ph (and the skin's acid mantle)

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skin  our skin is one of the best defenses against germs for several reasons. the top layers of cells are dry and densely packed. the dryness and close quarters of this first line of defense make it inhospitable to many bacteria. salty secretions from sweat glands create an environment that is hyperosmotic and thus discouraging to bacteria because the high salt concentrations pull water from inside the bacteria, dehydrating them.
bacteria  however, some bacteria do naturally associate with skin. rather than harming us these bacteria actually help protect us and are referred to as the microflora. first, colonization of the skin by harmless bacteria means that there are few resources available for pathogenic bacteria because the microflora outcompete the incoming pathogen. second, some resident microflora help lower the ph of skin.
ph  a slightly acidic ph (4-6) helps deter colonization by non-resident bacteria and pathogens because many bacteria can survive only in a narrow ph range near neutral. the acidic conditions of the skin are caused by secretions from sweat glands, skin oil, and the breakdown of fatty acids by staphylococcus epidermis. thus a resident microflora species is partly responsible for the acidic ph of skin.
the ph of the skin certainly differs throughout parts of the body; you would expect that regions with higher staphylococcus epidermis concentrations would be slightly more acidic. slight skin ph differences are also present in individuals because not everyone's skin is exposed to the same conditions such as weather and harsh detergents.
fyi: the initials "ph" stand for "potential of hydrogen" -- ph is [technically] the measure of hydrogen ion concentration. in lay terms, it's the measure of acidity / alkalinity. see the ph scale below.

sebum  sebum is an oily secretion produced by sebaceous glands, tiny ducts adjacent to hair follicles. sebum is secreted into the follicle, from which it spreads over the hair and skin. the main role of sebum is to waterproof the skin and hair. both excess and lack of sebum are undesirable. excess sebum is associated with oily skin and acne. it is particularly common in adolescents as the increased levels of sex hormones stimulate sebum production. lack of sebum, which is common in middle and older age, leads to skin dryness and accelerates wrinkle formation.
sweat  sweat is a salty, watery solution produced by sweat glands, numerous microscopic channels opening onto the skin surface. as sebum and sweat mix up on the skin surface, they form a protective layer often referred to as the acid mantle. the acid mantle has a particular level of acidity characterized by ph from about 4 to 5.5. a ph of 7 is considered neutral, above 7 is alkaline, and below is acidic. (the ph of acid in the human stomach, for example, is usually from 1 to 2, which is highly acidic. the skin, on the other hand, is mildly acidic.)
in addition to helping protect skin from "the elements" (such as wind or pollutants), the acid mantle also inhibits the growth of harmful bacteria and fungi. if the acid mantle is disrupted or loses its acidity, the skin becomes more prone to damage and infection. the loss of the acid mantle is one of the side effects of washing the skin with harsh soaps or detergents.
the ph scale  as shown in the chart below, each increase in ph by one level means a tenfold decrease in the concentration of hydrogen ions. therefore, the change is significantly greater from level 1 to level 0 than the change from level 7 to level 6. conversely, each decrease in ph by one level means a tenfold increase in the concentration of hydrogen ions. (In other words, although battery acid is only one level from stomach acid, it is considerably stronger than stomach acid as opposed to the difference between distilled water and saliva.)
hydrogen ion
acidic  100000000sulphuric / battery acid
  10000001hydrochloric / stomach acid
  1000002lemon juice / vinegar
  100003grapefruit / orange juice
  10004tomato juice
  1005[black] coffee
  106saliva / milk (6.7)
neutral  17distilled water
  0.18sea water
  0.019baking soda
  0.00110milk of magnesia
  0.0000112soapy water
  0.00000113bleach / oven cleaner
alkaline  0.000000114caustic lye