Children who are exposed to tobacco smoke at home are prone to antisocial behaviour, aggression and are more likely to drop out of school, a new study suggests.

The study by University of Montreal in Canada shows that the more children are exposed to household tobacco smoke in early childhood, the greater their risk of adopting antisocial behaviour towards others, engaging in proactive and reactive aggression, having conduct problems at school, and dropping out at age 12. Animal studies have suggested that exposure to tobacco smoke is toxic to the developing brain at a time when it is most vulnerable to environment input.

Abnormal brain development can result from chronic or transient exposure to toxic chemicals and gases in second hand tobacco smoke. These compounds eventually solidify and create third hand smoke. Antisocial behaviour is characterised by proactive intent to harm others, lack prosocial feelings, and violate social norms.

Such behaviours include aggression, criminal offences, theft, refusal to comply with authority, and destruction of property. In later childhood, antisocial behaviour is often associated with academic problems, as highlighted in the study. Deviance and dropout risk are costly to society as a whole.

"Young children have little control over their exposure to household tobacco smoke, which is considered toxic to the brain at a time when its development is exponential," said Linda Pagani, professor at the University of Montreal."The detection of early environmental factors that influence later child well-being represents an important target for individual and community health," Pagani said.

"Parents who smoke near their children and play often inadvertently expose them to second and third hand smoke. It was already known that environmental smoke places children at risk of short- and long-term health problems," she added."However, now for the first time, we have compelling evidence which suggests other dangers to developing brain systems that govern behavioural decisions, social and emotional life, and cognitive functioning," said Pagani.

Researchers examined data from a longitudinal birth cohort of Quebec boys and girls born in 1997 and 1998. The Quebec Longitudinal Study of Child Development is a public database administered and coordinated by the Institutde la statistique du Quebec in the US. Every year, parents of 1,035 children from the longitudinal study reported whether anyone smoked at home when their children were aged 1.5 to 7.5 years.

At age 12, their children self-reported their antisocial behaviour and academic characteristics. Overall, 60 per cent of families reported never being exposed to tobacco smoke, while 27 per cent reported intermittent exposure, and 13 percent reported chronic exposure. The study appears in the journal Indoor Air.