Women with jobs that involve metalworking fluids may have a somewhat higher risk of developing breast cancer, a preliminary study suggests.

Metalworking fluids help cool and lubricate machinery used in the manufacture of automobiles, farm equipment, aircraft and other products. The new study looked at women who spent at least three years working at one of three large automobile manufacturing plants in the U.S.

Among the nearly 4,700 women that researchers followed, the risk of breast cancer increased in tandem with exposure to soluble, oil-based metalworking fluids, but not to two other classes of the fluids.

“This preliminary investigation revealed weak evidence of an association between lifetime cumulative exposure to soluble metalworking fluids and breast cancer risk,” conclude Dr. Deborah Thompson of the Massachusetts Department of Public Health and her colleagues.

Their report is published in the February issue of the American Journal of Industrial Medicine.

The findings, say the researchers, warrant further study, but they also urge “cautious interpretation of the results” due to the limits of their research - including a lack of information on the women’s other risk factors for breast cancer.

Metalworking fluids have been linked to health problems before, with research suggesting they raise the risk of respiratory problems and cancers of the lung, throat, pancreas, stomach and rectum.

The fluids come in three main classes: so-called straight oils, which are refined from mineral oil; soluble fluids, which consist of oils emulsified in water; and synthetic fluids, which are water-based and contain no mineral oil.

According to Thompson’s team, there are several potentially toxic components to metalworking fluids. In the early 1980s, they note, the International Agency for Research on Cancer classified the mineral oils found in straight and soluble fluids as skin-cancer-promoting substances, due to their levels of polyaromatic hydrocarbons. Since then, these compounds have been largely removed from the fluids, though small quantities may still be present.

The current study included 4,680 women who had worked at an automobile manufacturing plant for at least three years between 1941 and 1985. They were followed through 1994 or until their deaths.

Overall, Thompson’s team found 99 cases of breast cancer among the women. In general, those with breast cancer had a history of greater exposure to straight or soluble metalworking fluids than women who did not develop the disease.

Further analysis showed that it was exposure to soluble metalworking fluids in the 10 years before diagnosis that was most clearly associated with breast cancer. The risk also appeared to be “concentrated” among women who developed the disease before the age of 51, according to the researchers. However, they add, this particular finding is based on only a small number of women.

A weakness of the study, the authors note, is that it lacked information on other breast cancer risk factors among the women.

“Additional studies of metalworking fluids and breast cancer, with data on known breast cancer risk factors, are warranted,” they write.

Identifying which components of metalworking fluids are toxic “will always be difficult,” Thompson and her colleagues point out, because the mixtures are complex and change frequently to meet companies’ needs.

It’s estimated that about 1 million U.S. workers are exposed to metalworking fluids, the compositions of which vary widely depending on the individual manufacturing plant.

SOURCE: American Journal of Industrial Medicine, February 2005.