines and wood-burning devices.

The region had been experiencing worsening pollution from 2007 to 2009, but the reports found that trend improving between 2013 and 2015.

High levels of pollution can trigger asthma attacks, heart attacks and strokes.

The city of Phoenix bettered its rankings or stayed the same as last year for several categories of pollution.