Children living in flats have 45% more exposure to tobacco smoke than those in detached houses, a US study says.
Researchers from Harvard and Rochester Universities say that is because the smoke seeps through walls and shared ventilation systems.
They tested cotinine levels in blood samples from 5,000 children across the US for the study in Pediatrics.
Action on Smoking and Health (ASH) said there was a "strong case" for making blocks of flats smoke free.
Researchers limited the sample in this study to children who live in a household where nobody smokes.
They looked for cotinine - a product of nicotine and a highly sensitive marker for tobacco - in the children's blood.
The study found that 73% of the 5,000 children analysed were exposed to second-hand tobacco smoke.
Overall, researchers found that 84.5% of children who were living in blocks of flats had a cotinine level that indicated recent tobacco-smoke exposure, compared with 79.6% of children who were living in attached houses and 70.3% who were living in detached houses.
Dr Jonathan Winickoff, study author and associate professor of paediatrics at Harvard Medical School, said: "If your neighbours are smoking then you are exposed if you live through the wall in a semi-detached house. In apartment buildings this effect is magnified. Smoke contaminates the whole building."
"This study is the last link in the chain of evidence. It demonstrates the overwhelming need for smoke-free buildings," he said.
He continued: "In years to come, people will shake their heads in disbelief that there was ever smoking in homes where children live, eat, sleep and breathe."
Previous research has shown that passive smoking is a major cause of morbidity and mortality, even at low levels of exposure.
Karen Wilson, assistant professor of paediatrics at the University of Rochester Medical Centre, said:
"Parents try so hard to protect their children from dangers, such as tobacco smoke. It's surprising to see these results and realise that too many parents have no control over whether their children are exposed to second-hand smoke in their own homes."
Martin Dockrell, director of policy and research at the UK group ASH, said: "There is a strong case for social and private sector landlords designating some entire blocks as smoke-free to respect the choice and the health of the great majority of their tenants."