More rheumatoid arthritis in those exposed to certain air pollutants, researchers say.

SATURDAY, Nov. 5, 2011 (HealthDay News) — Exposure to certain types of air pollution is associated with an increased risk for the painful joint disease known as rheumatoid arthritis, new research suggests.

This link is strongest for sulfur dioxide, one of the six most common air pollutants in the United States, according to the findings from two studies scheduled to be presented Wednesday at the American College of Rheumatology annual meeting, in Chicago.

In the studies, investigators looked at 2,092 rheumatoid arthritis patients and more than 93,000 people without the disease in the United States and Sweden, and used their home addresses to estimate their long-term exposure to several common air pollutants, both gaseous (for example, carbon monoxide, sulfur dioxide and nitrogen dioxide) and particulate (soot or dust).

There was no evidence of increased risk of rheumatoid arthritis associated with particulate air pollution. But increasing exposure to sulfur dioxide and oxides of nitrogen in the 10 and 20 years prior to onset of rheumatoid arthritis was associated with increased risk of the disease among the Swedish participants, the investigators found.

Low-, medium- and high-exposure to sulfur dioxide, nitrogen dioxide or nitrogen oxide were associated with an up to 7 percent, 11 percent and 7 percent increased risk for rheumatoid arthritis, respectively, according to the Swedish study.

These increased risks of rheumatoid arthritis were higher in people with less than a university education than in those with at least a university education. Education levels are a measure of socioeconomic status.

People "with a lower socioeconomic status are more likely to live in houses where more air pollution leaks in from the outside or other factors such as general health status that may make them more susceptible to the effects of air pollution," Dr. Jaime Hart, an instructor in medicine at Brigham and Women's Hospital in Boston, said in an American College of Rheumatology news release.

Hart was lead investigator of the U.S. study and was scheduled to present the U.S. and Swedish findings at the meeting.

The U.S. study found that only exposure to sulfur dioxide was associated with modest increases in rheumatoid arthritis risk. Those with a high exposure to sulfur dioxide had a 5 percent greater risk of rheumatoid arthritis than those with low exposure.

But Hart noted that the U.S. participants were part of the Nurses' Health Study, which meant they may have had a higher overall socioeconomic status than the Swedish participants.

Study data and conclusions presented at medical meetings should be viewed as preliminary until published in a peer-reviewed journal.

About 1.3 million Americans have rheumatoid arthritis, which typically affects women twice as often as men. Previous research has suggested a connection between environmental factors and rheumatoid arthritis.