Declines in death rates from most major causes - including heart disease and cancer - have pushed Americans' life expectancy to a record 77.6 years.

And people with a slow metabolism who become fat are also unlikely to benefit because their obesity puts them at greater risk of dying from heart disease, stroke and cancer.

Benzene, a chemical compound refined from crude oil, was found to cause leukemia in 1928 by two Italian doctors who detected the cancer in a worker exposed to benzene for five years.