The sky is a little bluer over the United States, as a new report suggests the air is slowly but surely becoming cleaner.

The air quality in many cities has improved markedly thanks to improved technology in fuel-burning mechanisms, although problem areas remain, the American Lung Association announced Wednesday.

"What we’re seeing in the report is progress nationwide in both particle pollution reduction and ozone pollution, but I will point out that we have a long way to go nationwide," says Lyndsay Alexander, assistant vice president of the American Lung Association's Healthy Air Campaign.

The biggest improvement came as counties studied across the United States lowered the levels of particle pollution in the air. Although weather patterns change air quality, 16 US cities hit their lowest levels of particle pollution ever for the entire year. This included Los Angeles, although it remains the nation's most polluted city for ozone pollution, while Bakersfield topped the list for particle pollution.

"You accept the victories and you always want things to be better,” Norman Edelman, senior scientific adviser to the American Lung Association, told ABC News. “They can be better and it would make a difference."

Many cities benefitted from both new practices at power plants fueled by coal and better emissions and engine technology in cars and larger vehicles.

“Retrofitting and retiring the old, dirty engines in construction equipment and school buses is really helping air pollution," Ms. Alexander says.

Improvement came across the United States, and many areas are seeing the effects of the 1970 federal Clean Air Act. Although some still have dirty air, many of the nation's most polluted cities were slightly cleaner than last year. In Ohio, for example, particle pollution readings improved in Cleveland, making it among 16 cities that reported their lowest levels of particle pollution on record, reported The Plain Dealer. Many Americans still live in cities with dirty air, however, as 166 million – 52.1 percent of the US population – reside in counties where the air quality needs improvement.

Cities that regressed generally did so as a result of changes to their climate in the form of drought, rainfall, and especially wildfires. California's reputation for dirty air remains, despite ongoing efforts to improve air quality, and the state holds the report's top seven areas with poor-quality air, the Guardian reported.

Los Angeles and Bakersfield, Calif., a nearby city, still reported the nation's worst air because of the area's high population, an urban plan that requires a car, and geography that collects rather than dissipates the smog. Cities throughout California have also suffered from intense drought and several major wildfires.

The American Lung Association lauded the federal Clean Air Act, currently on hold by the Supreme Court, but urged states to individually evaluate their air quality to determine paths to improvement. As scientific information has become more available, cities have been able to make specific plans because they know their targets for clean air.

“Air pollution doesn’t honor state boundaries and can travel hundreds of miles," Alexander says.

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On an individual level, the American Lung Association urged Americans to save electricity and gas where possible and avoid burning trash or wood.

“We want people to understand that in addition to supporting strong federal and state policies, that individuals can improve air quality as well," Alexander says.