A decision by the Obama administration to pull the plug on a stronger ozone pollution standard proposed by the EPA will likely reduce the benefits that Bay cleanup efforts reap from more stringent air pollution control measures.

Nitrogen oxides, a major contributor to ground-level ozone pollution, are also a large source of nitrogen pollution to the Bay. While exact figures are not available, the proposed ozone standard had the potential to reduce the amount of nitrogen pollution reaching the Chesapeake by several hundred thousand pounds a year.

On the Friday before Labor Day, though, the White House Office of Management and Budget sent a letter to the EPA asking that it reconsider the rule, citing the need to "promote predictability and reduce uncertainty" in the regulatory system.

The action was immediately blasted by environmental groups, which said the new standard would have prevented 4,300 deaths and 7,000 hospital visits a year. Besides endangering human health, ozone - a key element of smog - is harmful to crops and forests.

But the proposed regulation's withdrawal was praised by business groups that had led a furious campaign against the rule, which they contended could cost $90 billion a year.

Air standards are reviewed by law every five years, and the regular review of the ozone standard is scheduled for 2013. The OMB said that this has created the potential for the standard to be changed yet again in two years, and asked that the EPA's proposed change be postponed.

The ozone standard was last changed in 2008, when it was reduced from 0.085 parts per million to 0.075 ppm, under former president George W. Bush. But that action was criticized by health and environmental advocates, in part because the agency's own scientific advisory committee had recommended a stricter standard to protect public health.

After the election of President Obama, new EPA Administrator Lisa Jackson announced that the agency would review the ozone standard, and in August 2010 proposed a standard of 0.07 ppm.

Ironically, although that standard has been put off, states may still get some help in Bay cleanup efforts from the 2008 standard. That's because when the EPA calculated the total maximum daily load "pollution diet" that was finalized in December, its estimates for air pollution still incorporated the 2007 ozone standard of 0.085 ppm.

Lewis Linker, modeling coordinator with the EPA Bay Program Office in Annapolis, said that rough estimates suggest that the 0.07 standard would have reduced the amount of nitrogen entering the Bay from its watershed by about 1 million pounds a year, and reduced the amount of nitrogen landing directly on the Bay by about a half million pounds. The current standard of 0.075 ppm will achieve about two thirds of those reductions - something not factored into the TMDL.

"I think we will be able to reduce some nitrogen loads," because of the 0.075 ozone standard, Linker said.

The new ozone standard would have made the job easier, though, and several state watershed implementation plans indicated that future air pollution reductions could help them achieve Bay cleanup goals. New York's plan, which fell about 500,000 pounds a year short of meeting its Bay nitrogen goal, specifically said that "anticipated changes to the EPA's air quality standard for ozone" was one of the actions that would help it close that gap.

Under the pollution diet, states in the watershed need to reduce the amount of nitrogen reaching the Bay by about 25 percent, or 60 million pounds a year from 2009 levels.

Rich Batiuk, associate director for science with the EPA's Bay Program Office, said the agency will use its computer models to estimate the amount of nitrogen reductions that could accrue to each state through the implementation of the 2008 ozone standard, which could in turn help them meet their nutrient reduction goal.

Similarly, if there are reductions in air pollution from future federal or state actions, including any future revision to the ozone standard, states could get additional benefits.

The potential for new air pollution reductions is unclear. The EPA has several pending regulations that could affect nitrogen oxide emissions, but Congress is considering several measures that could block further air rules. On Sept. 23, the House passed a bill that would block new EPA rules for sulfur dioxide, nitrogen oxides and mercury as well as create a cabinet level panel to study the cumulative impact of new rules on the economy.

Between a quarter and a third of all nitrogen reaching the Chesapeake originates from air pollution. About two thirds of that originates from nitrogen oxides, formed by burning fossil fuels, and the rest is ammonia, which mostly originates from agriculture.