Cannabis use by the middle-aged may increase the risk of them suffering a heart attack, scientists warned yesterday.

Researchers in Boston, Massachusetts, said the chances of having a heart attack increased nearly five times in the first hour after smoking marijuana. Most at risk are baby boomers, who are more likely to use cannabis than previous generations and are now at the age when they are most prone to heart disease.

Dr Murray Mittleman, director of cardiovascular epidemiology at the Beth Israel Deaconess Medical Centre, said: "People's risk of coronary artery disease increases as they enter their 40s and 50s, so the risks associated with smoking marijuana ­ which might have been a trivial issue when people were younger ­ may now pose a significant health concern."

The researchers in Boston studied 3,882 patients, aged 20 to 92, as part of an investigation into the causes of heart attacks. Of the 124 patients who had used marijuana during the past year, nine had taken the drug in the hour before their heart attack, according to the study published in the American Heart Association's journal Circulation.

The risk of a heart attack was, on average, 4.8 times greater in the first hour after smoking marijuana and 1.7 times during the second hour. Although the average age of cannabis users was 44, nearly a quarter were aged between 50 and 69.

Two-thirds of them used cannabis at least once a month, while four out of 10 smoked the drug weekly. When compared with non-users, they were more likely to be men, cigarette smokers and obese.

Dr Mittleman said the study would add to the debate on using cannabis for medicinal purposes, by evaluating its potential risks. The research suggested that cannabis was much less likely to trigger a heart attack than cocaine, which carried a 25-fold increased risk of a cardiac arrest.

Dr Mittleman said: "The risk associated with marijuana use is about the same as the risk we found for strenuous physical exercise, and a little higher than that observed for sexual intercourse."

Being exposed to fine- particle air pollution for less than two hours also adds to the risk of heart attacks in vulnerable people, the Boston team reported yesterday. Tiny, invisible particulates contained in traffic fumes and industrial smog are one of the most dangerous forms of pollution because they get deep into the lungs, increase the likelihood of blood clots and restrict blood flow.

The Boston researchers studied 722 heart attack victims and measured for concentrations of several air pollutants, including fine particulates, at the time of their collapse. The analysis showed that people were nearly 50 per cent more likely to have a heart attack in the two hours after exposure to high levels of fine particles.

Dr Mittleman, who led the study, said: "Our findings suggest that people who have heart disease ­ or an elevated risk of heart attack ­ would be well-served to avoid being outdoors for extensive periods of time, when the air quality is poor. This is especially the case on the hot, hazy days of summer ­ when the problem is much more prominent."

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