Richard Woollons, of Christchurch, asks :-

Recently I had several X-rays on my dodgy hip. How many X-rays are safe?

Juergen Meyer, a medical physicist at the University of Canterbury, responded.

First some background information. X-rays have sufficient energy to remove an electron from an atom or molecule. This is known as ionization. Ionization of important cellular molecules, such as the DNA, can lead to biological damage. Fortunately, the human body is extremely capable of repairing radiation damage as we have always been exposed to natural radiation from the environment throughout history. In fact human cells on average experience over 100,000 DNA lesions per day not just as a result of exposure to ionizing radiation. Cells have developed specialized pathways to sense, respond to, and repair the damage. The body does not distinguish between natural and man-made radiation and therefore repairs any damage from man-made X-rays in the same way it would repair damage from natural radiation.

In general radiation effects can be classified into deterministic and stochastic effects. If cell damage is not adequately repaired, it may prevent the cell from surviving or reproducing or it may result in a change or mutation that it retains.

Most organs or tissues of the body are unaffected by the loss of a few cells. However, if a large number of cells are killed this can lead to a loss of tissue function. An example of such a deterministic effect is radiation introduced cataracts. Deterministic affects have a threshold and medical procedures are typically well below this threshold. So there is practically no risk of deterministic effects from a few X-rays.

If a cell is modified but viable it can lead to carcinogenesis and possibly hereditary effects. It is generally accepted that there is no threshold but the probability of cancer increases with dose. The severity of the cancer is not dose related. A cancer induced by a large amount of X-rays is no worse than one induced by a small amount but the probability of its induction is increased. This category of effect is called stochastic. So the risk of getting cancer from a few X-rays is non-zero but extremely small.

In general it is important to note that even though the risks are extremely small, every radiation exposure has to be justified and the benefits have to outweigh the risks. To put some numbers to it, we can use the exposure to natural background radiation as a reference:

The worldwide annual effective dose from background radiation is about 2.4 mSv per year. In some places the annual effective dose is much higher, say 10 or 30 mSv, whereas in NZ it is slightly lower with 1.1 mSv. Exposure from one conventional pelvic X-ray is about 0.8 mSv, whereas a CT scan of the pelvis results in about 10 mSv. Air travel between NZ and Europe is about 0.1 mSv.

In summary, below about 100 mSv there is no risk of deterministic effects and the probability of stochastic effects is very very small. Despite that I would still question your doctor every time he or she orders an X-ray to make sure that the exposure is justified.