Children whose parents smoked around them when they were toddlers, are more likely to have a larger waist and weigh more by the age of 10, a new study has found.

According to Canadian researchers, around 40% of children worldwide are exposed to secondhand (passive) smoke in their homes. They decided to look further into the effects of this.

Over 2,000 families were assessed for the study.

"By the age of 10, the children who had been intermittently or continuously exposed to smoke were likely to have waists that were up to three-fifths of an inch wider than their peers. And their BMI (body mass index) scores were likely to be between .48 and .81 points higher. This prospective association is almost as large as the influence of smoking while pregnant," commented Prof Linda Pagani of the University of Montreal.

She emphasised that while these increases may not seem like much, they occur during a critical period of children's development, known as the ‘adiposity rebound period'.

"This weight gain could therefore have serious long-lasting effects," she noted.

She said that the early exposure to secondhand smoke could lead to hormone imbalances, or neurodevelopment issues, which could have major consequences for the children involved.

"For example, young children have ventilation needs per kilogram of body weight that are approximately two to three times higher than adults due to their immature vital systems, resulting in more noxious effects given equal levels of household smoke exposure compared to adults," Prof Pagani said.

In other words, if a child and an adult are breathing in the same amount of cigarette smoke, the effects will be greater in the child.

"We suspect the statistics we've established linking childhood obesity to exposure to parents' smoking may underestimate the effect due to parents under-reporting the amount they smoked out of shame," she added.

Prof Pagani called for more public health initiatives to explained the dangers of secondhand smoke ‘during the critical early childhood years'.

Details of these findings are published in the journal, Nicotine and Tobacco Research.