We usually think of all of our hair as growing all the time, but actually it doesn't. At any given moment, about 90% percent of our hair is growing, which it does continuously for two to six years.

The other 10% of the hair is "resting", as it's called. This rest period lasts for two or three months.

After resting, a hair falls out; about 100 to 150 of them are lost each day. That may sound like a lot, but remember that a full head of hair consists of around 100,000 individual hairs.

After the hair falls out of a given hair follicle, a new hair starts growing there, as the cycle begins all over again. In other words, everyone is always losing and replacing hairs.

Which hairs are growing and which are resting or have just fallen out, changes all the time.

Because it's always true that the great majority of our healthy hairs are growing, it seems as if they all are. On the other hand, the fact that we may find some hairs in the shower or on our comb or brush doesn't automatically mean that we're going bald.

When we talk about hair loss, we don't mean this daily shedding of a few hairs, whose replacements start growing again right away.

Hair loss is what happens when not all of the hair being shed is replaced, so that the total number of hairs starts to decline. Hair loss can be either temporary or permanent.

Temporary hair loss can be caused by stress (sometimes in a delayed reaction- For example, people have been known to lose hair temporarily several months after major surgery), scalp infections or some other diseases (for example, diabetes), hormonal changes (including those associated with pregnancy and childbirth), use of inappropriate hair products, side effects from medication (including chemotherapy), and malnutrition. There are also some hair styles that involve pulling on the hair that can damage the scalp. This damage is temporary if the pulling is stopped in time, but can become permanent if the pulling goes on long enough to cause scarring of the scalp.

Temporary hair loss can be reversed if the underlying cause is ended, which of course is why we call it temporary.

What we usually think of as permanent hair loss, "baldness" as the word is commonly used, is known scientifically as "androgenetic allopecia". In normal English, this is "male-pattern baldness".

Male-pattern baldness is widely believed to have a genetic basis. It usually manifests itself in a receding hairline and in baldness on top of the head, and tends to be more extensive the earlier in life it starts.

There is a more rare condition, "female-pattern baldness", that usually results in thinner hair all over the scalp. Evidence suggests that an excess of a chemical called dihydrotestosterone (DHT) is associated with male-pattern baldness, shrinking the hair follicles and causing the hairs gradually to grow thinner and thinner, until they stop growing at all. Female-pattern baldness appears to have different causes, and to need treatment by different products.

Because serious hair loss is a much more common problem for men than for women, most hair loss products (including all top herbal hair loss products) work by trying to reduce production of DHT; trying to prevent existing DHT from having an effect by preventing it from binding to androgen receptors; reducing inflammation of the scalp that may inhibit hair growth; and stimulating new growth. In other words, the products may deal with temporary and permanent causes of new baldness, and with reversing hair loss that already has started.