Risk, or the chances that a hazard will give rise to harm,4 is generally couched in terms of numerical odds or probabilities (see table 1) yet research has shown that people find it difficult to digest such measures.5 One difficulty is that the range of risks is so wide—from, say, the greater than 1 in 10 risk that cancer will be our eventual cause of death to the less than 1 in 10 million chance per year of being killed by lightning. We all find it hard to grasp such extremes.