There's no doubt about the harmful effects of smoking, nor of being exposed to secondhand smoke. But did you know that "third-hand" smoke may also pose a threat, particularly to unborn babies and young children?

Thirdhand smoke refers to the residue of secondhand smoke that lingers on surfaces such as clothing, carpeting, furniture or inside cars. Because it's composed of smaller, ultrafine particles with a greater molecular weight, it poses a greater asthma hazard than firsthand or secondhand smoke, say experts.

Now, even if a smoker steps outside to have a cigarette, nicotine residues stick to his skin and clothing and those residues follow him back inside, which can persist on surfaces for days, weeks and even months.

These residual chemicals can have a serious impact on an unborn child's lung development, according to a new study at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center. The exposure to the constituents of thirdhand smoke is as damaging and, in some cases, more damaging than secondhand smoke or firsthand smoke, says Virender Rehan, M.D., the study's corresponding author. "We can conclude that prenatal disruption of lung development can lead to asthma and other respiratory ailments that can last a lifetime."

"Pregnant women should avoid homes and other places where thirdhand smoke is likely to be found to protect their unborn children against the potential damage these toxins can cause to the developing infants' lungs," advises Rehan.

Young children are especially susceptible, too. That's because exposure to toxic thirdhand smoke comes via touching surfaces contaminated with thirdhand smoke, as well as ingesting contaminated dust. "Among infants, it has been found that the rate of ingesting dust is more than twice that of adults, making babies especially vulnerable to the effects of thirdhand smoke," says Rehan.