Micromort

A micromort (from micro- and ) is a unit of risk defined as one-in-a-million chance of death. Micromorts can be used to measure riskiness of various day-to-day activities. A microprobability is a one-in-a million chance of some event; thus a micromort is the microprobability of death. The micromort concept was introduced by Ronald A. Howard who pioneered the modern practice of decision analysis.

Micromorts for future activities can only be rough assessments as specific circumstances will always have an impact. However past historical rates of events can be used to provide a ball-park, average figure.

Activities that increase the death risk by roughly one micromort, and their associated cause of death:

Increase in death risk for other activities on a per event basis:

An application of micromorts is measuring the value that humans place on risk: for example, one can consider the amount of money one would have to pay a person to get him or her to accept a one-in-a-million chance of death (or conversely the amount that someone might be willing to pay to avoid a one-in-a-million chance of death). When put thus, people claim a high number but when inferred from their day-to-day actions (e.g., how much they are willing to pay for safety features on cars) a typical value is around $50 (in 2009). However utility functions are often not linear, i.e. the more a person has already spent on their safety the less they are willing to spend to further increase their safety. Therefore, the $50 valuation should not be taken to mean that a human life (1 million micromorts) is valued at $50,000,000. Furthermore, the local linearity of any utility curve means that the micromort is useful for small incremental risks and rewards, not necessarily for large risks.

Government agencies use a nominal Value of a Statistical Life (VSL) - or Value for Preventing a Fatality (VPF) - to evaluate the cost-effectiveness of expenditure on safeguards. For example, in the UK the VSL stands at £1.6 million for road improvements. Since road improvements have the effect of lowering the risk of large numbers of people by a small amount, the UK Department of Transport essentially prices a reduction of 1 Micromort at £1.60 (US$2.70). The US Department of Transportation uses a VSL of US$6.2 million, pricing a Micromort at US$6.20.

Micromorts are best used to measure the size of acute risks, i.e. immediate deaths. Risks from lifestyle, exposure to air pollution and so on are chronic risks, in that they do not kill straight away, but reduce life expectancy. Ron Howard included such risks in his original 1979 work, for example an additional one micromort from …

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