Quoting Slovic et al (1982), Facts versus fears: Understanding perceived risk:
…Sowby (1965) provided extensive data on risks per hour of exposure, showing, for example, than an hour of riding a motorcycle is as risky as an hour of being 75 years old.
Slovic, P., Fischhoff, B. & Lichtenstein, S. (1982). Facts versus fears: Understanding perceived risk. In D. Kahneman, P. Slovic & A. Tversky (eds.). Judgement under uncertainty: Heuristics and biases. Cambridge: Cambridge University Press
Sowby, F.D. (1965). Radiation and other risks. Health Physics, 11, 879-887.