Hair loss can be caused by a number of factors, including genetics, age, lack of hair care and medical conditions – but what exactly is the science behind it all? Why do we lose our hair?

Hair is pretty complex stuff. It's made up of two key components: shafts and follicles. The follicle is nestled within the scalp and each of us have around 100,000 of them on our heads. Each and every follicle grows, on average, twenty new hairs in its lifetime. But each follicle isn't active all of the time – hair growth goes through periods of growth and rest – the cycles depend very much on the individual. Also, the follicles are out of synchronisation with each other – when some are growing, others are not. If you were to pull a hair out of a follicle, it would bring on the growth of a new one. Hair's made of keratin and comprises three layers - the medulla, the cortex and the cuticle, and it grows at a rate of about 6 inches per year.

So what's this all got to do with hair loss? Well, over time, the number of follicles we have on our heads decreases over time and with age. For men, it leaves common baldness a lot of the time particularly at the frontal/parietal scalp, the top (vertex) of the scalp and bi-temporal areas.  For women, a general thinning of the hair develops, usually at the frontal scalp, though the actual hairline often remains intact.

Here's the complicated bit: Androgen is a type of male hormone and is key to the production of hair. If Androgen reacts with a certain type of enzyme – 5 Alpha Reductase – an increase is noted in the levels of Dihydrotestosterone – otherwise known as DLT. DLT shrinks follicles – after a while, hair can't grow from them at all. With each hair cycle, the hair grows shorter and the diameter of each hair reduces. DHT is blamed for 95% of hair loss cases. This is the most common cause of hair loss and is usually referred to as androgenetic alopecia.

There are treatments out there to slow down the science at work. Finasteride is one, but it only works for men as it can have negative side effects on a woman's fertility. It works to block DHT and the production of androgen for the duration of the treatment – but it's not a miracle cure.  Women can try Minoxidil at 2% dosage. When applied to the scalp it causes a notable increase in hair growth.

Another cause of hair loss is telogen effluvium – considerable shedding of hair that often occurs after the body has been severely affected by a high level of stress. The hair follicles move en masse from the active to the resting stage of their cycle. After several months pass and the hair moves back into its normal pattern, the resting hairs are shed pretty much simultaneously, as new hair growth displaces them. Hair loss occurs across the scalp rather than in specific locations. The kind of stress known to cause this hair disorder is usually associated with one of the following: severe infection; childbirth; medications; high fever; thyroid abnormalities; malignancy, major surgery; very poor diet or nutritional/protein deficiencies. 

Alopecia areata is an autoimmune disorder that affects over one million people in the United Kingdom alone.  This is where a person’s own bodily defences, the immune system represented by white blood cells, mistakenly attack hair follicles, halting their growth. Alopecia areata often begins with small, smooth  round bald patches anywhere on the scalp.  Untreated, it can lead to complete hair loss (alopecia totalis) or even total loss of body hair (alopecia universalis).

Further reading:

http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair
http://ezinearticles.com/?The-Effects-of-DHT---Hair-Loss-and-How-to-Stop-It&id=1296545
http://knol.google.com/k/bryan-cho/common-causes-of-hair-loss-from-the/Lq59COk7/riVSLQ#