From acid rain and global warming to hazy skies and cloudy waters, there is one form of pollutant that shares at least some of the blame: nitrogen oxides.

A byproduct of fossil fuel combustion, NOx was barely on the radar screen of air quality regulators only a decade ago but is now implicated in at least 11 human health and environmental concerns, according to a new EPA report.

With nitrogen oxides now a major target for air pollution reductions, the report was undertaken to ensure that new regulations address the "unusually broad range of detrimental effects" resulting from NOx emissions.

"Not only will the EPA have this information as we are making policy decisions, but states and regional groups will also have that information," said Doug Grano, of the EPA's Office of Air and Radiation Policy and Guidance, who was in charge of the report. "We're hopeful that will lead to better decision-making in the sense that people will be able to think more about integrating their decisions and achieving the biggest bang for the buck, so to speak."

Nitrogen oxides-or NOx for short -are formed at high temperature combustion when nitrogen in the air (which is otherwise generally inert in the atmosphere) along with nitrogen in the material being burned reacts with oxygen. While some NOx is formed by natural activities, such as lightning, the overwhelming majority of NOx in North America comes from human activities, primarily fossil fuel combustion.

The term NOx actually covers several oxides of nitrogen, including nitrogen dioxide, nitric oxide and others which, in turn, can be converted in the atmosphere to nitric acid (a type of acid rain), nitrates (a fertilizer) and other forms of nitrogen. The various forms of nitrogen can contribute to a variety of problems, such as creating ground-level ozone, affecting the growth of plants and forests, contributing to the nitrate contamination of drinking water, reducing visibility and a variety of other ills.

Nitrogen deposition has been a growing concern for the Bay and other coastal waters in recent years because nitrogen spurs the growth of algae blooms which block sunlight to important underwater grasses that provide food and habitat for waterfowl, crabs, clams, juvenile fish and other species. When the algae die, they sink to the bottom and decompose, depleting the water of oxygen, a process known as eutrophication. Eutrophication is considered the primary water quality threat to the Bay and most coastal waters.

Recent estimates suggest that about 25 percent of the nitrogen entering the Bay results from airborne deposition, about 75 percent of which results from NOx emissions. (The rest results from ammonia, largely from agriculture, and naturally occurring organic nitrogen, such as pollen.)

Yet stringent controls on NOx emissions have only been emphasized in recent years. NOx emissions, the report notes, grew three-fold from 1940 through 1970, from 7.3 million tons a year to 20.6 million tons. New regulations stemming from the 1970 Clean Air Act only slowed the rate of growth, as emissions still climbed by nearly 12 percent between 1970 and 1994, reaching 23.6 million tons.

The increase took place largely because NOx was not specifically targeted for sharp reductions in the way that lead, carbon monoxide and other pollutants were under the air act.

That began to change in recent years as evidence has grown that NOx emissions must be curbed to help reduce acid rain and to solve the chronic problem of ground-level ozone-a key component of summertime smog-that blankets many cities.

New, acid rain controls on power plants that resulted from those concerns caused total NOx emissions to fall in 1995 and 1996.

In addition, new restrictions on auto emissions, including a 60 percent NOx reduction, was phased in over recent years, and further reductions are planned. The EPA has also initiated regulations on a host of never-before regulated "non-road" vehicles, such as construction equipment and locomotives, which are also expected to result in further reductions.

New, tougher air standards for ozone and tiny airborne particulates, as well as other recent actions aimed at controlling the ground-level ozone problem, are expected to result in additional NOx controls in future years.

But just as the impacts of NOx on acid rain and ozone formation were not fully appreciated in the past, the report is intended to alert policy-makers that new areas of concern regarding NOx emissions-such as impacts on the Bay and coastal waters-are emerging and should be considered when making regulations.

The report discusses each potential area of concern, the potential contribution of air pollution and scientific uncertainties. But it makes no recommendations about how air regulations should be altered to address those issues. "That's where the most difficult part is, to actually say we're going to integrate this stuff," said Rich Batiuk, associate director for science with the EPA's Bay Program Office, who worked on the report. "How we do that is a good question. I think it's offices and programs such as ours that are going to have to take the next step and really push it."

Indeed, how the anticipated future reductions are made will affect the degree to which other NOx-related problems benefit from the action.

For example, the report notes, curbing NOx emissions at power plants only in the summer months would help achieve the ozone standard. But controlling NOx year-round would provide increased protection for the Bay and other coastal waters, as well as for forests, lakes and streams impacted by acid rain.

Reducing acid rain damage in the Northeast, the report says, may require emission reductions in the winter because deposition that falls at that time is especially important in creating springtime acidic "pulses" in streams that can kill fish and their eggs.

Decisions about other pollution control efforts, such as the type of auto emission inspection programs being used, and the areas where they are required, could also affect NOx emissions.

The report states that decisions about NOx emission reductions should be "made in the context of the many environmental effects associated with NOx emissions" so they can better achieve "multiple public health and environmental goals." But the report also called for a "balanced approach" toward NOx reduction, and it noted that some programs, like year-round emission controls at power plants, can be far most costly than seasonal controls.

Ultimately, Grano said, how those reductions are achieved "is going to depend on who is making the decision. For example, the states adjacent to the Chesapeake Bay are going to be much more interested in developing year-round controls than Texas or Oklahoma might be."

Copies of the report, "Nitrogen Oxides: Impacts On Public Health and the Environment," may be downloaded from the EPA's Office of Air and Radiation Policy and Guidance web site: www.epa.gov/ttn