Which is more likely to give you lung cancer, inhaling polluted city air every day, or smoking a pack of cigarettes a day?

According to a telephone survey taken by American Cancer Society researchers, most of us either think pollution is more likely to cause lung cancer than smoking -- or don't know which is worse. Unfortunately, either answer is completely at odds with decades of scientific evidence that cigarette smoking is the number one cause of lung cancer by far.

And that's just one of the things we are confused about when it comes to cancer risk factors, the researchers say. Many people hold mistaken beliefs about what will and won't cause cancer, the survey results suggest. One common scenario is for people to give a lot of weight to unproven theories or to risk factors that make a relatively small contribution to cancer, but at the same time to ignore or minimize the cancer risk posed by other factors that really do have an impact.

The findings, published in the ACS journal Cancer, are based on a survey conducted in 2002 of nearly 1,000 US adults who had never had cancer. Participants were asked to respond to 12 statements about cancer with true, not true, or don't know. Discovery Health Channel and Prevention magazine helped design the survey.

"There are some gaps in the public's understanding of cancer risk factors," says study co-author Ted Gansler, MD, director of medical content at ACS. "The important thing is for people to know which things really make a difference for cancer."

The issue of smoking, pollution, and lung cancer is one example. Nearly 40% of people in the survey said pollution is a greater risk factor for lung cancer than smoking, and another 19% weren't sure.

Although pollution does contribute to lung cancer (as well as other problems like asthma, bronchitis, and heart disease), smoking trumps it -- by a long shot. Smoking is responsible for about 87% of lung cancer deaths.

Quitting smoking -- or never starting in the first place -- is the most important thing you can do to lower your risk of lung cancer. Yet more than 15% of those surveyed did not believe that long-time smokers can reduce their cancer risk by quitting, and another 6% weren't sure.

Another statement that raised a red flag with researchers: What someone does as a young adult has little effect on their chance of getting cancer later in life. Nearly 25% said that was true, and another 7% weren't sure.

In reality, says Gansler, many things people do in their youth can have a profound impact on their future cancer risk.

For instance, young people may start smoking, thinking they can quit at any time, but quitting is actually very difficult. Furthermore, the younger people are when they start smoking, the more likely they are to smoke as adults, raising their risk for lung cancer and other cancers. Likewise, young people tend to be less concerned about sun exposure even though getting sunburned is known to raise the risk of developing skin cancer later in life. Young people may also engage in risky sex behaviors that boost their chances of catching or spreading HPV (human papillomavirus), the virus responsible for most cases of cervical cancer.

Certain groups of people were more likely than others to hold misconceptions about cancer risk factors: those who were male, older, non-white, less educated, and of lower income.

In one curious finding, the survey participants who described themselves as knowing the most about cancer were more likely to have some inaccurate beliefs. For instance, people who rated themselves as "very informed" about cancer were more likely than those who were "somewhat informed" to believe that underwire bras cause breast cancer, a largely discredited theory based on "extremely weak evidence," says Gansler. Similarly, the "very informed" respondents were more likely to believe that smokers cannot reduce their risk of cancer by quitting.

The take-home message: Make sure you can trust your sources of cancer information, says Gansler.

"There's a substantial number of people who are very concerned about cancer but are not paying attention to the right issues," Gansler says. "And they are not getting their information from the right sources."

Trustworthy sources of cancer information include not-for-profit organizations like the American Cancer Society, and federal agencies like the National Cancer Institute and the US Centers for Disease Control and Prevention, Gansler says. Other sources you can count on to be reliable include the nation's leading cancer hospitals and treatment centers, he says.

And finally, he says, be very skeptical of any cancer information that is forwarded to you in email or that arrives unsolicited. Another red flag: Emails that mention a commercial product, particularly if they contain claims that their product is better somehow than the competition.

Citation: "Prevalence and Sociodemographic Correlates of Beliefs Regarding Cancer Risks." Published online July 26, 2007, in Cancer. First author: Kevin Stein, PhD, American Cancer Society.