The high energy level that allows X-rays to pass through tissue comes with an inherent downside. An X-ray photon has enough energy to knock an electron free from an atom in your body, upsetting the balance between the atom’s positively charged nucleus and negatively charged orbiting electrons. The result is an ion, an atom with a net electrical charge.
An ion’s electrical charge can trigger a range of chemical changes in cells throughout the body. Most notably, it can break apart DNA strands, causing a cell to mutate. Mutations, in turn, can make a cell cancerous, and this cancer can spread to other cells.
Doctors represent the risk of X-ray scans in terms of equivalent background radiation – the constant ionising radiation from naturally occurring radioactive materials and cosmic rays emanating in space. For example, one chest X-ray is equivalent to about 48 hours of natural background radiation, which increases your risk of cancer by only one in a million.