CHICAGO (Reuters) - Scientists have detected two substances in tobacco smoke that directly cause lung cancer, and they said on Sunday the finding may help one day predict which smokers will develop the disease.

They said people with high concentrations in their urine of a nicotine byproduct called NNAL had double the risk of developing lung cancer compared to smokers with lower NNAL concentrations in their urine.

And smokers who had high urine levels of both NNAL and another nicotine byproduct called cotinine had more than eight times the risk of lung cancer compared to smokers with the lowest concentrations of these two compounds.

The findings may help explain why some smokers get cancer while others do not, they said.

"Smoking leads to lung cancer, but there are about 60 possible carcinogens in tobacco smoke, and the more accurately we can identify the culprit, the better we will become at predicting risk," said Jian-Min Yuan of the University of Minnesota, who presented the findings at the American Association for Cancer Research in Denver.

Only about one in 10 smokers gets lung cancer.

Studies have found that laboratory animals with high concentrations of NNAL had higher rates of lung cancer, but its effect in humans has not been clear.

The researchers collected data from two large Chinese studies of 50,000 men and women aged 45 to 74. In addition to asking them how much they smoked, what they ate and other lifestyle factors, the researchers collected blood and urine samples.

Yuan and colleagues identified 246 smokers who later developed lung cancer and 245 smokers who did not during the 10-year period following their initial interview and exam.

The team, which included researchers at the National University of Singapore and the Shanghai Cancer Institute, divided NNAL urine levels into low, medium and high categories.

After accounting for number of cigarettes smoked per day, they found people who had medium levels of NNAL had a 43 percent higher risk of lung cancer than those in the lowest levels. And those with the highest levels had double the risk.

People with the highest levels of both cotinine and NNAL had an 8.5-fold higher risk compared with smokers who had the lowest levels of both.

Yuan said testing for NNAL and cotinine in urine may serve as a starting point for a new way of predicting lung cancer risk. "Our goal in the next three to five years is to amass this information so that it can be used as a screening test to alert smokers to their risks," he said.

Lung cancer kills 1.2 million people a year and is the top cause of cancer death globally.