Occupational exposure to motor oil and similar substances may increase the risk of arthritis, research suggests.
Motor oil may trigger the immune system
Exposure to mineral oils, in particular hydraulic or motor oil, was linked to a 30% increased risk of developing certain forms of rheumatoid arthritis.
The researchers, from Sweden's Karolinska Institute, believe mineral oils may contain molecules that activate the immune system.
The study is published in Arthritis Research and Therapy.
The Karolinska team focused on patients diagnosed with rheumatoid arthritis between 1996 and 2003.
Each patient provided details of their exposure to different types of mineral oils, including asphalt and engine oil.
The results suggested that exposure to hydraulic or motor oil was associated with an increased risk of two forms of rheumatoid arthritis.
The study confirmed previous tests in animals, in which exposure to mineral oil had been shown to cause arthritis in rats.
Odd risk factors
A spokesperson for the Arthritis Research Campaign said: "The results of this study are not as bizarre as they may seem.
"We know from our own studies that all kinds of odd risk factors for rheumatoid arthritis turn up in these studies, such as having a blood transfusion, and eating lots of red meat.
"There was even a study a few years ago that showed if you had a well at the bottom of your garden you were more likely to develop rheumatoid arthritis.
"Also, three times as many women as men have rheumatoid arthritis, and they rarely have any exposure to motor oils.
"However, this is a serious study in a prestigious journal and should be taken seriously.
"And we know there are many so-called environmental risk factors such as exposure to silica that triggers RA in some people - combined with a genetic predisposition."