There is evidence of a "gateway" effect, meaning that people who start by using smokeless tobacco may move on to cigarettes, said Peter Shields, deputy director of the Ohio State University Comprehensive Cancer Center. Those who switch to smokeless tobacco as a means of quitting cigarettes, meanwhile, may have a hard time, he said.
"If someone switched entirely, it would probably be better than if they continued to smoke," Shields said. "But, actually, what happens is that people don't switch entirely."
And even if a person uses only smokeless tobacco, the link between that product and oral cancer is "essentially certain," said Jeffrey Liu, an assistant professor at Temple University School of Medicine and an attending surgeon at Fox Chase Cancer Center.
The evidence comes from several sources, including animal and human laboratory studies, as well as epidemiology - the statistical analysis of population-wide disease rates, said Stephen S. Hecht, a cancer researcher and organic chemist at the University of Minnesota Medical School.
Hecht was one of the authors of a 2008 review of epidemiologic studies in the United States and Asia, published in Lancet Oncology. The analysis found that the rate of oral cancer in people who use smokeless tobacco was 2.6 times the rate in those who do not. The rates of esophageal and pancreatic cancer among users were also elevated, but to a lesser degree.
The authors found that 4 percent of oral cancers in U.S. men could be attributed to smokeless tobacco use. Other risk factors for oral cancer include smoking, heavy drinking, and human papilloma virus.
Northern European studies have not found a strong link between smokeless tobacco and oral cancer. Shields said that might be due to much lower levels of carcinogens in the tobacco sold there, due to a different curing process.
There also is not a clear link between smokeless tobacco and cancer of the salivary gland, which claimed baseball Hall of Famer Tony Gwynn in June. Liu said that statistical case is harder to make because salivary gland cancer is rare.
Yet overall, the link between smokeless tobacco and cancer is solid enough for Major League Baseball, which in recent decades has taken a variety of steps to curb its use, or at least its use in the public eye:
Teams cannot allow tobacco companies to leave free samples in clubhouses, as once was common.
Players may not use smokeless tobacco during televised interviews and cannot carry it on their person during games.
Annual physical exams for players must include an oral exam.
Smokeless tobacco is banned in the minor leagues.
Outgoing commissioner Bud Selig has said he wants further restrictions to be considered at the bargaining table; the players' union prefers additional education. Baseball officials say one-third of major leaguers use smokeless tobacco, down from 50 percent 20 years ago, citing surveys by the Professional Baseball Athletic Trainers Society.
Close to half of the current Phillies use smokeless tobacco, according to an informal count.
The product is less common in the general public, according to phone surveys conducted for the federal Centers for Disease Control and Prevention. In Pennsylvania, 9.7 percent of men and less than 1 percent of women reported using it in 2009, the most recent year available.
In New Jersey, 3.6 percent of men and 1.1 percent of women reported using smokeless tobacco in 2009.
In both states, high schoolers are rivaling the grown-ups, according to separate surveys conducted by state health departments. In Pennsylvania, 8.5 percent of students reported using smokeless tobacco in 2010, the most recent year available. In New Jersey, the rate was 5.4 percent.
U.S. retail sales of smokeless tobacco were $6.1 billion for the last 52 weeks, up 5.5 percent from the previous year, said Ken Shea, a senior analyst with Bloomberg Intelligence, the research arm of Bloomberg L.P.
The continued use of the product by teenagers worries Shields, because of the relentless attraction of nicotine.
"The problem is really with adolescents," Shields said. "They think they're immortal and they can quit anytime. It doesn't work that way."
BY THE NUMBERS
Of oral cancer in men can be attributed to smokeless tobacco.
Other causes: Smoking,
heavy drinking, and human papilloma virus.
9.7% Of men, and less than
1 percent of women, in Pennsylvania reported using smokeless tobacco in 2009.
Of men, and 1.1 percent of women, in
New Jersey reported using smokeless tobacco in 2009.
U.S. retail sales of smokeless tobacco for the last
52 weeks, up
5.5 percent from the previous year.
Inquirer staff writer Matt Gelb contributed to this article.