Radiation has received a mixed press since the discovery of x rays 115 years ago.

Wilhelm Roentgen’s exciting new finding was initially thought to be beneficial for health and was quickly embraced by scientists as well as the public, who went crazy for the “medicine” radithor – radium 226 and radium 228 in distilled water – which could apparently treat rheumatism, epilepsy, impotence, and insanity.

The case of the radium dial factory workers in the 1920s, who tried to sue their employers after developing radiation poisoning from licking their brushes when painting glow in the dark paint onto watch faces, was the first sign that radiation might not be as good for you as was previously thought.

But it wasn’t until the death of popular American golfer Eben M Byers, who died from radiation sickness in 1932 after consuming too much radithor (http://www.time.com/time/magazine/article/0,9171,743525,00.html), that the public began to think that radiation could be dangerous. Add to this the Cold War and the discovery that ionising radiation can cause cancer and all of a sudden radiation became something to be feared and avoided.

But we’re subjected radiation all the time and yet don’t seem to be dropping like flies. The population of the United Kingdom is exposed to “background radiation” of roughly 2.5 mSv a year thanks TO radioactive sources in food and the soil and cosmic radiation from space. And Tsutomu Yamaguchi, who was unfortunate enough to be caught in both the Hiroshima and the Nagasaki nuclear bombings, lived to the ripe old age of 93.

So what really is the health risk associated with exposure to radiation? Or could it potentially even be good for you?

These were the questions posed at a public lecture hosted by the Royal College of Radiologists at the Royal Society of Medicine last week.

The lecture was somewhat incongruously introduced by Jimmy Savile, resplendent in tracksuit and gold jewellery in a room full of medical types. He has been a volunteer hospital porter at Leeds Royal Infirmary since the “I’m backing Britain” campaign in the late 1960s. He spoke warmly of the pleasure he gets from talking with sick patients and his role in designing and building the National Spinal Injuries Centre.

Dr Bob Bury, consultant radiologist at Leeds Teaching Hospitals Trust, then delivered an engaging talk in which he outlined the level of risk posed by the number one source of man made radiation: gamma rays and x rays used in medical imaging.

A full body computed tomography scan gives a scary sounding 14-21 mSv dose of radiation, equivalent to eight years or so of background radiation. This procedure increases the lifetime risk of cancer by only 0.08% though. A chest x ray entails just 0.1 mSv and increases the risk of cancer by less than 1 in 1,000,000. In fact, most medical imaging techniques involve doses lower than the annual background radiation dose.

It’s important to pay close attention to the figures touted around too. For example, a 25 mGy dose of radiation to the pelvis in the first semester of pregnancy doubles the risk of childhood cancer – sounds pretty shocking. But when you look at the absolute risk, the odds are 1 in 1300.

So is it possible that radiation isn’t as bad as we think? To argue this possibility Dr Bury introduced the concept of hormesis – the idea that a low dose of something can be beneficial when a high dose is dangerous. Paracetamol is one example, radiation another. It is thought that low doses of ionising radiation could be beneficial by deactivating free radicals, stimulating DNA repair mechanisms, and stimulating immune surveillance.

A 100 year study of British radiologists found that those who registered after 1920 had a 28% lower incidence of cancer mortality than equivalent men in the general population and a 36% lower incidence of non-cancer death. There’s even a theory that people in the gulf states of the United States might be suffering radiation deprivation thanks to the low background radiation in the area compared with the mountain states.

One of the problems with radiation is that we just don’t know how dangerous it might be. The effect of radiation on the risk of cancer is just too small to measure and the background lifetime risk of cancer is confoundingly high at one in three.

When weighing up the risks associated with medical imaging, it’s important to consider the massive contribution radiation has made to diagnosis. Dr Bury recalled that in the 1970s doctors would often have to perform a diagnostic laparotomy to find out what was wrong, whereas these days a surgeon can determine a diagnosis using medical imaging without needing to make a single incision.

All that said, medical professionals take radiation protection very seriously. Radiologists work on the ALARA principle – that the dose used in any use of medical imaging must “as low as reasonably achievable.”

The bottom line of the lecture was that low levels of radiation are unlikely to be particularly harmful, and that the benefits of using medical imaging outweigh the risks. Providing that you don’t have several scans a year, the health risks from the doses used in medical scans are very small in relation to the underlying risks of cancer.

• You can watch a video of this lecture on the Royal Society of Medicine website by following this link.

Helen Jaques is a technical editor at the BMJ.