Roughly half of deaths from 12 smoking-related cancers may be linked directly to cigarette use, a U.S. study estimates.

While the largest proportion of deaths associated with smoking were for cancers of the lung, bronchus, trachea and larynx, about half of fatalities from tumors of the oral cavity, esophagus and bladder were also tied to cigarettes, the study found.

“The bottom line is that while we’ve made a lot of progress against the tobacco epidemic in the United States, there’s still much work to do,” lead study author Rebecca Siegel, a researcher at the American Cancer Society in Atlanta, said by email.

Siegel and colleagues estimated that out of 345,962 cancer deaths in 2011 among U.S. adults 35 and older, 167,805 were associated with smoking.

To estimate the proportion of cancer deaths attributable to smoking, the researchers used a standard formula to calculate the fraction of cases of specific cancer types that would not have occurred if there were no smoking.

Then they analyzed data from national surveys and in-person interviews asking people about their health history and tobacco habits. They adjusted their estimates to account for age, race, education level and alcohol use.

Based on smoking habits in the population and the proportion of cancer cases attributable to smoking, the researchers estimated that 125,799 lung, bronchus and trachea cancer deaths, representing 80 percent of the total, were linked to smoking. So were 50 percent of deaths from esophagus tumors and 45 percent of deaths from bladder malignancies.

The researchers also credit smoking with 17 percent of kidney cancer deaths, 20 percent of stomach cancer deaths, 22 percent of cervical cancer deaths and 24 percent of liver and bile duct cancer deaths in 2011.

One limitation of the study is that the survey and interview participants were generally more educated and less racially diverse than the U.S. population as a whole, the researchers acknowledge in JAMA Internal Medicine.

It’s also possible, however, that they underestimated cancer deaths tied to smoking because they didn’t have data on second-hand smoke exposure, which may cause an additional 5 percent of lung cancer deaths, or on use of cigars, pipes, or smokeless tobacco.

“While smoking prevalence continues to slowly decline, the use of alternative tobacco products is on the rise,” Siegel said.

Use of combustible forms of tobacco other than cigarettes, such as cigars and hookahs, doubled from the equivalent of 15.2 billion cigarettes in 2000 to the equivalent of 33.8 billion cigarettes in 2011, Siegel said.

“Although we can’t know exactly how many people are not starting to smoke cigarettes because they are using other tobacco products, e-cigarettes are now the most common form of tobacco use among high school students,” Siegel said.

Evidence doesn’t suggest that people who use alternative tobacco products are more likely to quit smoking or avoid starting on cigarettes, Dr. Michael Ong, author of a tobacco cessation editorial accompanying the study, said by email.

If there’s an upside to alternatives, though, it may be seen in the declining proportion of lung cancer deaths attributable to cigarettes, said Ong, a researcher at the University of California, Los Angeles and the Veterans Affairs Greater Los Angeles HealthCare System.

“But most of all lung cancer deaths are still attributable to smoking, and lung cancer makes up the largest cause of cancer-related mortality,” he said.

In an indication of the uphill battle smoking cessation can be, another study also published in JAMA Internal Medicine found that clinicians simply asking patients to quit and advising them of the lung cancer risks isn’t enough motivation to make it more likely that patients successfully quit.

Researchers followed more than 3,000 smokers for one year after lung cancer screenings to see if the amount of cessation support they received from clinicians might influence their odds of quitting.

The smokers who received assistance such as referrals to counseling or prescriptions for smoking-cessation drugs were 40 percent more likely to quit, while those who received follow-up care to monitor their progress were 46 percent more likely to stop smoking.

“Smokers face physical, environmental and social barriers to quitting,” lead study author Elyse Park, a researcher at Massachusetts General Hospital in Boston, said by email. “Primary care providers can assist smokers, particularly smokers with a heavy smoking history, to boost their confidence and obtain the counseling and medication support that can help them improve their odds of successful quitting.”