Smoking may pose a bigger health threat to women than men, say researchers. Women who smoke have a higher risk of cancer than men, Norwegian investigators found. They looked at the medical records of 600,000 patients and discovered the bowel cancer risk linked to smoking was twice as high in women than men. Female smokers had a 19% increased risk of the disease while male smokers had a 9% increased risk, Cancer Epidemiology, Biomarkers & Prevention reports. In the study, nearly 4,000 of the participants developed bowel cancer. Women who started smoking when they were 16 or younger and those who had smoked for decades were at substantially increased risk of bowel cancer. Biologically vulnerable? The University of Tromso team who carried out the research say it is the first study to show women who smoke less than men still get more colon cancer. But they were unable to take into account other factors that might affect the risk of this type of cancer, such as alcohol and diet. The findings suggest that women may be biologically more vulnerable to the toxic effects of tobacco smoke. Experts already know that women who start smoking increase their risk of a heart attack by more than men who take up the habit, although it is not clear why. A new piece of research published in the Journal of Clinical Endocrinology & Metabolism suggests a possible explanation. A team from the University of Western Australia found teenaged girls exposed to passive smoking had lower levels of the "good" form of cholesterol that reduces heart disease risk. Second-hand smoke Second-hand smoke did not appear to have the same impact on teenage boys, however. The study looked at more than 1,000 adolescents living in Perth, Australia. Lead researcher Chi Le-Ha said: "Considering cardiovascular disease is the leading cause of death in women in the Western world, this is a serious concern." Around one in every five men and women in the UK is a smoker. Although smoking rates have been falling among both sexes, the decline has been less rapid in women. In England in 2010, more than a quarter of secondary school pupils had tried smoking at least once and 5% were regular smokers. Girls were more likely to smoke than boys - 9% of girls had smoked in the last week compared with 6% of boys. Quitting smoking cuts your risk of many diseases, including cancer. According to research in more than one million women, those who give up smoking by the age of 30 will almost completely avoid the risks of dying early from tobacco-related diseases. Sarah Williams of Cancer Research UK said: "It's well established that smoking causes at least 14 different types of cancer, including bowel cancer. "For men and women, the evidence is clear - being a non-smoker means you're less likely to develop cancer, heart disease, lung disease and many other serious illnesses." June Davison, senior cardiac nurse at the British Heart Foundation, said more research was needed to understand the effects of second-hand smoke.