Three Massachusetts counties – Bristol, Essex and Suffolk – cut their ozone and particle pollution levels, and the Boston metro area reported some of the lowest pollution levels on record in spite of a national trend of higher ozone pollution, according to the American Lung Association, published by the State House News Service.

The association on Wednesday released its 2018 “State of the Air” report, tracking ozone, or smog, and year-round particle pollution, also called soot, from 2014 through 2016. It ranked Boston as the 47th most polluted city in the nation for ozone, calling that level “significantly improved” from last year’s ranking of 37th. Hampshire County was the only county in Massachusetts to have its ozone grade drop from 2017 to 2018, falling from a C to a D.

Hampden County recorded the worst ozone levels in the state, according to the report. “When older adults or children with asthma breath ozone-polluted air, too often they end up in the doctor’s office, the hospital or the emergency room,” Michael Seilback, the association’s vice president of public policy and communications for the northeast region, said in a statement.

Ozone can even shorten life itself. The association said more than four in 10 Americans, or 133.9 million people, live in counties with “unhealthful levels” of either ozone or particulate pollution. The report gave Berkshire, Bristol, Essex, Franklin, Hampden, Norfolk, Plymouth, Suffolk and Worcester counties “A” grades on particle pollution, while the other five counties either had incomplete monitoring data or had no monitor collecting data.