Why are we scared of West Nile Virus more than we're scared of being crushed by a vending machine? We don't have a good sense of perspective when it comes to risks.

One of my friends believes that the problem is that we evolved living in small groups, such that if we knew about only 1000 other humans. Therefore, we could expect anything we hear happening to humans should have at least a 1 / 1000 chance of happening to ourselves. Since our instincts haven't changed since then, we tend to overestimate the impact of small risks, since there are hard-wired parts of our brains telling us nothing we hear about can be less likely than about a 0.1% chance.

Let's compile a list of risks and categorize them by how likely they are to be a serious threat to you, in terms of powers of 10.

Code Red: between certainty and 1 in 10

Code Orange: between 1 in 10 and 1%

Code Yellow: between 1% and 10^-4

  • Odds of contracting HIV from one act of unprotected sex with an otherwise-healthy HIV positive person (5 to 50 in 10^4, depending on the sex act. [1])
  • Odds that drinking one 2L bottle of non-diet soda will kill you some time in the next 15 years (1.2 * 10^-4) see blog entry.

Code Green: between 10^-4 and 10^-5

  • Rough estimate on the risk of getting variant Creutzfeldt-Jakob disease from eating an entire cow with BSE (= 9 * 10^-4) [[2]], calculated from dividing the known number of vCJD cases by the number of known BSE cases. (Although there's still a lot of uncertainty about the strength of the BSE-vCJD link[3]) This estimate may be on the high side, considering that there are more uncounted cow BSE cases than uncounted human vCJD cases; it might also be on the low side due to catching some infected meat before it got consumed.
  • Average additional risk due to obesity an American will die in the following 15 years from eating one extra fast food meal ( = 9.4 * 10^-5) [[4]]
  • Average total risk of living one day, assuming an 80-year lifespan ( = 3.4 * 10^-5)
  • Getting killed in a traffic accident on a 1000-mile road trip ( = 1.5 * 10^-5) [[5] - .pdf warning, page 8]
  • Dying in a skydiving accident ( = 1.5 * 10^-5) [[6]]

Code Blue: between 10^-5 and 10^-7

  • Contracting HIV from having sex once with a randomly-chosen American ( between 2.0 * 10^-6 and 2.0 * 10^-5) [[7]]

Code Indigo: between 10^-7 and 10^-9

  • Risk of contracting vCJD from eating a single BSE-infected beef meal; as calculated above but assuming 1000 beef meals per cow ( = 9*10^-7)
  • Risk of being killed by Salmonella from eating a whole batch of raw cookie dough ( = 3*10^-8)

Code Violet: between 10^-9 and 0

  • Getting your eye blown off opening a plastic pop bottle top. (Risk level unknown, since there's no record of it ever happening in a study looking at 5 billion person-years of exposure[8] - .pdf warning. See my blog entry for an editorial.)

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