Yesterday, British authorities broke up an alleged terror plot to blow up as many as ten commercial airliners as they flew to the United States. In response, the Department of Homeland Security upped the alert level on commercial flights from Britain to "red" and boosted the alert to "orange" for all other flights. In a completely unscientific poll, AOL asked subscribers: "Are you changing your travel plans because of the raised threat level?" At mid-afternoon about a quarter of the respondents had said yes. Such polls do reflect the kinds of anxieties terrorist attacks, even those that have been stymied, provoke in the public.

But how afraid should Americans be of terrorist attacks? Not very, as some quick comparisons with other risks that we regularly run in our daily lives indicate. Your odds of dying of a specific cause in any year are calculated by dividing that year's population by the number of deaths by that cause in that year. Your lifetime odds of dying of a particular cause are calculated by dividing the one-year odds by the life expectancy of a person born in that year. For example, in 2003 about 45,000 Americans died in motor accidents out of population of 291,000,000. So, according to the National Safety Council this means your one-year odds of dying in a car accident is about one out of 6500. Therefore your lifetime probability (6500 ÷ 78 years life expectancy) of dying in a motor accident are about one in 83.

What about your chances of dying in an airplane crash? A one-year risk of one in 400,000 and one in 5,000 lifetime risk. What about walking across the street? A one-year risk of one in 48,500 and a lifetime risk of one in 625. Drowning? A one-year risk of one in 88,000 and a one in 1100 lifetime risk. In a fire? About the same risk as drowning. Murder? A one-year risk of one in 16,500 and a lifetime risk of one in 210. What about falling? Essentially the same as being murdered. And the proverbial being struck by lightning? A one-year risk of one in 6.2 million and a lifetime risk of one in 80,000. And what is the risk that you will die of a catastrophic asteroid strike? In 1994, astronomers calculated that the chance was one in 20,000. However, as they've gathered more data on the orbits of near earth objects, the lifetime risk has been reduced to one in 200,000 or more.

So how do these common risks compare to your risk of dying in a terrorist attack? To try to calculate those odds realistically, Michael Rothschild, a former business professor at the University of Wisconsin, worked out a couple of plausible scenarios. For example, he figured that if terrorists were to destroy entirely one of America's 40,000 shopping malls per week, your chances of being there at the wrong time would be about one in one million or more. Rothschild also estimated that if terrorists hijacked and crashed one of America's 18,000 commercial flights per week that your chance of being on the crashed plane would be one in 135,000.

Even if terrorists were able to pull off one attack per year on the scale of the 9/11 atrocity, that would mean your one-year risk would be one in 100,000 and your lifetime risk would be about one in 1300. (300,000,000 ÷ 3,000 = 100,000 ÷ 78 years = 1282) In other words, your risk of dying in a plausible terrorist attack is much lower than your risk of dying in a car accident, by walking across the street, by drowning, in a fire, by falling, or by being murdered.

So do these numbers comfort you? If not, that's a problem. Already, security measures—pervasive ID checkpoints, metal detectors, and phalanxes of security guards—increasingly clot the pathways of our public lives. It's easy to overreact when an atrocity takes place—to heed those who promise safety if only we will give the authorities the "tools" they want by surrendering to them some of our liberty. As President Franklin Roosevelt in his first inaugural speech said, "The only thing we have to fear is fear itself— nameless, unreasoning, unjustified terror which paralyzes needed efforts to convert retreat into advance." However, with risks this low there is no reason for us not to continue to live our lives as though terrorism doesn't matter—because it doesn't really matter. We ultimately vanquish terrorism when we refuse to be terrorized.