Football—the U.S. kind, played by physical giants—is a cold-weather sport. Some of the most memorable games of all time have been played with snow falling on ice-covered fields. But long before temperatures dip, athletes—some of whom weigh well over 135 kilograms—sweat it out in summer training camps where on-field temperatures often exceed 32 ºC.

These conditions test the endurance of behemoths getting into shape by doing sprint drills (aptly named suicides), tackling each other, and practicing plays over and over again. The time between two torturous workout sessions a day is frequently spent tending to bodies suffering from cramps and dehydration as they near the point of heat exhaustion [see photo, "Too Hot"].

In 2003, this annual hazing proved too much for Korey Stringer, a player on the National Football League's Minnesota Vikings. By the time the 167-kg player collapsed from heatstroke, his core body temperature had reached 42.7 °C. He never regained consciousness.

Since then, several pro and college teams have begun issuing "radio pills" to players who they think might be at risk for heatstroke. Once swallowed, the multivitamin-size pill acts as an internal thermometer, providing

continuous readings of a player's body temperature, which can be picked up by a sensor placed against the small of the player's back. Players take the pills a couple of hours before the start of practice, allowing the capsules time to reach an athlete's small intestine, where core body temperature readings accurate to within 0.1 °C can be taken.

A year after the Vikings player died, Philadelphia Eagles player Tra Thomas was saved from a similar fate during summer training camp when a radio pill reported that he had a core body temperature of 40.9 °C and trainers pulled him off the field. "He hadn't shown any signs of heat stress," said Derek Boyko, the Eagles' director of football media services. "Who knows if, without the device, the training staff would have known he was in danger before it was too late."

The radio pill, part of the CorTemp Physiological Monitoring System manufactured by Palmetto, Fla.­based HQ Inc., relies on a temperature-sensitive quartz crystal oscillator whose vibration frequencies are well known for temperatures ranging from ­60 °C to 150 °C. For instance, the crystal oscillates at 262.25 kilohertz at the normal body temperature of 37 °C. The electronic components calculate the temperature and transmit the data as a digital signal. Power comes from a silver oxide hearing aid battery that holds enough energy for nine days of temperature readings. The capsule remains in the body for only 24 to 36 hours before it is eliminated.

The temperature readings are transmitted wirelessly to a handheld receiver­data recorder. As the digital signal induces a voltage on the pill's communication coils, this voltage creates a quasistatic magnetic field with a radius of about a meter. When a coach or trainer holds the receiver to the small of a player's back, a magnetic coupling between the pill and the receiver induces a voltage in the handheld device's antenna, which is then demodulated to retrieve the original temperature data.

Because magnetic communication does not generate a propagating wave and there is strong attenuation of the signal with distance, the data are hard to intercept and virtually free from interference—even if there are dozens of other players running around the practice field with radio pills in their guts. Creating such a magnetic communication bubble also requires very little power, which allowed the radio pill's designers to use the tiniest of commercial batteries.

The technology was originally developed in the mid-1980s by NASA so the space agency could monitor the body temperatures of astronauts on the Space Shuttle. For instance, when former Mercury astronaut and retired U.S. Senator John Glenn returned to space in 1998 at age 77 aboard the Space Shuttle Discovery, a radio pill continually monitored his internal temperature.

HQ acquired a license to use the technology from the Johns Hopkins Applied Physics Laboratory in the 1990s as part of a NASA technology transfer program and began refining it for use in medical and industrial research. Bill Hicks, president of HQ, says the product has "proven itself as a diagnostic tool with which teams can determine whether their athletes are in danger."

The company is now branching out, marketing its temperature-sensing technology for use in applications including military clothing. Sensors would make it easier for commanders in the field to know when heat stress is limiting their soldiers' effectiveness.

Hicks wouldn't comment on whether the U.S. military has any plans to use the technology in Iraq, where daytime temperatures regularly soar above 50 °C. Six U.S. soldiers and one British soldier have died from heat-related illness since the conflict in Iraq began, according to iCasualties.org, a Web site that monitors combat deaths there.

The CorTemp system is also being aimed at monitoring another type of roasting. The device is helping food companies test their products in order to learn, say, exactly how much heat a hot dog can tolerate before it becomes overdone and leathery. It seems there really is a pill for everything.