Passive smoking makes children more likely to get serious infectious diseases, according to a new study.
Youngsters who had been exposed to second hand smoke in the first six months of life were 45 per cent more likely to need hospital treatment for an infectious disease by the age of eight.
And one in three children who had someone smoking within three metres of them during the first few months of life ended up in hospital.
The study, published on the Tobacco Control website, looked at 7,402 children born in Hong Kong in April and May 1997.
They were followed until they were eight.
Children with a low birth weight exposed to tobacco smoke were 75 per cent more likely to need hospital treatment for ailments such as meningococcal disease and respiratory illnesses by the age of eight.
Premature babies were twice as likely.
The study's authors, from the University of Hong Kong, say passive smoking could weaken the youngsters' immune systems as well as causing respiratory problems.
Their report says: "An excess risk of severe morbidity from both respiratory and other infections for all infants exposed to second hand smoke suggests that such exposure, as well as acting via direct contact with the respiratory tract, may also affect the immune system."