How To Rank Risks

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My favorite way of putting risks into perspective is to consider the average loss of life expectancy they cause, LLE (indicated in parentheses throughout this article). I present here a brief catalog of these, taken from my paper published in the September 1991 issue of Health Physics Journal.

Historically, diseases were one of the most important causes of life shortening, but now only heart disease (4.4 years), cancer (3.4 years), and stroke (250 days) cause LLE of more than six months. Aside from diseases, the principal direct causes of death are accidents (366 days), suicide (115 days), and homicide (93 days). Over half of all accident deaths are due to motor vehicles, and half of these are alcohol related. The most important other type of accidents are falls (28 days), suffocation (28 days), drowning (24 days), poison (20 days), and fires (20 days). From the standpoint of fatal occupational accidents, the most dangerous industries are construction (227 days) and mining (167 days); much safer are services (27 days), trade (27 days), and manufacturing (40 days).

Perhaps the best known risky behavior is smoking cigarettes (6.6 years for men, 3.9 years for women). Even more dangerous is being an alcoholic (12 years). Over-eating gives an LLE of about 36 days per pound, or one year for each 10 pounds overweight; being 20% overweight increases the fatality risk of heart disease by 29%, of cancer by 10%, of stroke by 15%, and of diabetes by 130%.

Having very poor, vs. very good, social connections correlates with LLE of 9 years. As one manifestation of this, remaining single rather than married has LLE of 5 years.

One of the greatest risks to an individual is living in poverty, LLE = 9 years for 19 large U.S. cities and for Montreal. In Britain, the difference in life expectancy between professional people and unskilled laborers is 7.2 years, and in Finland it is also 7.2 years. When Canadian men are ranked by income, those in the 90th percentile live 6 years longer than those in the 10th percentile. The latter have a higher mortality rate by 32% for heart disease and stroke, by 34% for cancer, and by 88% for accidents, poison, and violence. On an international scale, poverty plays a much bigger role life expectancy is typically 30 years longer in well-to-do countries than in poor countries.

Life expectancy varies substantially with occupation. Post office employees, university professors, and workers in clothing manufacturing and in communications industries live 1-2 years longer than average, and miners, policemen, firemen, truck drivers, and fishermen die 2-3 years younger than average. But the most dangerous job is no job at all unemployment. A 1% increase in national unemployment results in 37,000 deaths per year (plus 4200 admissions to mental hospitals and 3300 admissions to prisons).

If you believe the most dire warnings of environmental activists, you might add several other threats, though they would still be small compared to many of the problems listed above: air pollution (40 days), drinking water pollution (20 days), chemical residues in food (20 days), and chemicals released from consumer products (20 days). Media give wide publicity to cancer causing pollutants. Some of these are pesticide residues in food (12 days), tobacco smoke (8 days), other indoor pollutants (2 days), industrial air pollution (4 days), hazardous waste sites (2.5 days), drinking water contaminants (1.3 days), and all radioactivity releases from nuclear power including accidents (0.04 days). Note that nuclear power is an insignificant contributor to radiation exposure compared to radon in homes (25 days), other natural sources of radiation (10 days), and medical exposures (10 days).

Broiling meat produces carcinogens (0.1 day) and we produce similar carcinogens in bread crusts, toast, and fried potatoes. But everything man does, purposely or through pollution, is trivial in comparison with nature's contribution. All plants contain toxic chemicals to protect them from their natural enemies. Many of these chemicals can cause cancer, like nitrosamines in beets, celery, and lettuce; aflatoxin in peanuts, corn, and milk; sterigmatocystin in salami, ham, and wheat; hydrazines in mushrooms; allyl isothiocyanate in mustard, broccoli, and cabbage; safrole in pepper; tannins in coffee, tea, and wines; psoralens in celery and parsley; ethyl carbamate in bread, yogurt, beer, and wine; formaldehyde in fruits; benzene in eggs; methylene chloride in fats; coumarin in candy; diacetyl in coffee and butter; and flavonoids in fruits and vegetables. These are nature's pesticides, and per quantity ingested, they are typically as carcinogenic as man-made pesticides. But we eat 10,000 times as much of nature's pesticides as of man-made ones.

Natural catastrophes in the U.S. give the following relatively small LLE: hurricanes and tornadoes 1.1 days; lightning 0.7 day; storms and floods 0.9 day, earthquakes and volcanoes 0.2 day, heat waves 0.7 day, cold waves 2.1 days. Some similarly low risks are venomous plants and animals 0.5 day (half from bee stings, and only 15% from snakes, lizards, and spiders) and dog bites 0.12 day.

Historically, the great killers have been pestilence, war, and famine, with war often causing the other two. The best known pestilence epidemics have been the "Plague of Justinian" in AD 500-650 which killed 100 million, the "Black Death" in 1347-1351 which killed 75 million in Europe plus perhaps more than that in Asia, various diseases among American Indians due to contact with Europeans after 1492 that killed untold millions ( a large fraction of the Indian population), and the influenza epidemic of 1918-1919 which killed 20-50 million including a half million in U.S. AIDS is killing 12,000 Americans per year (LLE 55 days for the average American). Fortunately, it does not spread through such efficient channels as coughing or food, but we have no guarantee against development of a new, equally powerful virus that does spread efficiently. And the best-understood natural disaster that could wipe out nearly all of mankind is the impact on the Earth of a large asteroid, expected once in a million years.

To help put some of the risks we have described (plus others) into perspective, a bar graph is attached in which the length of the bars gives the LLE. Asterisks (*) refer to effects averaged over the entire population, while those without asterisks refer just to people involved in the activity. The largest risks are shown at the top: alcohol, poverty, smoking, poor social connections, heart disease, and cancer each take years off a person's life expectancy. The smallest risk in that left section (the smallest of the large risks), motor vehicle accidents, is also shown as the largest risk in the middle group for which bar lengths have been multiplied by 20. This middle group consists mostly of risks widely recognized but not greatly feared. The smallest risk in this middle group, bicycles (the most dangerous transport per mile traveled), is also shown as the largest bar in the bottom group, for which bar heights have been multiplied by another factor of 50, a total factor of (20 x 50 =) 1000 over the high risk group.

In a rational society, the low risks shown in the bottom group should receive little consideration, but the public's attention is determined more by media coverage than by results of scientific risk analysis. The most glaring example of this is nuclear power, which is widely perceived by the public as being dangerous. We see from the bar diagrams that its perceived risk is a thousand times inflated. Few people take the time to rank risks rationally.

Risks

Bernard L. Cohen is a Professor Emeritus of Physics, University of Pittsburgh