Swine flu, body heat, and airport scanners.

Science, technology, and life.
April 28 2009 10:22 PM

Heat Check

Swine flu, body heat, and airport scanners.

A thermal scanner. Click image to expand.
A thermal scanner

Six years ago, when Severe Acute Respiratory Syndrome swept the world, governments bought thermal-image scanners and set them up at airports. The idea was to spot overheated travelers and check them for fever before they infected others. Now the machines are being deployed again, this time to catch swine flu.

William Saletan William Saletan

Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

The World Health Organization thinks this is folly. According to WHO officials, the virus has already spread too far, and anyway, "Fever monitoring doesn't work because you don't get the cases which are still in incubation." For these and other reasons, dozens of thermal imagers in Canada and Australia are sitting unused.

That's a shame. Heat scanners certainly can't handle the current flu threat by themselves. But they can help.

A recent report from the Canadian government explains how the devices work: They "determine the temperature of an object by measuring the amount of infrared radiation emitted by that object; the higher the temperature, the more infrared radiation that is emitted." You can watch scanners in operation here and here. To see what a machine operator sees, check out the photos here, here, here, and here.

Canada seems chastened by a four-year-old study that found its scanners weren't cost-effective against SARS. But most of the imagers now being deployed are already paid for. They were bought during the SARS and bird flu outbreaks. Why not use them?

It's true that the current flu has already spread to four continents. But that's no reason to suspend vigilance. Air travel is the fastest, most effective way to transmit a contagious human-to-human disease. At any given time, half a million passengers are in the air. From Mexico alone, weekly air traffic is more than 1 million people. Every interception slows transmission and buys time to catch up with drugs and vaccines.

In fact, one reason why scanners didn't do more to stop SARS is that they were deployed too late. In Canada, they took weeks to acquire and set up. This time, countries already have them on hand and know how to use them. We have a head start. And we're wasting it.

The scanners have their flaws. As the Canadian report notes, they "measure the skin and not the core temperature." But they don't have to be perfect. They just have to spot the people worth pulling over for closer inspection. If you're flagged, the next step is a thermometer check, an exam for symptoms, and an interview about your medical history and where you've been traveling. (In Indonesia, you might be "sprayed with a 70 percent alcohol solution.") If you test positive for influenza A, you might be quarantined at a hospital till you recover. But in that case, your itinerary is the least of your worries.

It's also true that scanners don't catch non-feverish carriers. But alternative methods don't, either. Right now, U.S. airports are counting on Customs and Border Protection officers to spot visible symptoms. According to Homeland Security Secretary Janet Napolitano:

All persons entering the United States from a location of human infection of swine flu will be processed through all appropriate CBP protocols. Right now those are passive. That means that they're looking for people who—asking about, are you sick, have you been sick, and the like; and if so, then they can be referred over for further examination. Travelers who do present with symptoms, if and when encountered, will be isolated per established rules.

Looking for symptoms? Asking people whether they're sick? Come on. At least a heat scanner measures something quantifiable and catches more than the eye can see.

Skeptics at the WHO say border screening is disruptive. But scanners are far more efficient and less disruptive than labor-intensive alternatives. Earlier this week, Japan reported that officials, doctors, and nurses in that country were boarding flights to examine arriving passengers for flu symptoms. By comparison, the latest generation of thermal imagers can instantly scan travelers as they pass by.

If you think heat is a bad proxy for flu infection, ask yourself whether it's worse than nationality. Travel companies are canceling flights to Mexico. Today, Japan began denying visas to Mexicans on arrival. Governments and businesses want an easy way to identify, segregate, and scrutinize the people most likely to be carriers. Which group would you rather they target? People with excess body heat? Or Mexicans?

SARS and bird flu weren't the last plagues to spread across our planet. This flu won't be the last, either. Fortunately, all these viruses have one thing in common: fever. For now, late as it is, that heat signature is our best shot at catching them. Let's use it.

(Now playing at the Human Nature blog: 1. The mainstreaming of marijuana. 2) Emergency contraception and personal responsibility. 3. Outing executioners.)