Tobacco smoke is hazardous because it contains a number of unhealthy chemicals, including some 60 carcinogens (cancer-causing agents), tar, arsenic, carbon monoxide and nicotine.

Because rice accumulates arsenic, he says, even the background level "is a problem for people who eat much rice in their diet."

Although there's "a definite need to reduce arsenic levels in rice, " says Richard Loeppert, a crop scientist at Texas A&M University in College Station, "it's not an immediate hazard."