NEW YORK (Reuters Health) - Infrared thermometers that take a person's temperature with a sweep of the forehead may not be as accurate as old-fashioned measures, a study suggests.

The devices, known as temporal thermometers or temporal scanners, have become popular with doctors, parents and athletic trainers alike because they are quick and easy to use. The thermometers use an infrared scanner to gauge the heat radiating from blood vessels in the forehead, and use this to calculate the core body temperature.

However, Texas researchers found that the thermometers' readings were generally lower than volunteers' actual core body temperatures when they were under heat stress. The researchers measured actual core temperature with an ingested pill that gauges the temperature in the intestines.

The findings, reported in the journal Medicine & Science in Sports & Exercise, cast doubt on the ability of temporal scanners to diagnose fevers or heat-related illness.

"In my opinion, (people) should not use these thermometers," said study co-author Dr. Craig G. Crandall of the Institute for Exercise and Environmental Medicine at the Presbyterian Hospital of Dallas.

"The data simply don't support their accuracy," he told Reuters Health.

For their study, Crandall and his colleagues had 16 healthy adults wear special wetsuits that were gradually heated to raise their core body temperature. Their temperatures were repeatedly measured with a temporal scanner and with the ingested pill.

Before the suits were heated up, readings from the temporal scanner and pill generally matched up. But after 30 minutes of heat, readings from the scanner actually fell, despite the fact that volunteers' core temperature was climbing.

One concern, Crandall said, is that temporal scanners, used in the hospital or home, could miss many cases of fever.

Another is that athletes suffering from heat exhaustion will be misdiagnosed. Temporal scanners have been used in major sporting events, including the Boston Marathon, Crandall and his colleagues point out.

Some past studies have similarly raised doubts about the accuracy of infrared ear thermometers. Crandall said that he lacks confidence in both the ear and forehead measurements.

For the general public, the best way to gauge core temperature is through the old-fashioned rectal approach, according to Crandall -- though he acknowledged that most people old enough to refuse this method probably will.

Measurements taken under the tongue or in the armpit are "suitable" alternatives, he said.