A University of Otago study has shown that exposure to second-hand smoke doubles rates of asthma and wheezing in infants.
Researchers analysed nicotine levels in infants' hair.
They found those with high levels were twice as likely to have asthma or reoccurring problems.
Researchers say the purpose of the study is not to blame parents who smoke- as they often struggle with stress and addiction.
Instead, the study hopes to inform parents of the risk to their children's health.