Dear Dr. Komaroff: I am a 43-year-old woman whose hair has gone quite gray in the past year. I’ve had a fairly stressful year, and since the change in my hair felt quite sudden, I’m wondering if stress could have caused it. If not, what else might be the culprit?

Dear Reader: Each hair in your head keeps the color it has. If a single strand of hair starts out a certain color, it is never going to turn gray. If a hair is not gray, no amount of stress can turn that hair gray.

But stress definitely can affect hair color. It does so by influencing the color of new hairs. Each hair is produced by a single hair follicle. Hairs on the scalp typically live two to five years. When a person’s old brown hair dies, the hair follicle that produced the old hair now starts producing a new hair. After we reach age 35, the new hairs produced by a hair follicle tend to lose color, in comparison to the last hair produced by that follicle.

How does stress affect hair color? First, by causing hairs to fall out – to die young. Stress can cause hair to shed at about three times the rate it normally does. The hair grows back, so the condition doesn’t cause balding. But if you’re middle-age and your hair is falling out and regenerating more quickly because of stress, it’s possible that the hair that grows in will be gray instead of its original color.

Genetics also influences whether and when you turn gray. In fact, it’s probably a stronger cause of graying hair than stress. Look at past generations of your family. This will give you a better indication of when or if you’ll go gray than your current stress levels.