By the age of 40, almost half the elastin present at birth is lost, resulting in lines, wrinkles and thin, sagging skin. The dermis houses the hair follicles, from which new hair is made and sensory nerve receptors, responsible for the sense of touch, heat, cold and pain. It is also rich in tiny blood and lymph vessels to provide oxygen and nutrients for cell growth and repair and removes wastes.
Sweat and oil glands
Within the dermis are the sweat and sebaceous glands, whose secretions reach the surface through tiny openings or pores. Sweat glands mainly regulate temperature.
Sebaceous glands produce an oily secretion called sebum, which lubricates and waterproofs the skin and helps prevent moisture loss. Sebaceous glands are most concentrated on the scalp and face - particularly around the nose, cheeks, chin and forehead; hence these are usually the oiliest areas. Sebum production can increase temporarily during stress, before your period and around the menopause. As we get older we tend to produce less sebum and the skin becomes drier.
The acid mantle
Skin and hair are protected by a thin sticky fluid, the acid mantle formed from sebum, sweat and acidic secretions produced by skin 'good' bacteria such as Staphylococcus epidermis. The acid mantle created a hostile environment for 'bad' bacteria, which prefer an alkaline environment.
Any disruption to the acid mantle - e.g. : excess sunlight, poor diet, alkaline skin products and harsh soap - interferes with the protective outer layer of the skin and strips away the acid mantle. This leads to dehydration, roughness and irritation. Skin is left defenceless and susceptible to microbial invasion.
The normal acid mantle for skin and hair has a pH between 4.0 and 5.9. Once the pH rises above 6.5, bacterial invasion increases dramatically. Excess sunlight, poor diet, excessive sweating and strongly alkaline skin/hair products, soap or detergents can easily strip away the acid mantle. Citric and lactic acid are often incorporated into personal care products to help lower their pH and make them 'pH balanced'.