BEIJING, Feb. 8 -- People whose spouses smoke at home are more vulnerable to coronary heart disease, a Chinese University of Hong Kong (CUHK) study has found.

    The study should be a warning to the 350 million mainland and 840,000 Hong Kong smokers. Women are more exposed to the risk of second-hand smoke because a much higher percentage of men smoke, both on the mainland and in Hong Kong.

    In fact, 55 percent females aged 15 or above on the mainland are exposed to second-hand smoke.

    The CUHK study, conducted between 2004 and 2006, comprised 507 subjects, including 239 woman patients with coronary heart disease, whose husbands or partners smoked about 20 cigarettes a day, and 268 disease-free people.

    Results of a questionnaire given to the 507 people to collect data on their exposure to passive smoking showed 34 percent of the patients and 25 percent of disease-free women had been exposed to passive smoking at home. Which means passive smoking increases the risk of coronary heart disease more than one-fold.

    The risk increased nearly 3.6 times if the spouses smoked for 10 or more years, and 3.9 times if they smoked more than 20 cigarettes a day.

    CUHK Cardiology Division Head Yu Cheuk-man says the risk is proportional to the intensity and duration of exposure to passive smoking at home.

    "Most houses are a sealed place. Since land prices in Hong Kong are very high in Hong Kong, the majority of the people are compelled to live in small apartments. So whenever a member of the family smokes, the apartment becomes like a smoking room with poor ventilation, harming others in the house as well," Yu says.

    Coronary disease is the cause of more than half of the 5,000 heart attack-related deaths in Hong Kong every year. Smoking-related illnesses caused 1 million deaths, or 12 percent of total number, on the mainland in 2000. The proportion is feared to rise to 33 percent in 2020.

    Yu has warned that the problem could worsen after the special administrative region government banned smoking in public places such as parks, computer game centers and restaurants from January.