 |
 ph (and the skin's acid mantle)
|
|
|
|
| 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
concentration | ph
level | examples |
|
| acidic | 10000000 | 0 | sulphuric / battery acid |
 | 1000000 | 1 | hydrochloric / stomach acid |
| 100000 | 2 | lemon juice / vinegar |
| 10000 | 3 | grapefruit / orange juice |
| 1000 | 4 | tomato juice |
| 100 | 5 | [black] coffee |
| 10 | 6 | saliva / milk (6.7) |
| neutral | 1 | 7 | distilled water |
 | 0.1 | 8 | sea water |
| 0.01 | 9 | baking soda |
| 0.001 | 10 | milk of magnesia |
| 0.0001 | 11 | ammonia |
| 0.00001 | 12 | soapy water |
| 0.000001 | 13 | bleach / oven cleaner |
| alkaline | 0.0000001 | 14 | caustic lye |
|
 |
|
