By Martin Macias, Jr.
(CN) – Concentrations of fine particulate matter in U.S. skies lead to thousands of deaths annually and have shortened life expectancy despite efforts to curb pollution, a study found Tuesday.

Fine particulate matter, known as PM2.5, is primarily produced by vehicles, heavy industry and power plants. The tiny particles – some 30 times smaller than the width of a human hair – are absorbed deep in the lungs and have been linked to heart attacks and forms of lung disease.

While fine particle pollution has declined since 1999 in the U.S, where the standard for PM2.5 is 12 microgram per cubic meter of air, efforts to relax controls on emissions have run parallel to the work of rolling back regulations all together.

In order to estimate the direct health impacts of pollution, researchers with Imperial College London and Carnegie Mellon University combed through satellite images and data from 750 air quality sensors to chart out PM2.5 concentrations in U.S cities and counties between 1999 and 2015.

In a study published Tuesday in PLOS Medicine, researchers found that although reduced PM2.5 levels have lowered mortality rates in the largest U.S counties, fine particle matter still kills thousands.

In 1999, the highest PM2.5 concentration was found in Fresno County, California – at 22 micrograms per cubic meter of air – while the highest in 2015 was recorded in nearby Tulare County, California, at 13.2 micrograms per cubic meter of air.

Imperial College London researcher and study lead author Majid Ezzati said in a statement Tuesday that even at concentrations lower than the national standard, fine particle pollution can still kill thousands.

Between 2015 levels in Tulare County and concentrations recorded in Apache County, Arizona, the same year – 2.8 milligram per cubic meter – fine particle pollution led to the lung and heart-related deaths of 15,600 women and nearly 14,800 men.

“We’ve known for some time that these particles can be deadly,” Ezzati said in the statement. “Lowering the PM2.5 standard below the current level is likely to improve the health of the U.S. nation, and reduce health inequality.”

More than 18 million people died in the U.S from lung and heart-related diseases between 1999 and 2015.

Ezzati added that because PM2.5 levels in the U.S are generally lower than those recorded in Europe, there are likely substantial amounts of deaths there that are tied to air pollution.

As a result of the deaths, national life expectancy decreased by 0.15 years for women and 0.13 years for men, according to the study, which did not include figures from Alaska or Hawaii.

Los Angeles County experienced the largest drop in life expectancy due to elevated PM2.5 levels, though southern states including Arkansas, Oklahoma and Alabama also saw dramatic dips.

The U.S Environmental Protection Agency funded the study, which also found the decline in life expectancy was greater in lower income counties than in wealthier counties.