Regular or long term exposure to consumer insecticides may increase the risk of developing autoimmune diseases like rheumatoid arthritis and lupus. That’s the finding of a study presented at a meeting of the American College of Rheumatology in Philadelphia.

Previous research has suggested a link between exposure to agricultural insecticides and the development of rheumatoid arthritis and lupus; but in this study, scientists set out to determine if the same risks could apply to bug killing chemicals applied around the home, yard or garden.

The use of bug sprays is common. Surveys suggest that as many as three-quarters of households use them, and there are hundreds of millions of tons of such products produced every year.

For this study, researchers at the National Institutes of Health identified more than 76,000 women between the ages of 50 to 79 who had taken part in the Women’s Health Initiative Observational Study, which gathered information from generally healthy postmenopausal women. Scientists looked, in particular, at the women’s answers to questions about their personal or commercial residential insecticide use over their lifetime and whether or not they had lived or worked on a farm.

Scientists then determined which women later reported they had been newly diagnosed with either rheumatoid arthritis or lupus and also reported using disease modifying anti-rheumatic drugs, which typically treat rheumatoid arthritis and other connective tissue diseases.

Of the more than 76,000 women enrolled, 178 had newly developed RA; 27 had newly developed lupus, and eight had both conditions.

The risk of developing these diseases was higher for women who reported more insecticide use, whether they had ever lived or worked on a farm. And those who had personally mixed or applied insecticides were at the greatest risk of developing rheumatoid arthritis or lupus.

For example, compared to women who reported no insecticide exposure, disease risk was more than doubled in those who reported using insecticides six or more times per year and in women who had used them less frequently but over longer time period – for 20 or more years.

“Our data do suggest that regular application over the course of many years may be a greater concern than a single, short-term exposure, “ says Christine Parks, PhD, the lead investigator and an epidemiologist with the National Institute of Environmental Health Sciences in Triangle Park, NC.  “The study has generated a lot of interest and I think there’s a good reason people should be interested and concerned now and it raises the need for doing more studies on environmental factors.”

The study did not look at specific chemicals or products used or at the kinds of pests being killed. It also didn’t assess whether the chemicals are currently on the shelves or were available decades ago but are now banned. Parks also says researchers can’t rule out the possibility that these findings are due to some other factors.

“At this time it is hard to imagine what other exposures might explain what we see for insecticide use, as there are relatively few established environmental risk factors for these diseases,” Parks says.

But she says it could be that insecticide use may be a marker for other risk factors that are still hidden in the data. For example, women who spray to kill bugs may live in conditions where bug infestations are more common, and it could be, for example, that low socioeconomic status, living in apartments or condominiums or even keeping a more hygienic environment could be the real culprit.

“But none of these factors have ever been studied with respect to autoimmune disease risk.” Parks says.

Still, she stresses that autoimmune diseases are fairly uncommon in the general population, so a person’s individual risk of disease, even if they’ve been exposed to bug sprays, is still low. The 213 women who developed rheumatoid arthritis or lupus in the first three years of this study represented only about 3 per 1,000 of patients involved.

Andrew Ruthberg, MD, a rheumatologist and assistant professor at Rush University Medical Center in Chicago agrees that is an important point.

“The absolute risk is small. Lets say you doubled it and said it’s going to go up to six out of 1000 for people with the exposure. That means 994 out of 1000 are not going to develop the disease even if they have some exposure,” Dr. Rothberg says. “If you look at that absolute risk, it may not be so powerfully frightening."

Dr. Rothberg says that this kind of study doesn’t produce a lot of specific information but is important because it identifies areas of investigation for future studies.

Parks agrees that more research is needed. And says insecticide use can sometimes be important for getting rid of bugs that damage property, like termites, or that can cause disease or transmit disease, like fleas, mosquitoes or bed bugs.

But given these findings, she says it makes sense to use precautions and minimize unnecessary exposures.

Parks suggests wearing a mask or gloves when spraying for bugs and making sure to follow the recommended guidelines for use on the product labeling.