Scalp hair loss which is also known as hair fall, thinning of hair, alopecia, baldness etc can be characterized in several forms due to different scalp conditions. There are multiple causes of hair loss including:

Gradual scalp hair thinning is caused through switching of active hair follicles from growth (anagen phase) to a resting phase (telogen phase) which gives rise to shorter and fewer in number hair. This age usually starts from 35 years onward.

Genetics of an individual also responsible for hair loss. If an individual has both paternal as well as maternal side baldness then there are more chances to develop baldness. This is also known as hereditary cause i.e. passing of the disorder from one generation to the next one through genes.

Diet or nutrition:
Limited intake of eatables and nutrients deficiencies also cause thinning of hair. These mainly include protein, biotin deficiencies; however excessive intake of animal fats and vitamin A is also responsible for hair loss to some extent.

Phototoxic effects of sunlight in which sunburn is caused due to sun exposure and its ultraviolet rays also cause hair loss.

Mineral deficiency:
Zinc and iron deficiency.

Medications for blood pressure, diabetes, cancer, heart disorders, cholesterol, steroids, acne medications, contraceptive pills, hormone replacement therapy etc. Traumas such as major surgery, heart attack, childbirth, poisoning severestress.

Both male and female hormones and their imbalance gives rise to hair loss. Menopause is also responsible for it.

Pregnancy and childbirth:
During pregnancy the hair are thicker due to high levels of oestrogens which decreased after child birth and causes actual baldness. Intake of fertility-stimulating drug like clomiphene also gives rise to hair loss similarly.

Water and air pollutants also cause hair loss.

Radiation to the scalp during radiotherapy against some cancer treatments also causes hair loss.

Spot baldness or Alopecia areata:
It is a disorder in which baldness or hair loss takes place from one location to other damaging every hair of the body resulting total scalp baldness (alopecia totalis) or entire body hair loss (alopecia universalis).

Both Hypothyroidism and Hyperthyroidism cause hair loss.

Excessive oily scalp:
Excessive oily scalp may gives shelter to a microscopic mite called Demodex folliculorum which found in hair follicles and fed on sebum excreted by sebaceous glands and causes thinning of hair.

Lack of exercise, labor, massage and comfortable lifestyle is also responsible for not having healthy hair follicles.

Excessive use of chemicals for washing, curling, bleaching, dyeing, straightening etc gives rise to hair loss too.