Children who are exposed to second-hand smoke are more likely to suffer tooth decay, research suggests.
A study on more than 76,000 children found that exposure to tobacco smoke at four months of age doubled the risk of suffering tooth decay by the age of three.
The research was carried out on children born between 2004 and 2010 in Kobe City, Japan.
Published in the British Medical Journal, the study said the level of dental caries in baby teeth in developed countries remains high.
Questionnaires were filled out by mothers and used to assess second-hand smoke exposure from pregnancy to three years of age alongside lifestyle factors, such as diet and frequency of teeth brushing.
Decayed teeth were defined as having at least one decayed, missing or filled tooth assessed by qualified dentists.
The results showed that compared with having no smoker in the family, exposure to tobacco smoke at four months of age was associated with around a two-fold increased risk of dental decay.
The experts from Kyoto University said: "Exposure to second-hand smoke at four months of age, which is experienced by half of all children of that age in Kobe City, Japan, is associated with an increased risk of caries in deciduous (first) teeth.
"Although these findings cannot establish causality, they support extending public health and clinical interventions to reduce second-hand smoke."
Second-hand smoke has also been linked to major health problems in babies and children, including ear infections, coughs and colds, respiratory problems such as bronchitis and pneumonia, and SIDS.
When pregnant women are exposed to the smoke it can increase the chances of miscarriage and infant death.