It took decades to prove that cigarette smoking causes cancer, heart disease, and early death. It took additional years to establish that secondhand smoke also kills.

Now scientists are worried about another cigarette-related phenomenon: thirdhand smoke. It's real, and it's ubiquitous. Without knowing it, indoor smokers have left a toxic legacy that continues years after their last butt was stubbed out.

Researchers now know that residual tobacco smoke, dubbed thirdhand smoke, combines with indoor pollutants such as ozone and nitrous acid to create new compounds. Thirdhand smoke mixes and settles with dust, drifts down to carpeting and furniture surfaces, and makes its way deep into the porous material in paneling and drywall. It lingers in the hair, skin, clothing, and fingernails of