But the increased risk was seen only in white men, not white women or African Americans.

The findings are too preliminary to conclude that ammonia emissions cause salivary gland cancers, Dr. Terry A. Day at the Hollings Cancer Center at the Medical University of South Carolina in Charleston told Reuters Health.

"I don't think we can say a whole lot yet," Day said in an interview. "I think it's going to take a broader look at this across the United States" to determine whether ammonia emissions actually increase the risk of salivary gland cancers.

Day, along with co-author Dr. Edward D. Gorham of the Naval Health Research Center in San Diego, reported the results of the study on Sunday at the 6th International Conference on Head and Neck Cancer in Washington, D.C.

The salivary glands, which are located under the tongue and along the sides of the upper and lower jaw, produce saliva, which is essential for digestion.

There are several types of salivary gland cancers, but none of them is common. Only about 1,000 to 1,500 people develop salivary gland cancers each year in the U.S., according to the American Cancer Society.

When caught in the early stages, salivary gland cancer is highly treatable.

But salivary gland cancers can turn deadly when they go undiagnosed, Day said. He noted that even small salivary gland cancers can invade the facial nerves and eventually lead to death.

Unfortunately, there has not been much research into the causes and prevention of salivary gland cancers, Day said.

To investigate a possible link between exposure to environmental factors and salivary gland cancer, researchers reviewed fatal cases of salivary gland cancer over two decades in South Carolina.

From 1979 to 1998, there were 174 deaths from salivary gland cancer in South Carolina. Among whites, fatal cases were three times more common among men, but among blacks, there was no significant gender difference.

Air pollution, including ammonia, varied widely from county to county. But ammonia levels were significantly associated with salivary cancer deaths, at least in white men. White men were more likely to die from salivary gland cancer in counties with higher levels of ammonia in the air.

The association between ammonia levels and the risk of dying from salivary gland cancer persisted even after researchers accounted for air levels of sulfur dioxide, a chemical that has been linked to other types of cancer. Similarly, socioeconomic factors and access to medical and dental care did not affect the increased risk seen in white men.