In a study of more than 75,000 women, those who used insecticides six or more times a year had nearly two-and-a-half times the risk of developing the autoimmune diseases than women who adopted a live-and-let-live attitude toward bugs.
Similarly, the risk more than doubled if bug sprays were used in the home for 20 or more years.
Hiring a gardener or commercial company to apply insecticides also resulted in a doubling of risk, but only if they were used long-term, says Christine G. Parks, PhD, an epidemiologist with the National Institute of Environmental Health Sciences in Research Triangle Park, N.C.
"Our new results provide support for the idea that environmental factors may increase susceptibility or trigger the development of autoimmune diseases in some individuals," she says.
Although the study doesn't prove cause and effect, "we need to start thinking about what chemicals or other factors related to insecticide use could explain these findings," Parks tells WebMD.
The researchers used data from the Women's Health Initiative Observational Study of 76,861 postmenopausal, predominantly white women ages 50 to 79. Of the total, 178 of them had rheumatoid arthritis and 27 had lupus. An additional eight women had both disorders. As part of the study, the women were asked a number of questions relating to farming and insecticide use.
"Importantly, the relationships we observed were not explained by other factors that we considered, including farm history, age, race, ethnicity, socioeconomic factors such as education and occupation, smoking and other risk factors for disease," Parks says.
Interestingly, a history of working or living on a farm did not appear to increase risk of rheumatoid arthritis or lupus in the study, she adds. Previous studies have linked farming and agricultural pesticide exposure to the disorders.
The findings were presented at the annual meeting of the American College of Rheumatology.
Studies show that as many as three-fourths of U.S. households have reported using insecticides in the home or garden, and 20% of households have applied insecticides in the last month, according to Parks.
"Insecticide exposure in the home can be quite persistent because they don't break down in the home environment," Parks says.
"The findings are fairly compelling" because they show the greater and longer the exposure, the greater the risk, says Darcy Majka, MD, assistant professor of medicine at Northwestern University Feinberg School of Medicine.
"Now we have to go back to the bench. Which products pose a risk? Is skin exposure [to blame], or inhaling?" she says.
For now, Majka tells WebMD, "The important thing is to follow the directions [on the product] and take other measures to limit chemical exposure."