(Reuters Health) - Women exposed to secondhand smoke as children may be more likely to develop rheumatoid arthritis than people who didn’t breathe cigarette fumes growing up, a French study suggests.

Rheumatoid arthritis is an immune system disorder that causes debilitating swelling and pain in the joints. It’s less common than osteoarthritis, which happens when cartilage on the ends of bones wears down over time.

Smoking has long been linked to an increased risk of rheumatoid arthritis. But the new study suggests that secondhand smoke may also increase this risk.

Altogether, the study involved 71,248 women, including 371 who eventually developed rheumatoid arthritis. Current and former smokers who were not exposed to smoke as children were 38 percent more likely to develop rheumatoid arthritis than those who had never smoked. When current or former smokers were also exposed to secondhand smoke during childhood, they were 67 percent more likely to develop rheumatoid arthritis.

Among women who never smoked at all, exposure to secondhand smoke during childhood was associated with a 43 percent higher risk of developing rheumatoid arthritis compared with no secondhand smoke exposure growing up, although this difference was not statistically significant, meaning it was too small to rule out the possibility it was due to chance.

“In adults exposed to active smoking, the mechanism leading to rheumatoid arthritis onset is quite well understood,” said study co-author Dr. Marie-Christine Boutron-Ruault of the INSERM epidemiology and population health research center at Paris-Sud University in Villejuif, France.

Rheumatoid arthritis happens when the immune system that’s supposed to attack invaders like bacteria and viruses mistakenly attacks healthy cells. In adult smokers, changes in some proteins in the air cells of the lungs are thought to trigger this autoimmune activity, leading to rheumatoid arthritis, Boutron-Ruault said by email.

“It is highly likely that the phenomenon described in adult smokers occurs similarly in passively exposed children,” Boutron-Ruault added. “The triggering of autoimmunity in children might not be restricted to rheumatoid arthritis risk, and could possibly increase the risk of other autoimmune diseases.”

For the study, researchers examined survey data collected every three years, starting in the 1980s. Participants were 50 years old on average when they joined the study and about 54 percent of them had never smoked. About 14 percent were current smokers and 32 percent were former smokers.

The study wasn’t designed to prove whether or how secondhand smoke exposure during childhood might cause rheumatoid arthritis. Researchers also relied on women to report their smoking history and tobacco exposure during childhood, and self-reported information may not be as reliable as data from lab tests or medical records.

It’s also possible that secondhand smoke exposure during childhood increases the risk of rheumatoid arthritis just because it leads to more cumulative years of smoke exposure among people who smoke as adults, said Jill Norris, a researcher at the Colorado School of Public Health in Aurora.

While it’s not clear that avoiding smoke exposure during childhood can prevent rheumatoid arthritis in the future, there are many other good reasons not to expose kids to secondhand smoke, Norris, who wasn’t involved in the study, said by email.

“The general advice is that if a parent is going to smoke, they should not smoke in the home or in the car when their children are riding with them,” Norris said. “It is also important to ask any caregivers (nannies, relatives, etc) to do the same.”

SOURCE: bit.ly/2NHHJFm Rheumatology, online August 14, 2018.