Microlife

A microlife is a unit of risk representing half an hour change of life expectancy.[1]

Discussed by David Spiegelhalter and Alejandro Leiva, and also used by Lin et al.[2] for decision analysis, microlives are intended as a simple way of communicating the impact of a lifestyle or environmental risk factor, based on the associated daily proportional effect on expected length of life. Similar to the micromort (one in a million probability of death) the microlife is intended for "rough but fair comparisons between the sizes of chronic risks".[1] This is to avoid the biasing effects of describing risks in relative hazard ratios, converting them into somewhat tangible units. Similarly they bring long-term future risks into the here-and-now as a gain or loss of time.

"A daily loss or gain of 30 minutes can be termed a microlife, because 1 000 000 half hours (57 years) roughly corresponds to a lifetime of adult exposure."[1]

The microlife exploits the fact that for small hazard ratios the change in life expectancy is roughly linear.[3] They are by necessity rough estimates, based on averages over population and lifetime. Effects of individual variability, short-term or changing habits, and causal factors are not taken into account.

Graphic representation of microlives

Microlives gained/lost per day of exposure, based on estimated life expectancy effects of long term lifestyle and demographic risk factors, for men and women aged 35 years.[1]

Risk factor	Men	Women
Smoking
Smoking 15–24 cigarettes	−10	−9
Alcohol intake
First drink (of 10 g alcohol)	1	1
Each subsequent drink (up to 6)	−½	−1
Obesity
Per 5 units above body mass index of 22.5 each day	−3	−3
Per 5 kg above optimum weight for average height each day	−1	−1
Sedentary behaviour
2 hours watching television	−1	−1
Diet
Red meat, 1 portion (85 g, 3 oz)	−1	−1
Fruit and vegetable intake, =5 servings (blood vitamin C >50 nmol/L)	4	3
Coffee intake
2-3 cups	1	1
Physical activity
First 20 minutes of moderate exercise	2	2
Subsequent 40 minutes of moderate exercise	1	½
Statins
Taking a statin	1	1
Air pollution
Per day living in Mexico City v London	−½	−½
Geography
Per day being a resident of Russia v Sweden	−21	−9
Era
Per day living in 2010 v 1910	15	15
Per day living in 2010 v 1980	8	5