DEAR DOCTOR K:

Growing up, I was taught that 98.6 was the standard "normal" body temperature. I think we all learn this as kids. But I find my own personal normal temperature is a bit lower. I hover around 97.9 degrees, even when I feel perfectly fine. Is there really such a thing as one normal body temperature?

DEAR READER:

It's a fact still taught to schoolchildren all over the world: Normal human body temperature is 98.6 degrees F. More recent studies actually put "normal" a little lower: around 98.2 degrees F. But as with most measurements, "normal" has a range.

Normal temperature was based on the average temperature of hundreds of people using mercury thermometers placed in the mouth. But in some countries, the thermometer was placed in the armpit, where the temperature is a little lower than in the mouth.

Wherever you place the thermometer, body temperature varies over the course of the day. It's low when you wake up, goes up by 1 or even 2 degrees during the day, and starts to dip in the early evening. If you're like me, it tracks your energy level.

In addition, different people have different "normal" baseline temperatures. Most people have temperatures that can range from 97.5 to 98.9 when they are healthy. About 5 percent of people have temperatures that are even lower or higher.

Body temperature also varies with your age. When you're an infant, it's relatively high. It starts to slowly drop until you reach middle age. Then it starts to rise again. When you reach your 80s, it can be as high as it was when you were a baby. Why this change with age? Don't ask me.

Women tend to have slightly higher normal temperatures than men. This may be true because the immune system of women is more active than the immune system of men. Body temperature is affected by chemicals (called cytokines) released when the immune system is activated.

Body temperature also varies with the menstrual cycle. About halfway through the cycle, the ovary releases an egg. Within 24 hours of the release, body temperature rises about a half to a full degree. Couples who do not use contraception and do not want to conceive a child often use this fact to determine the time of the month to avoid having sex.

The most common cause of higher-than-normal body temperature is infection. Almost any infection in the body can cause fever. Many doctors use 100 degrees F as a convenient cutoff for where fever begins. But it can be different for each person. There are other causes of a high temperature, too, including heat stroke or a drug reaction.

So your question is a good one, and here's the answer: The idea of one "normal" body temperature doesn't really hold up in actual practice. And having a body temperature of 97.9, as you do, is perfectly normal.

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.)

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