hours during exposure (as well as SPF lip balms to
protect the lips). Dermatologists recommend sun-
screens that block both UVA and UVB lightwaves.
Because both sunscreen use and skin cancer are
on the rise, some researchers have questioned
whether sunscreens cause, rather than prevent,
skin cancer. Though there are few clinical studies
of such a correlation, so far there is no evidence to
support this concern. Nor is there evidence to sup-
port claims that sunscreens promote estrogenic
activity in the body, another concern that some
people have raised. Health experts agree that
proper application of sunscreen is the most effec-
tive defense to protect the skin from damage.
Time of ExposureThe sun’s ultraviolet light is most intense from 10
a.m. to 2 p.m. in the United States. Health experts
recommend staying out of the sun as much as
possible during that period of time, especially dur-
ing summer months. When this is not practical,
dermatologists recommend combining protective
clothing and sunscreen for maximum protection.
See also CANCER RISK FACTORS.
sweat glands Structures within the dermis layer
of the SKINthat produce sweat as part of the body’s
temperature-regulation mechanisms. There are
two kinds of sweat glands—eccrine and apocrine,
both of which arise from the dermis.
Eccrine sweat glands are functional from
shortly after birth and are present in all skin. An
individual has between two and three million
eccrine sweat glands that produce about 20 liters
of sweat in 24 hours and can double or triple their
production rate during strenuous exercise or heat
conditions. Eccrine sweat glands open through
pores directly onto the surface of the skin (pores),
releasing sweat for rapid evaporation to cool the
skin and lower body temperature.
Apocrine sweat glands are present only under
the arms and in the pubic region, though are
abundant in these regions. Although present from
birth, they are nonfunctional until PUBERTY acti-
vates them. The apocrine sweat glands empty into
HAIRfollicles rather than directly onto the skin’s
surface. The sweat the apocrine glands produce
contains lipids and proteins, which helps the
sweat mix with the sebaceous fluids in the hair
follicles to reach the skin’s surface. BACTERIAon
the surface of the skin consume the lipids and
proteins, creating waste byproducts that pro-
duce the characteristic odor associated with sweat-
ing.
For further discussion of the sweat glands
within the context of integumentary structure and
function please see the overview section “The
Integumentary System.”
See alsoHEAT EXHAUSTION; HEAT STROKE; HYPER-
HIDROSIS; MILIARIA.202 The Integumentary System