I spend most of my working life exposing patients to radiation in my job as a diagnostic radiologist. Radiation is invisible, and deadly, But... If I asked people to place radiation on a spectrum of risk to health, with the safe stuff on the left and the most hazardous over to the right, most would put it at the right hand end of the scale. Further to the left, they might put more innocuous activities like sunbathing or driving their cars. Would they be right? The whole question of how we perceive risk is a fascinating one, and reason plays very little part in it. Two important factors are our familiarity with the particular risk factor and the degree to which we have control over it. We don't worry about a trip down the motorway because it's familiar, and we're in control. Radiation, on the other hand, is mysterious, invisible and deadly. It inspires a lot of what the psychologists call 'dread'. It's also true that we're prepared to shrug off quite large risks if we incur them voluntarily: hence skiing.