Micromort

A micromort (from micro- and mortality) is a unit of risk defined as a one-in-a-million chance of death. Micromorts are usually assigned per event, or per unit of time a person is exposed to something. Micromorts can be used to measure the 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 mortality from an event, or the health impacts of an event. 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 rough estimate. A related and somewhat opposing unit is the microlife, which measures statistical improvements to life expectancy from an activity—one example being regular aerobic exercise.

Baseline

Leisure and sport

Travel The distance associated with a gain of one micromort, by mode of transportation:

Travelling 8.8 miles (14 km) by motorcycle (collision, UK) Travelling 30 miles (50 km) by walking (collision, UK) Travelling 44 miles (71 km) by bicycle (collision, UK) Travelling 370 miles (600 km) by car (collision, UK) Travelling 1,000 miles (1,600 km) by jet airplane (collision) Travelling 6,000 miles (10,000 km) by train (collision)

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

Hang gliding – 8 micromorts per flight Ecstasy (MDMA) – 0.5 micromorts per tablet, rising to 13 if using other drugs Giving birth (vaginal) – 120 micromorts Giving birth (caesarean) – 170 micromorts AstraZeneca vaccination against COVID-19 – 2.9 micromorts COVID-19 infection at age 10, in December 2020 – 20 micromorts COVID-19 infection at age 25, in December 2020 – 100 micromorts COVID-19 infection at age 55, in December 2020 – 4,000 micromorts COVID-19 infection at age 65, in December 2020 – 14,000 micromorts COVID-19 infection at age 75, in December 2020 – 46,000 micromorts COVID-19 infection at age 85, in December 2020 – 150,000 micromorts

Value of a micromort

Willingness to pay An application of micromorts is measuring the value that humans place on risk. For example, a person can consider the amount of money they would be willing to pay to avoid a one-in-a-million chance of death (or conversely, the amount of money they would receive to accept a one-in-a-million chance of death). When offered this situation, people claim a high number. However, when looking at their day-to-day actions (e.g., how much they are willing to pay for safety features on cars), a typical value for a micromort is around $50 (in 2009). This is not to say the $50 valuation should be taken to mean that a human life (1 million micromorts) is valued at $50,000,000. Rather, people are less inclined to spend money after a certain point to increase their safety. This means that analyzing risk using the micromort is more useful when using small risks, not necessarily large ones.

Value of a statistical life 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 is £1 million GBP in 1997 value (equivalent to £2 million in 2025). Since road improvements have the effect of lowering the risk of large numbers of people by a small amount, the UK Department for Transport essentially prices a reduction of 1 micromort at £1.60. The US Department of Transportation uses a VSL of US$6.2 million, pricing a micromort at US$6.20.

Chronic risks 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:

Drinking 0.5 liter of wine (cirrhosis of the liver) Smoking 1.4 cigarettes (cancer, heart disease) Spending 1 hour in a coal mine (black lung disease) Spending 3 hours in a coal mine (accident) Living 2 days in New York or Boston in 1979 (air pollution) Living 2 months with a smoker (cancer, heart disease) Drinking Miami water for 1 year (cancer from chloroform) Eating 100 charcoal-broiled steaks (cancer from benzopyrene) Traveling 6000 miles (10,000 km) by jet (cancer due to increased background radiation) Chronic risks are typically measured in terms of individual life expectancy, rather than death rates. Therefore, chronic risks are better measured with a related unit, the microlife.

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