A ban on smoking in some public places may be bad for children because more adults will smoke at home in instead.
A University College London (UCL) study found smokers lit up more at home if they couldn't in pubs and restaurants.
The study of data from the US showed that the problem of passive smoking at home is worse for poorer families.
Pro-smoking groups say the study proves bans don't achieve their aims, while anti-smoking groups said other studies showed bans reduced home smoking too.
The US data was a health study of 30,000 people of all ages, across a number of different states in the US, all with different laws on where it was okay to smoke.
It found that in states with a total smoking ban children aged four to eight showed more signs of breathing in other people's smoke.
There was also an increase for people aged eight to 20, but it wasn't as large, perhaps suggesting they spent less time with their parents.
The researchers suggest that the study proves a partial ban on smoking, like the one planned for England and Wales, is better than a complete ban, like the one the Scottish Parliament prefer and that Northern Ireland will get.