New York, Oct 5 (IANS): Most parents know or suspect when their kids smoke but they are much more likely to be in the dark if the child uses electronic-cigarettes, commonly known as e-cigarettes, says a study.

Parents or guardians were substantially less likely to report knowing or suspecting that their child had used tobacco if the child used only e-cigarettes, or smokeless tobacco, compared to smoking cigarettes or using multiple tobacco products, showed the findings published in the journal Pediatrics.

The researchers also found that when parents set strong household rules about not using tobacco -- applying to all residents -- their children were less likely to start tobacco use.

Just talking to kids about not smoking was far less effective.

"We know that tobacco-free homes are a key tool to help prevent smoking by kids," said corresponding and senior author Benjamin Chaffee, Associate Professor at the University of California San Francisco's school of dentistry.

"What studies haven't examined is how tobacco-free homes stack up against other approaches and how much tobacco-free home rules might help with other tobacco products beyond smoking."

Teenagers living in homes with the strictest rules prohibiting tobacco use were 20-26 per cent less likely to start using tobacco, compared to youth living in the most permissive homes, said the study which tracked more than 23,000 participants in the US aged 12 to 17 years old.

In addition to cigarettes and e-cigarettes, the study looked at non-cigarette combustible products (including cigars, pipes, hookahs, and bidis), and smokeless tobacco (including snuff, chewing tobacco, snus, and dissolvable tobacco).

It found that parents were more likely to know or suspect that their child was using a tobacco or nicotine product if the child was older, male, identified as white, and lived with a tobacco user, as well as if the parents were less educated.

Mothers were singled out as more aware than fathers.