Smokers face an increased risk of certain types of throat and stomach cancers, even years after they quit, a new study finds.

Combining the results of 33 past studies, Italian researchers found that current smokers were more than twice as likely as nonsmokers to develop cancer, either in their esophagus or in a part of the stomach called the gastric cardia.

In some of the studies, the risk of esophagus cancer remained high even when people had quit smoking three decades earlier.

The two cancers, both known as adenocarcinomas, are relatively uncommon in Western countries. Rates elsewhere are much higher, especially in less developed countries. But in recent decades, rates of the cancers have been rising in the U.S. and Europe, possibly related to growing rates of obesity.

Smoking has long been considered a risk factor for the two cancers.

But these latest findings offer a "better quantification" of the risks, said senior researcher Dr. Eva Negri, of the "Mario Negri" Institute of Pharmacological Research in Milan.

What's more, they suggest that the risks remain higher than average for some time after smokers quit.

"Stopping smoking is highly beneficial at any age, but it appears that for these cancers the risk decreases only slowly," Negri told Reuters Health in an email.

For their study, published in the journal Epidemiology, Negri and her colleagues pooled the results of 33 previous studies. In most of them, researchers had compared a relatively small group of patients with either esophagus or gastric cardia tumors against a cancer-free group. In three studies, researchers had followed large groups of adults over time, charting any new cases of esophageal or gastric cardia cancers.

Overall, Negri's team found, current smokers had more than double the odds of developing either of the cancers, compared to people who had never smoked.

And while that risk declined after people stopped smoking, it was still 62 percent higher in former smokers than in lifelong non-smokers. In some studies, the extra risk of esophagus cancer persisted up to 30 years after people had quit.

Since both esophageal and gastric cardia adenocarcinomas are fairly uncommon in the West, the absolute risks to any one smoker may be low.

According to the American Cancer Society, the average American has a one in 200 chance of developing any type of esophageal cancer over a lifetime, and a one in 114 risk of developing some form of stomach cancer.

By comparison, the odds of developing lung cancer are about one in 13 for men, and one in 16 for women, counting both smokers and non-smokers. Smokers would be at much greater risk than lifelong non-smokers.

Lung cancer, heart disease and other ills are "numerically more important" than esophageal and gastric cardia cancers when it comes to the health consequences of smoking, Negri noted.

The types of studies that were available for her team to analyze can't prove that smoking causes adenocarcinoma of the esophagus or gastric cardia. To do that, researchers would have to purposely expose some people to years of tobacco smoke and see what happens to them over time - and ethical reasons make a study like that impossible.

Still, Negri and her colleagues say, the risks seen in the current study offer smokers one more reason to quit, and non-smokers one more reason to never start.