Lingering residue from tobacco smoke which clings to upholstery, clothing and the skin releases cancer-causing agents, work in PNAS journal shows.
Berkeley scientists in the US ran lab tests and found "substantial levels" of toxins on smoke-exposed material.
They say while banishing smokers to outdoors cuts second-hand smoke, residues will follow them back inside and this "third-hand smoke" may harm.
Opponents called it a laughable term designed to frighten people unduly.
The scientists say nicotine stains on clothing, furniture and wallpaper can react with a common indoor pollutant to generate dangerous chemicals called tobacco-specific nitrosamines or TSNAs.
In the tests, contaminated surface exposed to "high but reasonable" amounts of the pollutant nitrous acid - emitted by unvented gas appliances and in car exhaust - boosted levels of newly formed TSNAs 10-fold.
Substantial traces of TSNAs were also found on the inside surfaces of a truck belonging to a heavy smoker.
The researchers say third-hand smoke is an unappreciated health hazard and suggest a complete ban on smoking in homes and in vehicles to eliminate any risk.
Toxic particles from cigarette smoke can linger on surfaces long after the cigarette has been put out, and small children are particularly susceptible because they are likely to breathe in close proximity, or even lick and suck them, they say.
Researcher Lara Gundel, of the Lawrence Berkeley National Laboratory, said: "Smoking outside is better than smoking indoors but nicotine residues will stick to a smoker's skin and clothing.
"Those residues follow a smoker back inside and get spread everywhere. The biggest risk is to young children.
"Dermal uptake of the nicotine through a child's skin is likely to occur when the smoker returns and if nitrous acid is in the air, which it usually is, then TSNAs will be formed."