5. What are the health effects of exposure to ionizing radiation?
Ionising radiation can cause direct harm to body tissues and the severity of the damage increases with the dose. Doses below a threshold value of about 0.1 Gy are usually safe. This type of direct short-term damage is negligible from X-ray scanners not only after single exposures but also during cumulative and repeated exposures.
Cancer and hereditary disorders don’t behave the same way, since there is no relation between the severity of the effect and the amount of radiation. Higher exposures to ionising radiation however, makes these events more likely to occur. It is generally considered that there is no safe threshold limit below which the effect does not happen.
Radiation can induce most, but not all cancer types. Typically, cancer develops 10 years after exposure; or 1 to 2 years for leukaemia and thyroid cancer. There are many epidemiological studies on the effects of low-dose radiation and these show that the risk of developing cancer rises uniformly with exposure. People exposed to radiation at a young age have a greater chance of developing cancer, particularly for thyroid, leukaemia and breast cancer. Health risks appear to be similar for different groups of the population although people with some rare inherited diseases are more likely to get secondary cancers after radiotherapy. The effect of low doses of radiation does not depend on whether the dose was given in a single exposure or in more spaced, lower exposures.
Epidemiological studies are subject to several sources of uncertainty. The quality of studies can be limited by bias in the information available or in the selection of the sample studied and there can also be other confounding factors, unrelated to the factor investigated. One of the major limitations of studies that deal with small effects is random errors that arise from variability unrelated to the exposure being investigated. In this case, diseases such as cancer and cardiovascular disease have many causes and it is impossible to tell which of the cancers occurring in a population are caused by radiation and which are caused by other factors. As a result, it is very difficult to demonstrate the effects of very low radiation doses, say below 100 – 200 mSv, in epidemiological studies. The information required to be able to pinpoint the effects caused by the radiation are impossible to achieve in practice. Even if the studies involve large populations, any potential increase in the number of cancer cases would be of the same size as random errors and therefore easy to miss.
The doses to individuals from X-ray scanners are considered negligible so there is no basis to consider separately the risk to potentially vulnerable groups such as children or pregnant women. The risk when considering the whole population is also negligible but not zero. As an estimate, radiation doses of the order of a mSv are expected to increase the occurrence of cancer by 1%.
As the radiation from X-rays scanners are several orders of magnitude lower, the risks can also be assumed to be smaller.
As a comparison, roughly 0.6% of the lifetime cancer risk in the UK might be attributable to diagnostic X-rays and the figure could be larger in many other countries. Approximately 4% of all cases of leukaemia, and 5% to 19% of cases of childhood leukaemia could be attributable to background radiation. More...