The human eye is sensitive to wavelengths
of light from about 400 nanometers, or nm (violet light) to
about 700 nm (red light). Light with a wavelength shorter
than 400 nm is called ultraviolet light or ultraviolet radiation.
A nanometer is 1/1,000,000,000 of a meter or 10-9
Ultraviolet light is often called ultraviolet radiation. The
word radiation does not refer to anything related
to radioactivity; it is the scientific term applied to all
forms of electromagnetic energy, including visible light,
infrared light, or ultraviolet light. These forms could equally
be referred to as visible radiation, infrared radiation, and
Ultraviolet light is produced by the Sun and penetrates into
the atmosphere. Ozone in the upper atmosphere completely absorbs
the shortest wavelength type UV light, called UVC. The UV
UVA 315-400 nm
UVB 290-315 nm
UVC 220-290 nm
UVB is partially absorbed by the atmosphere, but is very damaging
to cells. The longer wavelength UV A 360-400 nm is emitted
by so-called black lights." This light is not thought
to be particularly harmful, though wavelengths shorter than
black light emissions may be dangerous.
The amount of UVB light that reaches the ground depends on
the elevation of the Sun (the higher it is in the sky, the
more UVB), the amount of ozone in the atmosphere (more is
better), and the degree of cloudiness of the sky. Clouds must
be relatively thick to appreciably block UVB.
Because of the great potential for sunburn from ultraviolet
light (especially UVB) an ultraviolet index has been devised
to label UV energy exposure from the minimal to very high.
The UV Index is categorized by the Environmental Protection
Agency as follows:
|0 1 2
|7 8 9
|10 and Greater
The UV index can be calculated for local
noon (1 p.m. Daylight Savings Time)
For example, on a summer day in late August, the UV index
was 12, very high, for Honolulu, Hawaii and San Juan, Puerto
Rico. It was low, at a value of 1, for only Anchorage, Alaska.
Daily values for
major cities are given every day at the United
States Environmental Protection Agency's Ultraviolet Index
Some basic strategies for avoiding
Sun damage and aging to skin, as well as reducing the risk
of skin cancer are:
- Limit your time in the
sun between 10 a.m. and 4 p.m. These are the brightest times
of the day.
- Wear a wide-brimmed hat
and if possible, tightly woven, full-length clothing.
- Use a broad-spectrum sunscreen
with an SPF of at least 15.
- Seek shade whenever possible.
- Wear UV-protective sunglasses
(check the tag). Polycarbonate lenses block UV.
- Avoid sunlamps and tanning
- Check the UV Index for
your location daily.
Sun damage to skin is not hard to assess. Parts
of your body that are not exposed to the Sun have a very different
appearance and texture than parts that are exposed regularly