Even if X-ray body scanners would prevent terrorists from smuggling explosives onto planes, nearly half of Americans still oppose using them because they could cause a few people to eventually develop cancer, according to a new Harris Interactive poll conducted online for ProPublica.

Slightly more than third of Americans supported using the scanners, while almost a fifth were unsure.

The Transportation Security Administration plans to install body scanners, which can detect explosives and other objects hidden under clothing, at nearly every airport security lane in the country by the end of 2014. It's the biggest change to airport security since metal detectors were introduced more than 35 years ago.

The scanners have long faced vocal opposition. Privacy advocates have decried them as a "virtual strip search" because the raw images show genitalia, breasts and buttocks – a concern the TSA addressed by requiring software that makes the images less graphic. But in addition to privacy objections, scientists and some lawmakers oppose one type of scanner because it uses X-rays, which damage DNA and could potentially lead to a few additional cancer cases among the 100 million travelers who fly every year. They say an alternative technology, which uses radio frequency waves, is safer.

Some travelers like Kathy Blomker, a breast cancer survivor from Madison, Wis., have decided to forgo the machines altogether and opt for a physical pat-down instead. "I've had so much radiation that I don't want to subject myself to radiation that I can avoid," she said. "I decided I'm just not ever going to go through one of those machines again. It's just too risky."

After ProPublica published an investigation, reported in conjunction with PBS NewsHour, showing that the X-ray scanners had evaded rigorous safety evaluations, the head of the TSA told Senator Susan Collins that his agency would conduct a new independent safety study. He subsequently backed off that promise, prompting the senator to write the TSA pressing the agency to go ahead with the study and asking it to post larger signs alerting pregnant women that they have the option to have a physical pat-down instead of going through the X-ray scanners.

The TSA has repeatedly touted a series of polls showing strong public support for the scanners. But those polls and surveys – conducted by Gallup, The Wall Street Journal and various travel sites – largely dealt with the privacy issue.

Only one of those polls – by CBS News – asked specifically about X-ray body scanners, finding that 81 percent of Americans thought that such X-ray scanners should be used in airports. But that poll – like all the others – did not mention the risk of cancer.

When confronted with the cancer-terrorism trade-off, however, Americans took a much more negative view of the scanners.

Harris Interactive surveyed 2,198 Americans between Dec. 2 and Dec. 6. (Full survey methodology can be found here.) The international polling firm asked, "If a security scanner existed which would significantly help in preventing terrorists from boarding a plane with powder, plastic, or liquid explosives, do you think the TSA should still use it even if it could cause perhaps six of the 100 million passengers who fly each year to eventually develop cancer"

Forty-six percent said the TSA shouldn't use it, 36 percent said it should, and 18 percent weren't sure.

Asked to comment, TSA spokesman Michael McCarthy said in a statement that the X-Ray scanners are "well within national standards."

"TSA’s top priority is the safety of the traveling public and the use of advanced imaging technology is critical to the detection of both metallic and non-metallic threats," he said. "All results from independent evaluations confirm that these machines are safe for all passengers."

The number of potential cancer cases used in the poll comes from a peer-reviewed research paper written by a radiology and epidemiology professor at the University of California, San Francisco, and posted on the TSA's website.

The professor, Rebecca Smith-Bindman, concluded that 'there is no significant threat of radiation from the scans.' But she estimated that among the 750 million security checks of 100 million airline passengers per year, six cancers could result from the X-ray scans. She cautioned that the increase was small considering that the same 100 million people would develop 40 million cancers over the course of their lifetimes.

Another study by David Brenner, director of Columbia University's Center for Radiological Research, estimated that as airlines approach a billion boardings per year in the United States, 100 additional cancers per year could result from the scanners.

The TSA uses two types of body scanners to screen travelers for nonmetallic explosives. In the X-ray machine, known as a backscatter, a passenger stands between two large blue boxes and is scanned with an extremely low level of ionizing radiation, a form of energy which strips electrons from atoms and can damage DNA, leading to cancer. In the millimeter-wave machine, a passenger stands inside a round glass booth and is scanned with low-energy electromagnetic waves which don't strip electrons from atoms and have not been linked to cancer.

There is a great deal of uncertainty when performing cancer risk assessments from the very low levels of radiation that the backscatters emit. The U.S. Food and Drug Administration put the risk of a fatal cancer from the machines at one in 400 million. The U.K. Health Protection Agency has put it at one in 166 million.

Some experts say such estimates of population risk create a distorted picture of the danger because humans are constantly exposed to background radiation and already accept risks that increase exposure, such as flying on a plane at cruising altitude.

In the authoritative study on the health risks of low levels of radiation, the National Academy of Sciences concluded that the risk of cancer increases with radiation exposure and that there is no level of radiation at which the risk is zero.

Given that risk, Brenner and some in Congress have argued that the TSA should forgo in the X-ray scanners in favor of the millimeter-wave machine.

European officials have gone so far as to prohibit the X-ray body scanners, leaving the millimeter-wave scanner as the only option. But some countries, including Germany, have reported a high rate of false alarms with the millimeter-wave machines.

The TSA has said that keeping two technologies in play creates competition, encouraging the manufacturers of both technologies to improve the detection capabilities, efficiency and cost of the scanners.