In November 2007, two radiation scientists, David Brenner and Eric Hall, wrote a review in the New England Journal of Medicine [1]. They estimated that more than 62 million CT scans were done each year in the United States. In 1980, the number was about 3 million.
A CT (computed tomography) scan takes X-ray pictures from many angles and builds them into 3D views of the body. Brenner and Hall warned that CT gives much larger doses than regular X-rays. They also wrote, plainly, that “the risks for any one person are not large.”
People still ask how much radiation a scan really gives. The honest answer is a range, not one number. I think most people need a rough sense of scale, a frank word about uncertainty, and good questions.
What does a millisievert measure?
When X-rays pass through your body, some of the energy is absorbed. That absorbed energy is what counts toward your radiation dose. RadiologyInfo.org, a patient website from radiology societies, explains the usual unit [2]. It is the millisievert (mSv), which measures effective dose.
Effective dose also weighs how sensitive each tissue is. If an exam “includes tissues or organs that are more sensitive to radiation,” the guide says, the effective dose will be higher. This puts very different exposures on one scale, so a scan can be compared with natural radiation.
The scale is useful but rough. Brenner and Hall noted that it “provides only an approximate estimate of the true risk” [1]. For risk, they preferred the dose to each organ.
How does a CT scan compare with an X-ray?
The guide says the average American gets about 3 mSv per year from natural sources [2]. The largest part, about 2 mSv, comes from radon gas in homes.
One adult chest X-ray is about 0.1 mSv, or about 10 days of natural radiation. CT doses are larger. Here are the guide’s typical values for an average-sized adult.
A CT of the brain is about 1.6 mSv, similar to 7 months of natural radiation. A low-dose CT for lung cancer screening is about 1.5 mSv, or about 6 months. A CT of the chest is about 6.1 mSv, similar to 2 years. A CT of the abdomen and pelvis is about 7.7 mSv, or about 2.6 years.
By simple division (6.1 divided by 0.1), a typical chest CT gives about as much radiation as 60 chest X-rays. A 2008 dose catalog by Mettler and colleagues lists average CT doses of about 2 to 20 mSv [3].
Why is there no single CT dose?
Those numbers are typical values, not promises. The guide says the actual dose “can vary substantially, depending on a person’s size, the reason for imaging, and differences in imaging practices” [2].
An abdomen and pelvis CT done twice, with and without contrast dye, is about 15.4 mSv. That is double the single scan. Brenner and Hall listed other factors [1]. They include the machine settings, the length of body scanned and the scanner’s design.
The imaging team controls many of these settings. Matching them to each patient’s size, they wrote, was “increasing but is by no means universal.” There is also a trade-off. Lower doses make grainier images, so the lowest dose is not always best.
Imagine two coworkers who each had a CT last month. One had a quick scan of the head. The other had the belly and pelvis scanned twice, with and without dye. Both tests were called a CT, yet one received several times the dose of the other.
So when people ask how much radiation a CT gives, the fair reply is another question. A CT of what, on whom, and how?
What is the cancer risk, and how sure are we?
At these doses, the main concern is a small added chance of cancer later in life. Brenner and Hall drew much of their evidence from survivors of the 1945 atomic bombs in Japan [1]. Survivors with low doses, from 5 to 150 mSv, showed a significant rise in cancer risk. These were doses to body organs, which are not the same measure as the scan values above. Children face greater risk from the same dose, because they are more sensitive and have more years ahead.
They also made a rough estimate for the whole country. Based on US CT use at that time, they suggested that perhaps 1.5% to 2.0% of all US cancers might be caused by CT radiation. That is a rough projection for the US population of 2007, not anyone’s personal risk.
Other experts read the evidence more cautiously. A 2023 position statement from the American Association of Physicists in Medicine (AAPM) accepts a theoretical effect at any dose [4]. But for most imaging tests, it finds “no conclusive epidemiological evidence” of harm. Epidemiological studies track health across large groups of people. The AAPM adds that risk predictions “are subject to significant uncertainty and should not unduly influence the decision for a justified procedure.”
I read these two views as closer than they first appear. Both treat the risk to one person from one needed scan as small. They differ on how sure we can be about small doses across millions of people. For a patient, the point is the same: no one can turn your millisieverts into a precise personal risk.
Questions to ask before a CT scan
Missing a diagnosis is a risk too. The guide says “a person is at risk if the doctor cannot accurately diagnose an illness or injury” [2]. Brenner and Hall agreed that when a scan is medically needed, “the associated risk is small relative to the diagnostic information obtained” [1].
Their worry was the unneeded scan. An informal poll of pediatric radiologists, which they cited, suggested that perhaps one third of CT studies could be replaced or skipped. For some cases, options without CT’s X-rays exist, such as ultrasound and MRI (magnetic resonance imaging). They also noted that needed scans were sometimes repeated as patients moved through the medical system, often “simply because of a lack of communication.”
I am a researcher trained in medical physics, and the scan that worries me is the one that did not need to happen. If a CT is suggested, these questions are fair to ask:
- How will this scan change my care?
- Have I had a recent scan that could answer the same question?
- Would ultrasound, MRI or a lower-dose scan work instead?
- For a child, will the settings be adjusted for their size?
These questions are not a reason to refuse a scan your doctor recommends. When there is a clear reason for it, the benefit usually comes first. Asking simply helps make sure each scan is the right one.
A CT scan is neither harmless nor something to fear. I would like people to know the rough size of the dose and accept that the risk estimate is uncertain. Save your concern for scans that are not needed.
The numbers here are typical values with real limits. Your own dose depends on your body, the machine and the reason for the scan. Ask the imaging team about your exam, talk it over with your doctor, and decide together.
References
[1] D. J. Brenner and E. J. Hall, “Computed tomography: An increasing source of radiation exposure,” The New England Journal of Medicine, vol. 357, no. 22, pp. 2277-2284, Nov. 2007, doi: 10.1056/NEJMra072149.
[2] RadiologyInfo.org (Radiological Society of North America and American College of Radiology), “Radiation dose,” last reviewed Apr. 15, 2025. [Online]. Available: https://www.radiologyinfo.org/en/info/safety-xray
[3] F. A. Mettler, W. Huda, T. T. Yoshizumi, and M. Mahesh, “Effective doses in radiology and diagnostic nuclear medicine: A catalog,” Radiology, vol. 248, no. 1, pp. 254-263, Jul. 2008, doi: 10.1148/radiol.2481071451.
[4] American Association of Physicists in Medicine, “AAPM position statement on radiation risks from medical imaging procedures,” Policy PS 4-B, Nov. 16, 2023. [Online]. Available: https://www.aapm.org/org/policies/details.asp?id=3615
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