Harvard Health Letter

Body heat: Older is colder

The normal temperature of 98.6° F is anything but, especially as we get older.

Normal, when it comes to body temperature, is 98.6° F, right?

Not really. The 98.6° F benchmark comes to us from Dr. Carl Wunderlich, a 19th-century German physician. In a precomputer tour de force of data collection and analysis, Dr. Wunderlich tabulated over a million axillary (armpit) temperatures for 25,000 patients and arrived at 98.6° F as the average.

Some of Wunderlich's observations have stood up. Researchers have confirmed that body temperatures do climb a degree or so during the course of a day. But his definition of normal has been debunked. An oft-cited 1992 study in the Journal of the American Medical Association (JAMA) pegged the average normal temperature at 98.2° F, not 98.6° F.

By Wunderlich's reckoning, temperatures of above 100.4° F were beyond the upper limit of the normal range and therefore indicated fever. The JAMA paper authors proposed abandoning that benchmark too, and replacing it with thresholds based on the time of day. In the early morning, temperatures above 98.9° F would indicate fever. Later in the day, the tip-off temp would be 99.9° F. Authoritative textbooks, such as Harrison's Principles of Internal Medicine, followed their suggestion and define fever in this way.

One limitation of the JAMA study was that it included only people from ages 18 to 40. Doctors have long known that children tend to have higher temperatures than adults because young metabolisms run a little faster. Similarly, they've known that the temperature of older adults is a bit chilly.

Taking a temperature

Our bodies don't have a uniform temperature. Because we lose heat through the skin, areas closer to the surface of the body and further from the center can be several degrees cooler than the interior, "core" parts.

Traditionally, rectal temperature has been the most reliable way to assess core body temperature. Oral temperatures taken in the traditional way, with the tip of the thermometer under the tongue, are about 1° F lower than rectal temperatures. They can be unreliable, thrown off by mouth breathing and recent eating or drinking.

Ear thermometers are especially popular with parents. They measure the temperature of the eardrum (tympanic membrane), a reading that is supposed to approximate the body's core temperature. But like oral temperatures, readings for ear thermometers tend to be lower (1.6° F lower in some studies) than rectal temperatures. Ear thermometers can be set so they automatically convert the ear reading to the equivalent of the oral reading. Getting an accurate measurement can be tricky. The thermometer's sensor must be aimed right at the eardrum. Otherwise the reading is influenced by cooler parts of the ear canal. Sometimes earwax gets in the way.

Hospitals are getting away from taking rectal temperatures. Patients never liked the experience, and there's concern that the practice could spread infectious disease. Instead, doctors and nurses are using a thermometer that measures the temperature of the temporal artery in the head. They get a reading by lightly tracing the infrared sensor across the patient's forehead and then behind the ear. It takes seconds and is completely noninvasive. You can buy these thermometers for home use for about $50.

Just how chilly was the subject of a study published in the December 2005 Journal of the American Geriatric Society. A research team led by Dr. Irving H. Gomolin, chief of the Division of Geriatric Medicine at Winthrop University Hospital in Mineola, N.Y., measured the temperatures of 150 older people (average age, 80.7), 100 of whom lived in a nursing home. The researchers took oral temperatures of the nursing home residents at three different times (6 a.m., 4 p.m., and 10 p.m.) for three consecutive days and a single midday reading of the people living independently.

The average body temperature of the nursing home residents never reached 98.6° F. At 6 a.m., it was 97.3° F; at 4 p.m., just a tenth of a degree higher; and at 10 p.m., 97.8° F — still close to a full degree below 98.6° F. The results for people living independently were similar: The average midday temperature was 97.7° F.

Older people have lower temperatures for several reasons, including inactivity. A considerable amount of the energy used for exercise ends up elevating body temperature.

Why is this important?

When someone's body temperature is low to begin with, an increase may not reach fever levels, especially if you have Wunderlich's outdated standard of 100.4° F in mind. So older people can be quite ill even though they don't seem to be running a fever. Moreover, the fever response, like the overall immune response, weakens with age, so the body temperatures of older people may not increase all that much when they are ill.

The flip side is that doctors need to be on the lookout for temperatures below about 95° F, especially in older patients. A low body temperature can be a sign of illness too, and it's often not noticed by the patient.

As Dr. Gomolin and his colleagues noted, individual variations in body temperature need to be taken into account. Ideally, you and your doctor should have enough temperature measurements at various times of day to establish a baseline body temperature for you. Short of this, recognizing that the familiar 98.6° F isn't the grand benchmark that we've long believed it to be is a good place to start.