After a five-year review, U.S. EPA has proposed leaving unchanged its two primary air quality standards for nitrogen oxides.
EPA Administrator Scott Pruitt, acting under a court-ordered deadline, signed a rulemaking notice Friday, and then it was quietly posted on an agency website.
"This proposed decision has been informed by a careful consideration of the full body of scientific evidence and information available in this review," the draft rule says, citing the recommendations of EPA's Clean Air Scientific Advisory Committee and an agency assessment of recent research into the health effects of nitrogen oxides.
The proposal will have a 60-day comment period upon publication in the Federal Register, accompanied by a public hearing if requested. Under a settlement clinched earlier this year in response to a lawsuit by two environmental groups, EPA must have the final rule in place by next April (Greenwire, May 1).
NOx — with nitrogen dioxide serving as a regulatory stand-in for the broader class of gases — is a key ingredient in smog. Major sources are cars, trucks and coal-fired power plants. Exposure to low levels can irritate the eyes, nose and throat, while inhalation over a day or two can lead to fluid buildup in the lungs, according to the Agency for Toxic Substances and Disease Registry.
Under the Clean Air Act, primary air quality standards are intended to protect public health. For nitrogen oxides, EPA's annual standard of 53 parts per billion dates back to 1971, supplemented by a one-hour 100 ppb limit added in 2010.
The decision to leave the current thresholds in place represents a victory for carmakers and other industry groups. Dissenting was the American Lung Association, which had argued in a filing in November that the one-hour standard should be tightened to allow for a "margin of safety" for asthma sufferers.
NOx is among a half-dozen "criteria pollutants" named in the act. EPA is supposed to review and — if warranted — revise the thresholds for those pollutants every five years in light of available scientific research. In practice, the agency is chronically late. Under the law, for example, the NOx review should have concluded two years ago. It took the lawsuit brought last year by the Center for Biological Diversity and the Center for Environmental Health to prod the agency into agreeing to hard deadlines.
While Pruitt, like other Republicans, has derided what he calls "sue-and-settle" deals, he made no attempt to undo the consent decree that had been tentatively reached under the Obama administration and was made final in April. The same consent decree also applies to a new review of the standards for sulfur oxides, with a proposed rulemaking due by next May.
The NOx review, launched in 2012, was a relatively sleepy affair in comparison with the tumult that surrounded the agency's decision two years ago to reset the ozone standard.
By EPA's estimate, total NOx releases have fallen more than 60 percent since 1980, with the decline expected to continue as stricter vehicle emissions limits take effect. There are no areas in the United States now in nonattainment for either the annual or hourly benchmarks.
Agency officials had signaled their intent to stick with the status quo last fall with the release of a draft policy assessment that found current evidence "does not call into question any of the elements of the current standards" (Greenwire, Sept. 28, 2016). In a March report, the Clean Air Scientific Advisory Committee, a group of outside experts, seconded that conclusion. The policy assessment was made final soon afterward.
The secondary NOx standard, designed to protect ecosystems, was not part of the review. It remains at 53 ppb on an average annual basis.
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