Prognosis: Washington and New York to develop Post-Traumatic Stress Syndrome, from multiple causes. These include the “orange” terror alert, just ended, and 20 inches of snow, now slowly seeping down the drain.

And as this latest case of snow panic recedes, it’s worth noting the risk it posed to life and property vs. the risk of terrorism.

D.C. is famous for snow panic. The threat of an inch of snow will send a crush of people to the grocery stores, which will soon be denuded of bread, milk and beer. The city’s largest snow disasters occurred in 1922, when 96 people died, crushed by the snow-stressed roof of the Knickerbocker Theatre, and in 1982, when Flight 90 foolishly attempted to fly a plane with snow-laden wings. It wound up in the Potomac with only a handful of survivors. Aside from that, the snow’s pretty benign, with daily death rates only slightly elevated from baseline values. In the latest terror panic, both New York and D.C. ran out of bottled water and duct tape. Like the situation with snow, this was caused by an obvious misperception of risk, which the federal government is doing little to dispel.

The major terrorist threats are from biological and radiological weapons. Of the former, the most likely are anthrax and smallpox.

Let’s start with anthrax. Very few people will die unless it is spread as an aerosol. But this requires specific meteorological conditions — a stable, calm atmosphere — that are generally hard to come by in the jet stream torn east during the winter. This probably explains why, in 2001, anthrax was mailed, killing few. If you’re concerned, talk to your doctor, get some doxycycline — there’s plenty around — or Cipro, or whatever appropriate antibiotic you tolerate well.

Smallpox? No amount of duct tape is going to keep you from running into the hot terrorist-on-the-street, but immunization will keep him from infecting you. And, as noted in the Feb. 19 Wall Street Journal, there’s plenty of vaccine. The odds of subsequent illness from the vaccine itself are around one-in-a-million, far less than your chance of getting killed in a car accident this year. Have you stopped driving yet? Also, many adults, vaccinated when smallpox was a real global threat, may still be immune. The strength of circulating antibodies can be determined by medical test.

A radiological bomb is easier to accomplish than a major biological attack. But, as in the case of biological weapons, the perceived threat is greater than the actual one, and stems from a profound lack of basic information about how such things work. While the facts aren’t hard to come by, it is hard to understand why the government (read: Homeland Security) has been so reluctant to be forthcoming.

There are three major types of radioactivity. The first, alpha particles, are stopped by a piece of paper. That makes them useless as a terror weapon. Beta emitters, slightly stronger, are stopped by a half-inch of Plexiglas. In a highly concentrated form and in the absence of some type of minimal shielding, however, beta emitters could at least cause terror. In reality, they’re common for medical imaging and tracing. That’s right — people eat these things or are injected with them, with no apparent harm, and with the long-term goal of improving their health.

The third type of radiation, gamma rays, is very high energy. They do kill cells at close range. Very close: Cesium-137 is implanted into men to control prostate cancer.

In bomb world, beta radiators are the easiest to get in quantity. But, because the dosage is low, whatever pile of beta rads that is worth exploding has to be a large, fairly obtrusive one. If it’s concentrated enough to do any damage, it will cause incapacitating radiation poisoning within a day to whoever puts it together. This problem becomes worse for gamma emitters. So how does one get enough together without incapacitating the terrorist? Good question.

Then there’s the problem of dispersal. The power of radiation, like the power of light, falls off as the square of the distance from the source. And an explosion disperses the radiation at the same proportion. In other words, enough to kill a person at close range, yet when exploded to a mere 100-feet radius the radiation is about one ten-thousandth as deadly as the un-exploded material, i.e. it’s not going to kill you! And, for a person about 100 feet away from that, dilute the dose by another factor of 10,000. The number of people killed in such incidents would likely be far less than the number that died in the snowstorms of 1922 and 1982.

Terrorists know the peculiarities of American culture and its propensity to over-react to media-hyped threats. All they have to do is watch what happens when it snows in D.C. So, we should all be asking, where is Homeland Security? Why are they so reluctant to tell us the reassuring truth?