X-rays are a form of radiation. Specifically, they are electromagnetic radiation with enough energy to knock electrons off atoms in your body, which is why they’re classified as ionizing radiation. This puts them in the same broad family as visible light and radio waves, but at a much higher energy level, one that can penetrate soft tissue and interact with your DNA.
What Makes X-Rays Ionizing
All light exists on a spectrum of energy. Radio waves sit at the low end, visible light in the middle, and X-rays near the top. What separates X-rays from harmless forms of electromagnetic energy is their ability to ionize atoms, meaning they carry enough energy to strip electrons away from the molecules they pass through. When that happens inside living tissue, it triggers chemical changes in cells.
The energy of X-rays ranges from about 10 electron volts up to extremely high levels. Most of the biological damage doesn’t come from the X-ray photon itself but from the secondary electrons it sets in motion as it travels through tissue. These fast-moving electrons are what actually disrupt the chemistry inside your cells.
How X-Rays Affect Your Cells
When X-ray energy reaches your DNA, it can cause damage in two ways. The direct route is a photon or secondary electron physically hitting a DNA strand and breaking it. The indirect route, which accounts for roughly two-thirds of the damage, involves the radiation splitting water molecules near DNA into highly reactive fragments called free radicals. These radicals then attack the DNA strand chemically.
The most consequential type of injury is a double-strand break, where both rails of the DNA ladder are severed at nearly the same spot. Your cells have repair machinery for this, but the process isn’t perfect. Misrepaired breaks can cause pieces of chromosomes to rearrange or rejoin incorrectly, which can sometimes lead to mutations. If those mutations affect genes that control cell growth, cancer becomes a possibility, though this outcome is rare from the doses used in medical imaging.
Radiation Doses in Medical Imaging
Not all X-ray exposures are equal. A standard adult chest X-ray delivers about 0.1 millisieverts (mSv), a unit that measures the biological impact of radiation. That’s equivalent to roughly 10 days of the natural background radiation you absorb just by living on Earth. A chest CT scan, by comparison, delivers about 6.1 mSv, roughly 60 times more than a single chest X-ray.
For context, the average person receives about 2.4 mSv per year from natural sources: cosmic rays, radon gas in the ground, and trace radioactive elements in food and water. Over a 65-year lifetime, that adds up to around 160 mSv with no medical imaging at all. So a single chest X-ray adds a tiny fraction to what nature is already delivering, while a CT scan adds the equivalent of about two and a half years of background exposure in one session.
How Risky Is the Exposure
Radiation risk from medical imaging is real but small, and it scales with dose. A widely cited estimate is that a cumulative dose of 1,000 mSv carries about a 5% excess risk of dying from cancer. Since a chest X-ray delivers 0.1 mSv, you’d need thousands of them to approach that threshold. CT scans, with their higher doses, carry more meaningful individual risk. One estimate found that a 40-year-old woman undergoing CT coronary angiography (about 20 mSv) has roughly a 1 in 270 chance of developing cancer from that single scan. For a 40-year-old man, the risk is about 1 in 600, reflecting the fact that women’s breast and thyroid tissue are more sensitive to radiation.
To put lower doses in everyday terms: the additional cancer risk from 0.1 to 1.0 mSv is comparable to the risk of death from a 4,500-mile flight. Doses between 1 and 10 mSv carry risk roughly comparable to driving 2,000 miles. These are not zero-risk events, but they fall well within the range of minor risks most people accept without a second thought.
Researchers have estimated that CT scans performed in the United States in a single year (2007) could be linked to about 29,000 future cancers, representing roughly 2% of all cancers diagnosed annually in the country. That number sounds large in absolute terms but reflects the enormous volume of CT scans performed, not an outsized risk from any individual scan.
Diagnostic vs. Therapeutic Radiation
X-rays serve two very different purposes in medicine, and the doses involved are vastly different. Diagnostic imaging uses just enough radiation to create a useful picture of the inside of your body. The goal is the lowest dose that still produces a clear image.
Radiation therapy for cancer is the opposite approach. It deliberately uses high, focused doses of radiation to destroy tumor cells. Therapeutic radiographers carefully calculate the exact dose needed and precisely map where the beam should enter and exit the body, concentrating damage on the tumor while minimizing exposure to surrounding healthy tissue. The doses in radiation therapy are orders of magnitude higher than anything used in a chest X-ray or CT scan.
Occupational Limits for Radiation Workers
People who work around X-ray equipment, such as radiologists, dental hygienists, and nuclear plant workers, are subject to annual dose limits. International guidelines cap occupational exposure at 20 mSv per year averaged over five years, with no single year exceeding 50 mSv. These limits are set conservatively, well below doses where measurable health effects have been observed in studies, to keep cumulative lifetime exposure at safe levels.