Heterosexual AIDS risk versus being struck by lightning

Newspaper reports have recently compared the risk of heterosexual AIDS to that of being struck by lightning. These statements are rather misleading about the real risk involved. To see the flaw, note that computing the probabilities shows that the risk of dying in a single car trip is much less than the risk of ever getting hit by lightning.

The Wall Street Journal front page article of Wednesday, May 1, 1996, "Health Hazard: AIDS Fight Is Skewed By Federal Campaign Exaggerating Risks", Amanda Bennett and Anita Sharpe, begins: In the summer of 1987, federal health officials made the fateful decision to bombard the public with a terrifying message: Anyone could get AIDS. While the message was technically true, it was also highly misleading. Everyone certainly faced some danger, but for most heterosexuals, the risk from a single act of sex was smaller than the risk of ever getting hit by lightning. In the U.S., the disease was, and remains, largely the scourge of gay men, intravenous drug users, their sex partners and their newborn children.

This statement then mutated, as in The Washington Times article, "Inflating the lie can be worth millions", by Wesley Pruden: The Wall Street Journal concludes that, for most heterosexuals, the risk of AIDS is something less than the risk of getting hit by lightning.

The Wall Street Journal statement is technically true, but highly misleading. The Washington Times incorrectly restates the Wall Street Journal by leaving off the qualifiers.

The basic problem with the Wall Street Journal statement is that it boils down to "The risk of a small amount of X is smaller than a lifetime of Y". Obviously, this statement can be true even if X is much more dangerous than Y.

As an example, the risk of dying from a 1000 mile car trip is less than the risk of ever getting hit by lightning. This doesn't mean that driving is safe, and in fact motor vehicle accidents kill about 50,000 Americans a year. In addition, it is clearly wrong to say that the risk of dying in a car accident is less than the risk of getting hit by lightning.

Thus, you can see that the Wall Street Journal statement sounds impressive but doesn't really prove anything. In addition, the Washington Times totally changes the meaning by removing the qualifiers "single act" and "ever".

To get an accurate comparison of AIDS and lightning, we have to look at the same time period. Since most people have sex more than once, it makes more sense to look at the risk per year than the risk of a single act. In a typical year, about 4000 Americans are diagnosed with AIDS obtained through heterosexual contact, about 300 are struck by lightning, and about 90 die from lightning strikes. Thus, you can see that the risk of AIDS is much higher than that of lightning; the AIDS rate is about the same as the death rate from significant killers such as bronchitis or non-gas poisoning or anemia or drowning or fires. (Are people needlessly worried about the chances of dying in a fire?) If we exclude the half of these AIDS cases attributed to heterosexual sex with partners who use IV drugs, AIDS still is much more risky than lightning. Even if we exclude in addition all the partners with unknown risk, AIDS still comes out ahead.

Therefore, comparisons between AIDS and lightning need to be examined very closely. The Wall Street Journal, by using a carefully constructed comparison, has a correct but misleading statement. Rephrases, such as in the Washington Times, are wrong. To see the problems with these comparisons, note that the risk of dying from a 1000 mile car trip is less than risk of ever getting hit by lightning.

Appendix: calculations and references.

There were 94 lightning deaths in the US in 1980 out of 225 million people. Thus, you have odds of about 1 in 2.4 million of dying of lightning per year and 1 in 32,000 of ever dying of lightning (assuming a 75 year lifespan). From a NOAA reprint, about 1/3 of lightning strikes are fatal, so this would give odds of 1 in 800,000 of being struck by lightning per year and about 1 in 10,000 of ever being struck by lightning. (The NOAA, by the way, calls lightning "The underrated killer" and points out that it kills more people in the US than tornadoes, floods, or hurricanes.) There are about 2.4 motor vehicle fatalities per 100 million miles travelled. Thus, in a 1000 mile trip, you have about 1 in 42,000 odds of dying.

Lightning deaths from "The Injury Fact Book", fraction of fatal lightning strikes from NOAA Reprint Vol 6 Number 2, 1976 (http://aws.com/lightnin.txt), other US statistics from "Statistical Abstract of the United States, 1990" The AIDS statistics are from the CDC's HIV/AIDS Surveillance Report, Fourth Quarter 1992. I used 1992 statistics because the statistics from 1993 on are artifically inflated by a change in the AIDS definition, so the 1992 statistics are the most accurate available.

Ken Shirriff: shirriff@eng.sun.com
This page: http://www.righto.com/theories/lightning.html Copyright 2000 Ken Shirriff.