Narrator:       This is Science Today. Obesity is now thought to be the second largest cause of cancer that's preventable, next to smoking. Michael Jerrett, an associate professor of environmental health sciences at the University of California, Berkeley helped conduct a study looking into how factors in the built environment, like traffic, affect children's activity levels and their body mass index, or BMI.

Jerrett:           Our study looks at the impact of traffic density in proximity to children's homes and we control for a wide range of individual and build environment factors that are thought to influence the progression of BMI.

Narrator:       Children who live in neighborhoods that have a lot of traffic around their homes tend to have higher BMIs by the time they are 18 than those who live in lower traffic areas.

Jerrett:           Traffic in and of itself exerts an effect that's significant and the effect size is about five to 10 percent. That might not sound like a lot, but given that this is a global problem and that traffic is such a pervasive exposure, we could have a very big burden of illness.

Narrator:       For Science Today, I'm Larissa Branin.