The first international study into the effects of passive smoking has found it causes 600,000 deaths every year.
The study by the World Health Organisation analysed data from nearly every nation.
It found that passive smoking, which causes heart disease, respiratory illness and lung cancer, is particularly dangerous for children who are exposed to it in their homes.
The WHO admits that legislating about what happens in the home is difficult.
But it says urgent action is needed to encourage parents to protect their children from second-hand smoke.