Smoking is the main lung cancer cause but only in some smokers, according to data presented at the American Association for Cancer Research 100th Annual Meeting 2009. Jian-Min Yuan, Ph.D., and M.D., associate professor of public health at the University of Minnesota, said: "A history of smoking has always been thought of as a predictor of lung cancer, but it is actually not very accurate. CigarettesSmoking absolutely increases your risk, but why it does so in some people but not in others, is a big question."

Yuan and his colleagues reported that the presence of the metabolite NNAL in a patient's urine might forecast the lung cancer risk. This metabolite has been shown to produce lung cancer in laboratory animals, but the effect in humans had not yet been studied.

That’s why for to assess levels of cigarette smoking, dietary and other lifestyle factors, researchers collected blood and urine samples from more than 50,000 patients. But for to evaluate the impact of NNAL, researchers identified 246 current smokers who later developed lung cancer and 245 smokers who did not develop lung cancer during the 10-year period following initial interview and collection of urine samples.

At the end of the investigation, researchers divided smokers with NNAL in the urine into three groups. They found that patients with a mid-range level of NNAL had a 43 percent increased risk of lung cancer, while those at the highest level had a more than two-fold increased risk of lung cancer after taking into account the effect of number of cigarettes per day, number of years of smoking, and urinary levels of cotinine on lung cancer risk.

The levels of nicotine in the urine were also calculated. And researchers found that those with the highest levels of nicotine and NNAL had an 8.5-fold increase in the risk of lung cancer compared with smokers who had the lowest levels after accounting for smoking history.

These findings may help explain why some smokers get cancer while others do not.