Exposure to paint, varnish and other solvents is associated with an increased risk of multiple sclerosis (MS), according to a study.

People with exposure to paint or other solvents are 50 percent more likely to develop MS than people without it. People with exposure to solvents who also carry the genes that make them more susceptible to MS are nearly seven times more likely to develop the disease as people with no solvent exposure who do not carry the MS genes. For people who have been smokers, the risk is even greater. Those who have been smoking with solvent exposure and the MS genes are 30 times more likely to develop MS than those who have never smoked or been exposed to solvents and who do not have the genetic risk factors.

"These are significant interactions where the factors have a much greater effect in combination than they do on their own," said study author Anna Hedström from the Karolinska Institutet in Stockholm, Sweden. "More research is needed to understand how these factors interact to create this risk. It's possible that exposure to solvents and smoking may both involve lung inflammation and irritation that leads to an immune reaction in the lungs."

For the study, researchers identified 2,042 people who had recently been diagnosed with MS in Sweden and matched them with 2,947 people of the same age and sex. Blood tests were used to determine whether the participants had two human leukocyte antigen gene variants, one of which makes people more likely to develop MS and the other reduces the risk of MS. The participants were also asked whether they had been exposed to organic solvents, painting products or varnish and whether they had ever been a smoker.

In the group with neither of the MS genes and no smoking or exposure to solvents, there were 139 people with MS and 525 people without the disease. In the group with the MS genes and exposure to solvents but no smoking, there were 34 people with MS and 19 people without the disease.

In the group with MS genes and exposure to solvents and smoking, there were 40 people with MS and five people without the disease. The researchers determined that the MS genes and exposure to solvents combined were responsible for an estimated 60 percent of the risk of developing MS. "How this cocktail of MS genes, organic solvents and smoking contributes so significantly to MS risk warrants investigation," said Gabriele C. DeLuca.

"In the meantime, avoiding cigarette smoke and unnecessary exposure to organic solvents, particularly in combination with each other, would seem reasonable lifestyle changes people can take to reduce the risk of MS, especially in people with a family history of the disease." The study appears in the journal Neurology.