Passive smoking may be much more dangerous than had been thought, researchers said on Wednesday in new study that is likely to boost demand for a ban on smoking in public places.

Scientists in Britain studied exposure to passive smoke by measuring cotinine, a breakdown product of tobacco smoke, in the blood of non-smokers.

“People who were non-smokers but had relatively high levels of cotinine had a heart disease risk of about 50 percent higher than those people who were exposed to low levels,” Professor Peter Whincup, of St. George’s Hospital Medical School in London, said in an interview.

Whincup, who reported the findings in the British Medical Journal Online First edition released Wednesday, said the research provides further evidence that passive smoking has adverse effects that may have been underestimated in the past.

Previous studies had estimated the increased risk of heart disease from passive smoking in non-smokers at 25 to 30 percent. Earlier research into the effects of second-hand smoke had focused on non-smokers living with smokers.

Supporters of a ban on smoking in the workplace, bars and restaurants described the findings as further evidence for new smoking legislation.

“The need for a ban on smoking in public places in the UK has never been better illustrated than by this potentially pivotal study. We have known for some time that passive smoking was strongly associated with increased risk of coronary heart disease (CHD), but this study strengthens the evidence considerably,” Dr Tim Bowker, of the British Heart Foundation, which partly funded the research, said in a statement.

“The evidence is now compelling. The government should not delay any further in introducing legislation to protect non-smokers from this unnecessary risk,” he added.

Ireland recently became the first country to introduce a national ban on smoking in public places. New York and parts of Australia have taken similar measures.