Over the past ten years, Jill Nelson, 52, a health coach, personal trainer and counselor in Chicago, has received at least seven computed tomography (CT) scans and close to 30 sets of X-rays for a variety of health ailments—from two separate disk fusions in her spine to a worrisome-looking spot on her lung. That’s on top of the 10 or so mammograms she has had since age 35, plus dozens of dental X-rays. “With all that radiation, I’m surprised I don’t glow,” she says. “It makes me a little uneasy—in trying to get my health problems diagnosed, did I increase my risk of cancer?”

Jill’s concern is shared by a growing number of doctors and medical organizations, who are worried about the soaring use of medical imaging tests that rely on ionizing radiation. This radiation can damage your cells’ DNA, which may, over time, lead to cancer. The more you’re exposed to, the riskier it is. And thanks to the increase in CT scans—which typically emit far higher doses of radiation than traditional X-rays or even other imaging tests like mammograms—exposure has risen dramatically. In 1980, only about 3 million CT scans were performed in the United States. By 2013, that number had skyrocketed to 76 million.

Exactly how dangerous are all those zaps? In 2009, National Cancer Institute researchers estimated that the 72 million CT scans performed in 2007 could lead to as many as 29,000 future cases of cancer. And a couple of years ago, when the Institute of Medicine looked broadly at the environmental causes of breast cancer, it concluded that one factor that’s strongly associated with risk of developing the disease is ionizing radiation.

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That sounds scary—and it is. “Limiting exposure to medical radiation should be on every woman’s cancer-prevention list,” says Rebecca Smith-Bindman, MD, professor of radiology, epidemiology, biostatistics and health policy at the University of California, San Francisco. Yet the tests are widely overused, research finds. “About a third of CT scans are clinically unnecessary or could be avoided by using conventional X-rays or an imaging test that doesn’t use radiation, like ultrasound or MRI,” says David J. Brenner, PhD, director of the Center for Radiological Research at Columbia University Medical Center.

The challenge is figuring out whether the CT your doctor wants you to have is essential or not—a judgment call that is difficult for the average person to make. CT scans can, in fact, be lifesaving. “They’ve revolutionized medicine in almost every area you can imagine, including helping prevent unnecessary exploratory surgeries and diagnosing and treating cancers, heart disease and stroke, ” Brenner says. Price and time can also be a factor since CT scans are cheaper and faster than an MRI. (For a cost comparison of common imaging tests, go to health.com/scan-costs.)

Understanding the risks of medical radiation—as well as the real benefits—will better prepare you to make the best decision no matter when you’re faced with it. Here’s what you must know to avoid unneeded radiation.

Weighing the rewards and risks

When you receive a traditional X-ray, a small amount of radiation passes through your tissues in order to create a two-dimensional image of your insides in shades of gray. Air is black because it doesn’t absorb any X-rays, while bones are white because they absorb a lot, and organs are somewhere in between.

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CT scanners, on the other hand, rotate around the body, sending numerous X-ray beams (and multiple times the amount of radiation) from a variety of angles. A computer processes the data to create three-dimensional pictures, providing a far more detailed view. “CTs allow us to see behind and around structures in the body in three dimensions with exquisite resolution,” Brenner says. As a result, they’re an indispensable tool in diagnosing all sorts of frightening health problems, such as finding small, early cancers (particularly in the lungs, liver and kidneys) or spotting internal injuries after a serious accident.

“They can detect differences between normal and abnormal tissue about 1,000 times better than a traditional X-ray,” says Richard Morin, PhD, professor of radiologic physics at the Mayo Clinic in Jacksonville, Fla. “Before CTs, if we suspected cancer in the abdomen or internal organs, we had to cut the patient open and do exploratory surgery, which could mean a weeks-long hospital stay. Now with a single scan we can confidently make the call in minutes, and the patient walks out the door afterward. If it’s an appropriately ordered exam, the benefit is far, far greater than any radiation risk.”

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But the ease and accuracy of CTs has also fueled an alarming level of overuse. Have a headache that prompts a trip to the ER? Odds are good you’ll get a CT, even though current guidelines say that doctors shouldn’t perform imaging tests on patients with migraines or chronic headaches. Brain scans, whether a CT or an MRI, are worthwhile only if you have a headache with other worrisome symptoms, such as weakness or numbness on one side of your body, explains Brian Callaghan, MD, a neurologist at the University of Michigan. Even so, he and his colleagues recently found that about one in eight headache-related doctor visits result in a brain scan—and nearly half those patients are getting CTs, even though MRIs are more effective for peering into the brain.

“The goal isn’t to eliminate CTs but to use them more prudently,” Dr. Smith-Bindman says. “When my son did a head-dive out of a tree and was vomiting afterward, the ER doc recommended a CT scan to rule out a brain bleed, and I was happy to have the test. Five years later, when he hit his head skiing, it was pretty clear he just had a concussion, and the ER doc didn’t think a CT was necessary, so we didn’t get it. Doctors and patients need to step back a little and say, ‘Yes, this is a great test, but is it really necessary?’ If you have a CT when it’s not necessary, it won’t do any good—which means it can only do harm.”

The radiation equation

X-rays and CT scans use so-called ionizing radiation, which contains enough energy to penetrate the body—and can damage DNA in your cells. Any damage that isn’t repaired can lead to DNA mutations, and those glitches in a cell’s programming center can, over many years, lead to cancer.

And we know that it does. “There’s not a single cancer-causing agent that has been studied more thoroughly than ionizing radiation,” Dr. Smith-Bindman says. Survivors of the atomic bombs in Hiroshima and Nagasaki who were exposed to even very low doses were more likely to get practically every type of cancer, from leukemia to lung cancer. Nursing mothers who were treated with radiation for breast infections—a common practice in the 1920s and ’30s—developed breast cancer at higher rates than those who weren’t. Those of us who’ve had more sunburns (caused by the sun’s ultraviolet radiation) are at higher risk of getting skin cancer. And the newest studies reveal that children who undergo CT scans of the head, abdomen or chest are more likely to develop brain cancer and leukemia over the next 10 years.

For reasons that are unclear, women seem to be slightly more sensitive to radiation than men. Children are more vulnerable than adults; not only do their growing bodies and rapidly multiplying cells put them at a higher risk, they also have far more years ahead of them during which they could develop cancer.

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However, it’s important to note that our bodies are able to repair damage done to our cells by low levels of radiation. “If they weren’t, everyone who goes out in the sun would get skin cancer,” points out James Brink, MD, radiologist in chief at Massachusetts General Hospital.

The poison is in the dose, says John Boice, ScD, president of the National Council on Radiation Protection and Measurements and professor of medicine at Vanderbilt University. And the effects of exposure might be cumulative. “What may happen is that our bodies repair damage from small doses, but at higher doses our repair mechanisms are overwhelmed,” Dr. Brink explains. “And after that, subsequent exposure to radiation may propel the damaged cells farther down the path toward cancer.”

The actual danger to an individual receiving a scan (or even two or three) is relatively low. The overall risk of the average woman getting cancer at some point in her lifetime is about 38 percent; getting a single CT scan raises that risk to perhaps 38.001 percent, Boice explains. But since no one knows who is most likely to be affected, there is an element of radiation roulette at play.

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What’s more, we’re marinating in low-level radiation every day. The average person in the United States receives about 3 millisieverts (mSv) of radiation per year (more if you live at a high altitude) from the sun and naturally occurring radiation in the environment, such as radon gas. To put medical radiation in that context, a dental X-ray is equivalent to about one day of natural radiation, while a single chest X-ray is equivalent to about 10 days. A mammogram adds up to about seven weeks of natural radiation—but even that level, doctors say, poses a relatively small risk, especially when compared with the danger of missing a malignant tumor already growing in your breast. A regular-dose chest CT, on the other hand, exposes you to about two years’ worth of natural background radiation, or 7 mSv. Some of the Japanese survivors of atomic bombs were probably exposed to between 5 and 20 mSv on the low end. The trouble is, we don’t fully know how much our bodies can handle.

What doctors don’t know can hurt us

The issue of medical radiation is now on most physicians’ and medical societies’ radars; just this fall, the American Heart Association called for doctors to learn about, and discuss with patients, the risks of radiation exposure from cardiovascular imaging tests. So it’s surprising—and concerning—how spotty regulations still are. For instance, dosages aren’t standardized across imaging centers, which means that one hospital or clinic may be delivering up to 50 times as much radiation as another facility, according to Dr. Smith-Bindman. “If machines are set too low, they provide blurry, unusable images, but the vast majority are set higher than they need to be,” she says. This is in part because it’s not a simple matter of pressing one button and lowering the dose. “There are formulas you need to use to set up a new protocol,” Dr. Smith-Bindman explains.

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And because most older machines, many of which are still in use, don’t have warning systems to alert technologists when radiation levels are set too high, mistakes can happen. The most publicized CT-related overdoses occurred between 2008 and 2010, when several hospitals in California and one in Alabama seriously over-radiated more than 400 patients. The problem was discovered after patients reported losing their hair. Since then, new technology has been created to alert technologists if the dose is too high—and new federal legislation is in the works that would require radiology centers to adopt modern imaging equipment standards by 2016.

Still, the more common problem is that too many scans are being done in the first place, particularly in the ER, where doctors sometimes order CTs before they’ve fully evaluated a patient, Dr. Smith-Bindman says. But doctors in general have come to rely heavily on these tests. One reason: Many MDs today have a lower tolerance for ambiguity than ever before and have learned to trust images to give them definitive answers, even when other methods, including a risk-free physical exam, can provide the information needed.

In addition, physicians in private practice may feel financial pressure to recoup the cost of expensive equipment. “Research has found that if a neurologist, say, owns a CT scanner, the percentage of patients getting scans is higher than what’s typically done in a radiology clinic and much higher than in similar doctors’ practices without scanners,” Morin says. Add to the mix the possibility of being sued over a misdiagnosis, and you have a recipe for overuse.

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Making scans safer

Avoiding unnecessary medical radiation starts with speaking up and being your own best advocate (see 5 Questions to Ask Before You Have That Scan, page 117). At the same time, multiple industry efforts are under way to lower exposures from CTs. One initiative, Choosing Wisely (choosingwisely.org), helps doctors and patients understand which procedures and tests—including imaging tests—are unnecessary or commonly overused. Radiologists are leading the charge to make scans safer: Image Wisely, a program created by the American College of Radiology (ACR) and the Radiological Society of North America, is focused on optimizing the amount of radiation used in imaging studies and eliminating inessential CTs and other scans. The ACR has also created the Dose Index Registry in an effort to compare dosage information across facilities. About a third of the 3,000 or so scanning facilities in the U.S. are members, which means they get updates on the dosages other centers are using for similar tests, explains Morin, who was the founding chair of the registry. (For more on finding the best place to get a scan, go to health.com/safe-scan.)

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Meanwhile, companies that manufacture scanners are developing new technology to lower the radiation doses. “They’ve fine-tuned the equipment so you can produce high-quality images with lower doses,” Morin says. Even so, when you need to get a CT scan, it’s always a good idea to ask if they can scan using the lowest dose possible, Dr. Smith-Bindman says. If you’re smaller or thinner, technicians can often get a clear image at a lower dose. (The bigger your body, the more radiation you require, since fat absorbs some of the beams.) Avoid unnecessary radiation from even low-level sources, like dental X-rays, which you probably don’t need every year unless you have ongoing problems with tooth decay.

The idea is not to refuse all medical radiation but to do your best to discriminate between what’s essential and what’s not. “I always tell my friends to say to their doctors who recommend CTs, ‘I’m happy to have the test, but I’d like you to help me understand why I really need it,'” Dr. Smith-Bindman says. “Medicine often doesn’t change until patients start asking questions. And when it comes to medical radiation, it’s time to start asking.”

5 Questions to ask before you have that scan

“When a doctor prescribes a medication, she always talks about the risks and benefits,” says Rebecca Smith-Bindman, MD. “Now we need to start having that same kind of discussion about medical imaging.” In addition to the obvious “Why do I need this test?” ask these key questions—especially if your doc suggests a CT scan.

1. “Will the outcome of the test change the treatment I’m likely to receive?” If the answer is no, the test may not be necessary, Dr. Smith-Bindman points out.

2. “Are there alternatives without radiation, like ultrasound or MRI?” In some cases, such as many abdominal CT scans, other scans work as well or better, Dr. Smith-Bindman says.

3. If you’ve just had a scan at another facility, ask, “Is there a reason to repeat the scan I just had?” Notes John Boice, ScD: “It doesn’t make sense to do tests twice, yet it does happen.”

4. If a CT scan is crucial, ask, “Is there a way to minimize the dose?” Doctors may be able to use a lower-dose technique, particularly if you’re petite.

5. After a CT scan, ask, “How much radiation was I exposed to?” Write it down so you have a record.

You probably don’t need a CT for… Question your doc if she recommends a CT for these health problems.

Concussion: Concussions can be diagnosed by symptoms alone. But it’s valid to do a CT if the doctor suspects a skull fracture or brain bleed, says Robert Cantu, MD, clinical professor of neurosurgery at Boston University School of Medicine.

Sinus infection: This everyday health problem can generally be diagnosed through symptoms and a physical exam, says the American Academy of Allergy, Asthma & Immunology.

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Headache: If you do need a scan, MRI is the test of choice, unless a doctor suspects a stroke or brain hemorrhage, according to the American Headache Society.

Appendicitis in children: It’s best to use ultrasound first, then follow up with a CT if the ultrasound is inconclusive, according to the American College of Radiology.

Back pain: Most cases improve on their own within a month, so it doesn’t make sense to expose yourself to unnecessary radiation. If the pain continues, ask your doctor about an MRI.

How much radiation you get from…

Airport backscatter scan: .0001 mSv

Bone-density scan: 0.001 mSv

Dental posterior bite-wing X-ray series (two to four images): 0.005 to 0.055 mSv*

Two days in Denver: 0.006 mSv

Panoramic dental X-rays (standard single image): 0.009 to 0.024 mSv*

Cross-country flight: 0.04 mSv

Single chest X-ray: 0.1 mSv

Digital mammogram: 0.4 mSv

Average yearly dose from the sun and other environmental sources: 3 mSv

Chest CT: 7 mSv

Virtual (CT) colonoscopy: 10 mSv

PET/CT (often used to diagnose cancer): 25 mSv

Smoking a pack a day for a year: 53 mSv

*Dose can vary based on the type of machine used.

Your anti-radiation diet

Antioxidants from food can sop up the free radicals that cause DNA damage. And some research has hinted that what you eat may shield your body from radiation’s harmful effects. A 2009 study of airline pilots, who tend to be exposed to elevated levels of ionizing radiation, found that those with diets highest in vitamins C and E, beta-carotene, beta-cryptoxanthin (found in pumpkin, papaya and red peppers) and lutein-zeaxanthin (in leafy greens, egg yolks and squash) had fewer biomarkers of cumulative DNA damage.

Researchers in Toronto have recently shown that taking antioxidants before a scan can reduce the number of DNA breaks caused by the radiation. Published results are expected within the next six months. Says researcher Kieran Murphy, MD, professor of radiology at the University Health Network Toronto: “In light of what we’ve found, making sure you have a diet rich in antioxidant-packed fruits and vegetables could be beneficial.”

This post originally appeared on Health.com

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