SIRC spokesman Joe Walker believes that a cancer listing could have a devastating effect, particularly in the composites industry, where it has the potential to create fear among workers and the communities where composites manufacturing facilities using styrene resins are located. It also has the potential to have profound effects on other industries, in particular on the use of expanded polystyrene food service products, which have been used for 50 years without a single adverse health effect attributed to them, Walker observes.

He relates that the question concerning styrene’s carcinogenicity has been researched in studies using rats and mice and epidemiology studies on people working in styrene-related industries since SIRC was founded. “The bottomline of all of that work is that styrene is not a cause of human cancer,” he adds, noting that the evolving research continues to point away from a human cancer concern for styrene.

During the 1990s, researchers using laboratory rats and mice in styrene inhalation studies found lung tumors in exposed mice, but none in rats that had been exposed to much higher levels, recounts George Cruzan, a toxicologist working with SIRC. Scientists over the ensuing years continued to study styrene’s cancer-causing potential.

In late 2009, the peer-reviewed Journal of Occupational and Environmental Medicine published a SIRC-commissioned review by a panel of internationally recognized scientists who found that the available epidemiologic (human) evidence does not support a causal relationship between styrene exposure and any type of human cancer.

At the recent Society of Toxicology meeting, Cruzan and other toxicologists presented a poster based on the latest research with genetically modified mice. “This work—relying on techniques that won the Nobel Prize in physiology and medicine in 2007—showed no toxicity from styrene whatsoever,” reports Cruzan.

“Overall, eight cancer studies have been done with rats and mice exposed to styrene. In mice, three of five studies have shown either increased lung tumors or the suggestion of increased lung tumors, but none in rats,” he continues. “So we were left with the questions: Is there something about mice that’s causing these tumors, and are humans more like rats or more like mice?”

That work led researchers to identify an enzyme, CYP2F2, prevalent in mouse lungs but not found at the same levels in rat lungs, and even less so in human lungs. Experiments were conducted to examine the metabolism of styrene by that enzyme, finding that styrene is metabolized in the mouse lung by the enzyme.