Security Scanners

4. How is radiation exposure measured and assessed?

Frequent flyers and airline crews might be exposed to more radiation than the limits set for the general public

When dealing with radiation, the absorbed dose gives the amount of energy that reaches a very small amount of material, and is measured in Grays (Gy). Different types of radiation have different biological effects so a weighting factor is introduced to account for this. In radiation protection, this leads to a new quantity, the “equivalent dose” which is measured in Sieverts (Sv).

If the incident X-rays have high energies, one could assume that the dose would be uniformly distributed around the whole body. However, at low energies, organs which are closer to the surface (such as the lense of the eye, the female breast or the testes) will be more exposed than those deeper inside the body so it is necessary to calculate the dose that each different organ would receive. This is called the organ dose. The risk that ionising radiation is going to lead to cancer or mutations, varies between different organs and this is also considered to calculate the potential risks for each individual organ and then added together to work out the risk for the whole body, the “effective dose”. This dose is an average for the population but the risk to individuals varies depending on their gender, how old they were when they were exposed and other risk factors.

It is difficult to measure doses of ionising radiation directly within the body so organ doses are typically evaluated using computer models of humans called Voxel phantoms. These have very realistic anatomies and there are phantoms that represent different ages and both genders. It is very difficult to give reliable estimates on children under 14 years of age because they vary widely in height and size. Similarly, the doses calculated for adults are just an average but depending on the physical characteristics of the individual, the range of doses to an adult can vary by up to a factor of two.

The amount of radiation that a person would receive from a scan is similar to what they would get from background radiation on the ground for 1 hour, or during 10 minutes of flying in a commercial airplane. These are average values but it is important to consider that some people, because of their age, sex or other factors, are more sensitive to radiation than others.

Some groups of people such as air crews, airport staff, frequent flyers and couriers are likely to be scanned frequently so to assess the maximum dose they would receive from security scanners, it was assumed that a member of one of these groups could be scanned up to three times a day , every working day, so 720 times a year. For a typical X-ray backscatter scanner this would result in an annual dose of 0.3 mSv, which is still below the limit set for members of the public. However, if all the scans were done with an X-ray transmission scanner, the cumulative dose would be nearly 3 mSv, which is well above the limit. More...