"First came doctors’ warnings about cigarettes. Then came discoveries about the danger of secondhand smoke. Now, a growing number of scientists are raising the alarm about thirdhand smoke — residual chemicals left on indoor surfaces by tobacco smoke," the WashPost's William Wan reports.

Why it matters: "Public health advocates worry that those most vulnerable to the harmful effects of thirdhand smoke are also those who most likely to come into contact with it."

• What it is: "[C]hemicals from tobacco smoke often linger on clothes, surfaces and even skin. ... Mounting research has shown such potentially hazardous residue can be absorbed through the skin, ingested and inhaled months and even years after the smoke has dissipated."
• "A baby crawling on the ground ... has much more contact with carpets where cigarette residue often resides."
• "And because of increasing socioeconomic disparities in smoking, low-income families are more likely to live in homes and neighborhoods where decades of smoking have led to thirdhand smoke accumulation."