Although air quality has improved significantly since the 1990s, a new study claims that pollution still causes lung cancer, heart attacks and strokes that kill more than 30,000 Americans a year.

Researchers examined the concentration of fine pollution particles across the country from 1999 to 2015. The particles, which according to the EPA are 30 times smaller than the width of a human hair, come from coal-fired power plants, cars and other sources.

When humans inhale the particles, they become lodged in the small blood vessels in the lungs and can lead to lung disease over time. They can also reach the bloodstream, leading to heart attacks, according to the researchers.

"In every county, some people are dying too early at current levels of air pollution, which would make further improvements a truly national priority," Majid Ezzati, professor of global environmental health at Imperial College London and the senior author of the PLOS Medicine paper, said in a press release on Science Daily.

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The scientists' work revealed a drop in life expectancy for men and women. That drop was highest in areas where poor people lived and lowest in wealthy areas, according to UPI.

"The ubiquitous and involuntary nature of exposures, and the broadly-observed effects across sub-populations, underscore the public-health importance of breathing clean air," said Arden Pope, professor of economics at Brigham Young University and the lead author of the Environmental Health Perspectives paper, in a press statement.

The study, which was published in Environmental Health Perspectives, used public data from 28 years of National Health Interview Surveys linked with the National Death Index to create a large cohort of 1.6 million U.S. adults.

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