Re: Can uv b rays pass through a shirt?

Dear Cathy,

Thank you for your questions, and I'm glad that you are concerned
with the effects of the Sun's rays.  Even though it used to be
thought that getting a tan was healthy, we now know that exposure
to the Sun can cause skin cancer (as well as wrinkles!) Most of 
this damage is done by UV-B rays (290 to 320 nanometers in
wavelength.  The human eye sees light from about 400 (violet) to 
700 (red) nanometers in wavelength, so these UV (or ultraviolet) 
rays are compeletely invisible.  The warmth you feel from the Sun is also
invisible, and comes from the Sun's infrared radiation (wavelengths longer 
than 1000 nanometers).  So how bright sunlight looks or or how warm the Sun 
feels won't necessarily tell you how quick you'll get a sunburn.  
However, it's a general rule that if you can see your shadow 
clearly, you are at risk for sun damage.

The quick answer to your question is that it depends on what your 
shirt is made out of and how loosely woven it is!   As far as
I could find out, both UV-A and UV-B rays would be blocked
by very tightly woven clothing.  I'm sure that you've 
noticed that you don't get a suntan underneath a bathing suit, which
is usually made of a tightly woven material.   
On the other hand, clothing with a lot of holes will
let through both UV-A and UV-B rays because sunlight
can travel directly to your skin through the holes.

Don't forget also to wear sunglasses to protect your eyes from
UV-ray damage.  And a wide-brimmed hat is also a very good idea
to protect your much of your face, including your ears and neck.

For more details on ways to protect yourself from the Sun, keep reading!

Susan Stolovy
----------------------------------------------------------------------
 
Here are some more details from J.W. Haefemeyer M.D., M.S.
(see  http://www.albinism.org/publications/sun-protection.html)

Up to 50% of the ultraviolet rays can go through wet lightweight cotton 
tee shirts worn for swimming. Colored clothing and denser-woven clothing 
allow less light penetration. Several manufacturers are promoting densely 
woven "breathable" synthetic fiber clothes for sun protection. These 
allow protection with long sleeves in warm weather. With time this type 
of clothing may become less expensive and more common.

     Latitude: A person who can tolerate one hour of sun in Florida
without burning can tolerate two hours of sun in New Jersey under
     the same conditions.
     Season: The greatest intensity of ultraviolet light occurs at the
summer solstice, about June 22. May 1 has as much intensity as
     August 15.
     Altitude: Each 1000-foot increase in altitude adds 4% to the
intensity of the sunburning rays. The intensity of sunlight at 5000 feet
     is about 20% greater than at sea level.
     Weather: A bright day with a thin cloud cover has 60 to 80% of the  
ultraviolet rays that are present on a clear day. Clouds can cool
     and give a false impression that there is little risk of sunburn. 
 
In addition to the UV forecast, consider your surroundings: Sand reflects
25% or more of ultraviolet rays, so that it is possible to get
sunburned while sitting in the shade on a beach. Fresh snow reflects 70
to 90% of ultraviolet rays. Water can also reflect UV. 
Reflected light may burn areas which are usually shaded, such as those 
under the
nose or chin. As much as 96% of ultraviolet rays can penetrate
clear water.
 


Ever wonder how sunscreens work?  Here's some information from:
http://www.healthanswers.com/database/ami/converted/001869.html#
Sunscreens act by providing a physical or chemical barrier to sunlight. 
Sunscreens that provide a physical barrier either reflect or scatter
light in the visible and ultraviolet (UV) range (290 to 700 nanometers 
(nm)) prevent penetration of the light into the skin where it may
cause damage to the skin. 

Chemical sunscreens act by absorbing ultraviolet light in the UVB range 
(290 to 320 nm). The UVB range is the range that is primarily
responsible for sunburning and causing skin cancer. The UVA range (320 to 
400 nm) is responsible for suntanning and photosensitivity
reactions (increased sensitivity to sunlight as the result of certain 
medications, cosmetics, soaps, or plants). UVA is also responsible for
serious skin damage. 

Sunscreen effectiveness is based on what wavelengths of the UV spectrum 
it covers, its concentration, what liquid the medication is
dissolved in, and what happens to it during swimming or sweating. 

Sunscreens absorb about 95 percent of the UVB light in the most dangerous 
wavelengths (290 to 320 nm). Suntanning products absorb
between 85 to 95 percent of the UVB light, allowing the tanning process 
to occur faster but also increasing the risk of burning. 

Sunless tanners that temporarily dye the skin do not protect the skin 
from sunburn unless sunscreen agents are also contained in the
product. If time in the sun is planned, a regular sunscreen product 
should be used.