Air quality in western Pennsylvania improved in 2016, according to three regional monitors tracked by the Allegheny County Health Department.

“It was a good year in ozone,” said Jayme Graham, the department's air quality manager.

Allegheny County keeps an eye on harmful, ground-level ozone throughout the summer and small particulate matter in the air year-round.

Complete 2016 data for particulates will not be available for a few months, but ozone monitor readings show Pittsburgh only broke federally-set limits five times in 2016. That's great news for a city once known for its perpetual black clouds, Graham said.

“Emissions are much less than they were before with controls on power plants, better running vehicles,” she said. “The other thing, even though we had high temperatures in August, we also had more rain and rain essentially washes the air out.”

Ground-level ozone is created when air pollution, often in the form of urban smog, is bombarded with sunlight. It can cause respiratory irritation and damage crops and other vegetation.

Warnings are issued by the state whenever there is an expectation that ozone levels could become dangerously high. Only one such warning was issued for Allegheny County in 2016.

“Predicting an air quality action day is a little bit difficult," Graham said. "It’s actually predicting the weather and then predicting chemistry in the weather, so we don’t always match exactly."

The American Lung Association gave Allegheny County an "F" in its annual State of the Air report released in April, ranking the region the 8th worst in the nation.

Pennsylvania receives transport pollution from nearly 300 coal-fired power plants in 15 states and the District of Columbia, much of which originates in plants lining the Ohio River for hundreds of miles.

U.S. Steel's Clairton Coke Works and Shenango Coke Works, which shuttered one year ago, have historically been major contributors, according to the Heinz Endowments' Breathe Project launched in 2014.

Graham said she expects air quality to continue improving for the next several years.