Hair Loss Advice


Biology of Hair Loss -Learn more about hair loss and what causes it


The average human scalp is covered by 100,000 hair follicles.

If you've got all your hair, that may seem like a big number but when you start to lose it, the fate of each follicle becomes more important. What makes hair fall out? Join our two experts as they cast some light on the issue.

Webcast Transcript

biology hair loss DAVID FOLK THOMAS: The average human scalp is covered by 100,000 hair follicles.

Now, if you've got all your hair, that may seem like a big number, but when you start to lose it, the fate of each follicle becomes more important. What decides that fate and what makes hair grow or fall out?

Here to shed some light on the issue are two experts in the field. On my left is Dr. Angela Christiano.

She's an associate professor of dermatology and genetics at Columbia University, and next to Angela is Dr. Animesh Sinha. He's an assistant professor in the department of dermatology at Weill Medical College of Cornell University and New York Hospital here in New York City.

Why do we have hair? What's all this about?

ANIMESH SINHA, MD, PhD: Human hair is not absolutely vital for survival, but it does play several important functions. Perhaps the most important function is its protective role in protecting us from heat loss, so it's a heat insulator.

As well, human hair generally protects the skin from minor abrasions or cuts and against ultraviolet radiation from the sun, perhaps. The eyelashes and eyebrows are protective to the eyes, and hair in the ear canal or the nasal passages can help filter out particles and pathogens and protect our internal organs.

Additionally, hair is a tactile organ. There are many free nerve endings associated with hair follicles and they're sensitive to light touch. Finally, hair is a marker for identity.

It serves for social communication as well as for sexual attraction, and we know these are important issues.

DAVID FOLK THOMAS: You can allow your hair to grow and style it and do different things.

ANIMESH SINHA, MD, PhD: Sure. Our hair is a big part of our identity, what we're given naturally, and also the way we groom it, so these are important functions for daily life.

DAVID FOLK THOMAS: What exactly is hair made out of?

ANIMESH SINHA, MD, PhD: Hair is actually part of a follicular unit, and we can think of the follicular unit as a stocking-like invagination that comes down from the surface of the skin or the epithilium, and it's broken down, technically, into three basic parts: a lower part called the bulb, where the hair shaft originates from; a middle part called the isthmus, one region of which is probably the source of the stem cells or regenerative cells for the hair follicle; and the top part is the infundibulum, where the hair shaft exits the hair follicle through to the surface externally.

DAVID FOLK THOMAS: You talked about the phases of the hair cycle. What are those?

ANGELA CHRISTIANO, PhD: Hair follicles are unique in human bodies, I think, because they're one of the only organs that actually undergoes a cycle, that goes through a cycling through sort of an immature phase, a growing phase and then a resting phase.

So in the hair cycle those are called anagen, that's the growing phase; catagen is the transition between growing and resting, and telogen is when your hair has pretty much stopped growing, it's just sitting there waiting to fall out.

So usually you lose between 50 and 100 hairs a day normally, and you don't notice that, because they fall out in a random pattern. But when you start suffering from hair loss, as you mentioned earlier, each one of those becomes important.

DAVID FOLK THOMAS: Now, Ani, specifically, what is happening in those cycles that Angela just mentioned?

ANIMESH SINHA, MD, PhD: I think Angela said it very nicely, that what's unique is that hair continues to cycle -- there may be evolutionary reasons for that -- and the predominant number of hairs on our body are in anagen phase or growing phase.

That lasts anywhere from two to five years, and 85 to 90% of the hairs on our body are in that phase.

After that point they undergo a short regression phase, or catagen phase, and that lasts two or three weeks, and about 1% of our hair follicles are in that phase.

Finally, there's the resting phase, or the telogen phase, which accounts for about 10 to 15% of follicles and lasts about 100 days or 150 days. Over the course of the lifetime, a particular follicle might undergo 10 to 20 of these anagen-catagen-telogen cycles and keep cycling in this manner.

DAVID FOLK THOMAS: So at any given time you're actually having new hairs growing into your scalp for the first time -- a new cycle of the hair, if you will.

ANIMESH SINHA, MD, PhD: Yes, in some percentage of follicles.

DAVID FOLK THOMAS: And as I mentioned, I have 100,000 hairs up here right now? Is that right?

ANGELA CHRISTIANO, PhD: You look pretty good.

DAVID FOLK THOMAS: In the course of this I might lose one of them, based on the 50 per day, if we extrapolate that out? There it goes.

ANIMESH SINHA, MD, PhD: Right. The average daily hair loss is about 50 to 100 per day. Now, if there's a consistent loss of more than 150 hairs per day, if that's persistent and consistent over some time, then that might push us into the disease state, temporary or otherwise, and that would be termed alopecia.

DAVID FOLK THOMAS: How long can hair grow? For men, I guess we cut it at a certain point, but is there a limit if I decided not to get a hair cut, how long it could grow, and then will it stop eventually?

ANGELA CHRISTIANO, PhD: You do hear people say, "My hair only grows to this length, and never any longer," and then you hear people who say, "I once grew my hair down to my ankles, and it could have kept going." It's believed that the length of anagen, the length of the growth phase, is actually different in different people, and we don't know exactly what might determine that, although of course we believe it's genetic. DAVID FOLK THOMAS: If anagen can be -- is it six to ten years, usually, for that stage?

ANIMESH SINHA, MD, PhD: It varies, anywhere from two to maybe even ten years, and that's precisely that the length of hair is determined by the length of time that the hair follicle is in anagen or growing phase.

DAVID FOLK THOMAS: So does everybody's hair grow at basically the same rate, and then if that's the case, then if you have a shorter anagen phase that therefore you would only be able to grow your hair to a certain length, somebody with a longer phase?

ANIMESH SINHA, MD, PhD: In general, that is true, that the rate, the actual rate of the hair growth, is pretty constant -- anywhere between 0.3 to 0.5, so a third to a half a millimeter a day, and that works out to about a centimeter a month or six to seven inches per year, and that varies a little bit, depending on the site.

For example, the hairs grow a little faster on the top of our scalp versus the beard region, but that's pretty uniform. And then what determines the length of hair, again, as we talked about, is the length of anagen, and a number of factors, including genetic factors, related to age, race, sometimes medications, even seasonal variation -- all these factors affect the length of anagen and affect how long the hair will grow.

DAVID FOLK THOMAS: Can anything affect the rate of the hair cycle, any outside influences or internal influences?

ANIMESH SINHA, MD, PhD: Again, certain medications or hormonal factors might contribute to a decreased or increased anagen phase, and perhaps irreparably stop hair growth, and there are certain diseases associated with that.

Again, seasonal variations -- usually in the late summer, early fall is the shortest anagen time -- and that might make sense, because we might need a little less hair in the summer months and more in the winter months.

DAVID FOLK THOMAS: Just to wrap things up, you see people with straight hair, like myself, others very curly hair. What determines what's going on once it leaves the follicle?

ANIMESH SINHA, MD, PhD: The shape of the hair, whether it's straight or curly, is dependent on the shape of the follicle itself, so when the follicle is circular or round, the hair is straight. When the follicle looked in cross-section is oval or elliptical, that results in a curly hair at the surface.

Biology of Hair Loss