Researchers at University College London and St George's, University of London measured recent exposure to tobacco smoke in non-smoking middle-aged men taking part in the British Regional Heart Study by measuring the levels of cotinine - a compound carried in the blood - at two time points 20 years apart.
A blood cotinine level above 0.7ng/mL is associated with a 40% increase in the risk of a heart attack, and other studies have suggested that even a level of 0.2ng/mL may increase the risk. The researchers found that while in 1978-80, 73% of men had a cotinine level above 0.7ng/mL, by 1998-2000 that proportion had fallen to 17%.
However, despite the number of non-smoking men at risk having fallen, half of those who still had a high cotinine level (above 0.7 ng/ml) in 1998-2000 lived with a partner who smoked. Non-smoking men who had a partner who smoked had average cotinine levels of 1.39ng/mL, almost twice the level associated with an increased risk of a heart attack. Their cotinine levels were nearly eight times higher than the cotinine levels of men whose partner did not smoke.
During the period the study looked at, national data shows that the prevalence of smoking amongst adults across the UK declined from 40% to 27% and the number of cigarettes consumed by smokers fell from 114 to 97 per week. Restrictions on smoking in public spaces and workplaces were also introduced, although the study period was before the national legislative bans on smoking in public places introduced between 2006 and 2007.
Dr Barbara Jefferis, from University College London who led the research,
said: "The decline in smoking together with restrictions on smoking in public places has created an environment where people are exposed to far less tobacco smoke. This has resulted in the dramatic fall in the number of non-smokers at an increased risk of a heart attack.
"However, we can clearly see that living with someone who smokes puts you at a heightened risk. If we are going to reduce people's exposure to tobacco smoke further then we will need to focus efforts on reducing smoking in the home."
Professor Peter Weissberg, Medical Director at the BHF, said: "This research shows that a great deal of progress has been made in reducing exposure to potentially damaging environmental tobacco smoke over the past 20 years.
Importantly, it also shows that people are now more at risk of exposure in their own homes than in public places.
"We cannot stop people smoking in their own home, but we would urge smokers to think of the risk they're exposing their non smoking friends and relatives to when they have a cigarette in the house."
"The BHF are calling for a proper plan to reduce the harm from smoking including measures in the NHS Bill that will put an end to point of sale displays and prohibit cigarette vending machines, which are disproportionately used by underage smokers."
Explore further: Studies may have overestimated effect that smoking bans play in reducing hospitalizations