What’s your single biggest risk for cancer?

Most Americans get the answer wrong. If you’re a woman, obesity is the single biggest cause of cancer, and if you’re a man, it’s the second leading risk.

The American Society of Clinical Oncology just surveyed 4,000 Americans and found a troubling disconnect between what people think about cancer and what’s true.

Just 30 percent of those polled knew that obesity is a major risk factor for cancer, ASCO found.

“Of the four most common cancers — breast, lung, colon, and prostate — three of them have a pretty clear association with obesity,” said Dr. Clifford Hudis, CEO of ASCO.

Just 20 percent of those polled knew viruses can cause cancer — the human papillomavirus or HPV causes almost all cases of cervical cancer and a growing number of cases of oral cancer. And only 30 percent knew alcohol use is a risk factor.

Obesity is a major cause of cancer CDC / Center for Disease Control and Prevention

Most people — 78 percent of them — knew that tobacco use is the single biggest cause of cancer. And 66 percent knew sun exposure can cause cancer.

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Cancer is neck and neck with heart disease as the No. 1 killer in the United States. The American Cancer Society projects that nearly 1.7 million people will be diagnosed with cancer in 2017 and 600,000 will die of it.

How to cut risk

Most Americans don’t realize that cancer can be prevented. Research shows that as many as 40 percent of cancer cases, and half of cancer deaths, come down to things people could easily change, including smoking, drinking too much, being overweight and not exercising enough.

Earlier this month, the Centers for Disease Control and Prevention found that almost 40 percent of American adults and nearly 20 percent of adolescents are obese — the highest rates ever recorded for the U.S. More than 70 percent of Americans are either overweight or obese, meaning that an unhealthy weight has become the norm, with healthy-weight Americans — who have a BMI of less than 25 — now in the minority.

People worry that cellphones might give them cancer, but there's no good evidence for that whatsoever. Meanwhile, fewer than half of Americans use sunblock to protect their skin from the sun, and possible skin cancer. Shutterstock

And the CDC says people who are overweight or obese have higher risks of 13 types of cancer, which account for about 40 percent of all cancers diagnosed in the United States in 2014.

“55 percent of all cancers diagnosed in women and 24 percent of those diagnosed in men are associated with overweight and obesity,” the CDC says.

What we get wrong

Yet people are often worried about the wrong things, the ASCO survey found.

“For example, people worry about cellphone usage. And that's a surprisingly persistent belief, notwithstanding the lack of clear evidence one way or the other,” Hudis said.

“There's really no good evidence that it causes cancer and there are a number of observational studies now that suggest no special risk,” he added.

“In addition to that, and this one maybe surprised me the most, is that there's a number of people who think caffeine's a risk factor for cancer and there's really no good evidence for that whatsoever.

And 53 percent blamed pollutants for cancer, although pollution is not a major contributor to cancer cases.

So it may come as no great surprise that the survey found most Americans are missing out on chances to prevent cancer.

“Only 48 percent say they use sunblock or limit their exposure to the sun; 41 percent say they maintain a healthy weight; and 38 percent say they limit alcohol consumption in order to prevent cancer,” ASCO said in a statement.

The statistics on obesity show at least some people are fooling themselves if they think they’re keeping a healthy weight. And 21 percent said they limited caffeine intake to reduce cancer risk, a finding that indicates people’s understanding of risk factors is fuzzy at best.