How many medical x-rays are safe?

We put this to Phil Clarke and Stuart Yates:Phil - Hi, I'm Phil Clark from the Particle Physics group in Edinburgh University. And first of all, when you're discussing radiation dosage, it often gets quite complex due to the different ways to measure radiation and there's often an abundance of different units like, rems, grays, sieverts, Röntgens, Becquerels, Curies and so on so that can confuse things so much. But the important unit of measurement is what known as the Gray (Gy) and that's the unit of absorbed dose.It corresponds to one joule of energy absorbed by a kilogram of material. Now the different types of radiation like alpha, beta and gamma decays, result in different biological effects. So what you do is you have to take the grey number and multiply it by what's often called the Q factor and an example would be for x-rays and electrons, the Q value would be one. So, if you multiply those two together, you get what's known as the dose equivalent and the scientific measurement for that is a sievert.And one sievert is actually quite a large value so you typically measure in millisieverts, so thousands of sieverts. Now a typical standard chest x-ray produces about 0.1 millisievert and the dosage that are recommended for people working at CERN or the maximum dose is about 6 millisieverts. And if you're a radiation worker it goes up to about 20, or if you're an airline staff member, the usual measurement is 5 millisieverts. So the amount of radiation you get from an x-ray is actually quite small.Diana - That's the physics of x-ray doses but what about the different types of x-ray scans?Stuart - My name's Stuart Yates and I'm a radiation protection advisor working at Addenbrookes Hospital. Well, you get a very wide range of different x-rays giving different amounts of radiation dose, but taking a typical example chest x-ray, its' very common lots of people might be referred to by the GP or hospital doctor.And a typical x-ray gives you about the same amount of radiation dosage you'd get in three or four days from natural sources of radiation in the environment and also natural radioactivity in food that we eat, for example, Brazil nuts contain radium and so they're slightly radioactive. And so typically, a chest x-ray is about the same as eating three or four bags of Brazil nuts in terms of radiation dose.The CT scans I think can give you more radiation dose or your equivalent perhaps to a few years of natural radiation but then the benefit is also that much greater because the doctors will get that much more information and so one of the key things in all x-rays is that, you will only get that x-ray if the benefit outweighs the risk.Because radiation comes naturally from cosmic rays from outer space we're actually quite well-protected at ground level from that radiation because of absorption in the atmosphere. But when we fly, we're less protected because we're higher up in the atmosphere and so typically you'd get the same amount of radiation dose from a chest x-ray as you would from say, a return flight to Southern Europe.Diana - So, a simple chest x-ray will give you 0.1 millisieverts. That's the 60th of the dose limit for someone at CERN. However, a CT scan can give you up to 20 millisieverts of radiation which is four years worth of background radiation and that's unless you live in some parts of Cornwall where it's only two years worth because the rocks there emit lots of lovely radioactive radon.