The average emissions of six major pollutants in the U.S. remained unchanged from 2016 levels to 2017, the latest edition of the Environmental Protection Agency’s report on national emissions data reveals.

The report tracks broad changes in air quality in the United States by calculating average emissions from the most common pollutants every year. U.S. emissions have declined significantly over time, about 73 percent from 1970 to 2017, according to the report released Tuesday. (RELATED: Our Air Is Cleaner Than Ever, So Why Do People Think It’s Getting Worse?)

“Through federal and state implementation of the Clean Air Act and technological advances in the private sector, America has achieved one of the great public-private successes of our time – dramatically improving air quality and public health while simultaneously growing the nation’s population and economy,” EPA Acting Administrator Andrew Wheeler said in a statement. “This report details a remarkable achievement that should be recognized, celebrated, and replicated around the world. A 73 percent reduction in any other social ill, such as crime, disease, or drug addiction, would lead the evening news.”

The EPA’s 2016 “Our Nation’s Air” report, an earlier edition of the emissions study, shows that from 1970 to 2016, emissions from the most common pollutants decreased 73 percent. That broad calculation remained unchanged in the recent report on emissions in 2017.

The U.S. economy has roughly tripled in growth, including higher energy production, more vehicles on the road and a larger population in the same amount of time, according to the EPA.

Emissions of sulfur dioxide decreased year to year from 2016 to 2017. Those gains were tempered by increased pollution from lead and particular matter over the same timeframe. Carbon monoxide, nitrogen dioxide and ground level ozone emissions remained stable.

The increase in particulate matter emissions year over year likely stemmed from spot events such as wildfires that burned through much of the West in 2017, Bill Wehrum, the assistant administrator of the Office of Air and Radiation, told reporters during a press call Tuesday. The West, and especially California, suffered through a historic wildfire season in 2017 that set a spending record on firefighting by the federal government.

“You see a significant, consistent downward trend over time but, for instance, events in a particular year can show relative increases year to year notwithstanding the long term trends,” Wehrum told reporters. “So for particulate matter, which shows a small incremental increase year over year, we think a lot of that is attributable to the wildfires that are occurring out west, and that’s had enough of an affect that it is actually influencing the national average numbers that we are reporting here.”

The increase in lead pollution found in the study is due to increased monitoring and more comprehensive data used in 2017 versus previous years. The lead pollution was also measured over a shorter timeframe than in previous studies, Office of Air Quality Planning and Standards scientist Liz Naess, who oversaw the study, told reporters during the press call.

“In previous reports, we went back to a larger timescale, but what that did is it decreased the number of lead monitors included in that analysis,” Naess said. “The number of lead monitors we had drastically increased by 2010 and we made the choice to shorten the time horizon for the trends analysis.”

That “drastically increased the number of monitors and data points that were in that trend analysis and so that’s why you see the difference between last year and this year,” Naess added.

This post has been updated with comments from a press call.

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