Chemicals in paint and varnish may increase people's risk of developing the crippling nerve disorder multiple sclerosis (MS), according to a study.
The research, published in the journal Neurology, found that people with exposure to paint or other solvents are 50 per cent more likely to develop MS than those with no exposure.
People with exposure to solvents who also carry the genes that make them more susceptible to MS are nearly seven times as likely to develop the disease as those with no solvent exposure who do not carry the MS genes, the researchers said.
For people who have been smokers, the risk is even greater, they said.
Those who have been smokers 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 Anna Hedstrom from the Karolinska Institutet in Stockholm, Sweden.
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"More research is needed to understand how these factors interact to create this risk. It is possible that exposure to solvents and smoking may both involve lung inflammation and irritation that leads to an immune reaction in the lungs," said Hedstrom.
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.
There were 34 people with MS and 19 people without the disease in the group with the MS genes and exposure to solvents but no smoking, researchers said.
In the group with MS genes and exposure to solvents and smoking, there were 40 people with MS and five people without the disease, they said.
The researchers determined that the MS genes and exposure to solvents combined were responsible for an estimated 60 per cent 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 of the University of Oxford in the UK.
"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," DeLuca said.