Townsend said the new study -- a compilation of information from a variety of existing scientific studies -- resulted from a meeting about a year ago of scientists in a variety of fields whose research kept running into links between the rapid increase of nitrogen in the environment and potential public health problems.

One of the easiest effects to recognize, Townsend said, is an increase in number and size of dead zones such as the one that forms in the Gulf of Mexico each summer.

The algae blooms that cause these low-oxygen areas can leave people who come into contact with them with sicknesses including cholera.

They also can sicken fish and shellfish, which in turn can cause health problems when eaten.

Less obvious, he said, are health effects that have resulted from massive changes in the foods people eat that have been driven by changes in the way nitrogen fertilizer is used.

Nitrogen fertilizer has grown the crops in the Midwest that have contributed to a dramatic decrease in starvation and malnutrition in developing countries, Townsend said. But it also has spurred a worldwide boom in the use of beef, pork, chicken and other poultry that has led to unhealthy lifestyles.

"There are strong arguments that meat proteins may be an important part of human diets," the study said.

"However, high consumption of meat and some commercial grain products such as corn sweeteners can be linked to a range of health problems that are common in developed countries, including heart disease and diabetes."

Half of the inorganic nitrogen ever used on the planet has been applied during the past 15 years, the study said. Much of that has been used in the United States, where nitrogen fertilizers have revolutionized the way corn and other grains are grown.

Much of the corn is used as cattle feed, often in factory-style "concentrated animal feeding operations."

Wastes from those operations often join nitrogen runoff from farmland in local streams and rivers, increasing the risk of overfertilizing water bodies, a process called eutrophication.

And scientists are finding that some mosquitoes that carry diseases such as malaria, West Nile virus and other forms of encephalitis may favor such overfertilized streams, the report said.

The link between nitrogen and air pollution has been known for a long time. Nitrogen from automobile exhausts, electric power plants, factories and agricultural fertilizer creates ozone pollution that has been linked to asthma and other lung diseases.

The report said the frequency of asthma and lung disease has been increasing around the world.

Nitrogen fertilizer, in combination with climate changes and increased amounts of carbon dioxide in the atmosphere, may also be playing a part in rising pollen counts around the world, which has been linked to increases in the number of cases of hay fever, rhinitis and allergenic asthma, the study said.

And high levels of nitrate, formed when fertilizer seeps into groundwater, has been linked to reproductive problems; methemoglobinemia, or "blue baby syndrome," in which blood lacks the ability to carry sufficient oxygen to individual body cells; and cancer, including non-Hodgkin's lymphoma, bladder and ovarian cancers, the study said.

Although the changes in the use of nitrogen and its effects have been rapid, "the potential to back off the use of nitrogen without massive economic and social costs is clearly there," Townsend said.

For instance, the report suggests that getting Americans to switch to a more "Mediterranean" diet that is less dependent on meat will both reduce nitrogen use and reduce public health problems.

And Townsend said simply getting farmers to stop overfertilizing their fields would be an easy first step.

"Using a bit less fertilizer will not necessarily harm crop yield, but could make major differences in the environment," he said.