Ultraviolet
Radiation
Ultraviolet
Radiation
What is ultraviolet radiation?
Ultraviolet radiation, abbreviated as "UV radiation", is a form of non-ionizing
radiation.
Before you go on, I'm not sure I really understand the difference between
ionizing and non-ionizing radiation. Can you give me a hand here?
Sure. To make a long story short, non-ionizing radiation does not have enough
energy to ionize atoms. However, it can cause molecules to vibrate and rotate,
which means things will start heating up. Typically, the different types
of non-ionizing radiation are classified by frequency, which is stated in
units of "hertz", abbreviated as "Hz".
What are these classifications?
These include ultraviolet radiation, visible light, infrared radiation
(abbreviated as "IR"), radio- frequency (abbreviated as "RF") and microwave
radiation, and ultrasound. In addition, lasers use many of these frequencies
to form coherent light. All of these are electromagnetic radiations.
What's the difference between ultraviolet radiation and the other forms
of non-ionizing radiation?
The region of the electromagnetic spectrum just below that of visible light
is called the "near ultraviolet" region. This type of radiation is absorbed
(blocked) very strongly by most solid substances, and even absorbed appreciably
in air. The shorter UV wavelengths can reach the ionization energy for certain
types of molecules, so that region, termed the "far ultraviolet" region,
may trigger similar considerations as other ionizing radiations.
So what does UV radiation do?
Generally speaking, and depending upon the amount of UV radiation that we
are exposed to, it can burn the skin. However, there are also some therapeutic
effects as well.
Is ultraviolet radiation able to penetrate solid surfaces like ionizing
radiation?
Well, near ultraviolet radiation is absorbed very strongly in the surface
layer of the skin by electron transitions. But you need to have much higher
energies in order to trigger the photoionization process. When this occurs,
there can indeed be penetration into surfaces.
I understand that ozone can protect us from the harmful effects of UV
radiation. Is that correct?
Absolutely. The ozone layer in the upper atmosphere is important for human
health because it absorbs most of the harmful ultraviolet radiation from
the sun before it reaches the surface. However, the higher frequencies in
the ultraviolet region can still pass through the ozone layer and produce
harmful physiological effects ranging from sunburn to skin cancer.
Was ultraviolet radiation discovered at the same time as forms of ionizing
radiation, like x- rays?
No. Scientists were measuring the effects of ultraviolet radiation long before
they discovered x- rays and gamma rays.
How far back?
Ionizing radiation was discovered in the late 1800s by scientists like Roentgen
and Curie. However, as far back as 1666 Sir Isaac Newton allowed a sunbeam
to enter into a darkened room through a small hole, pass through a prism,
and fall upon the opposite wall. What appeared was a spectrum of very vivid
and intense colors. By means of further experiments he was able to prove
that white light was made up of several different components.
But that wasn't UV light, was it?
No. However, in 1800, Sir William Hershel discovered that the solar spectrum
extended out beyond the portion which was visible to the human eye. He formed
a spectrum much like Sir Isaac did, then passed a thermometer through the
various colors. He then compared the reading with that of a similar thermometer
shaded from the spectrum. What he found was a temperature rise as he passed
from the violet to the red end. Still more astounding was the fact that the
thermometer showed a higher temperature in the dark space beyond the red
region of the spectrum than it did in any part of the visible spectrum.
What did this mean?
This meant that there was some intense radiation beyond the region which
was visible to the eye. This invisible radiation beyond the red end of the
spectrum is now known as infrared radiation.
Interesting. So is that how UV radiation was first classified?
No. It wasn't until 1801 that a scientist known as J.W. Ritter investigated
the other end of the spectrum. He showed that chemical action was caused
by some form of energy in the dark portion beyond the violet. At that point
in time, the region became known as the "ultraviolet" region.
Tell me where I can find ultraviolet radiation.
There are both natural and artificial sources of UV radiation. Although there
are many artificial sources of this energy, sunlight is the only natural
source.
Tell me a bit more about sunlight.
The sun emits a wide variety of electromagnetic radiation, including infrared,
visible, and what is termed "ultraviolet A" radiation (UVA; 320 to 400 nm),
"ultraviolet B" radiation (UVB; 290 to 320 nm), and "ultraviolet C" radiation
(UVC; 10 to 290 nm). Blue visible light lies in the range of about 400-500
nm. The only UV wavelengths that reach the Earth's surface are UVA and UVB,
with UVA radiation being 1,000-fold less effective than UVB in producing
skin redness. However, its predominance in the solar energy reaching the
Earth's surface (tenfold to one hundredfold more than UVB) permits UVA to
play a far more important role in contributing to the harmful effects of
sun exposure than previously suspected.
Can I run into UVB at any time?
Yes, whenever the sun is out. However, the intensity of UVB is typically
at its highest when the sun is directly overhead (i.e., between 10:00 a.m.
and 2:00 p.m.). On the other hand, UV-B is attenuated (reduced) by the ozone
layer to the extent that reductions of a factor of 10 at 9:00 a.m. and 3:00
p.m. are observed, relative to the intensity at noon.
Does all UV radiation affect the body in the same manner, regardless of
wavelength?
The UV light spectrum can be subdivided into three subsections, based upon
the biological effects identified within each. These are the regions I told
you about previously, meaning the UVA, the UVB and the UVC regions.
What will the UVA region do?
The UVA region is more commonly known as the "black light region". You have
probably run into this kind of radiation in amusement park rides where UVA
is used to enhance visual thrills. There are no real health effects associated
with exposure to UVA.
What about UVB radiation?
Light in the UVB region will cause skin to tan. However, if you're not careful,
it can also cause serious sun burns.
And UVC radiation?
The UVC region is sometimes called the "germicidal region". This is because
UVC light is sometimes used for sterilization.
So, which of these wavelengths can I see with my eyes?
All UV radiation, regardless of its wavelength, is invisible to the eye.
The only way to measure the intensity of UV radiation is to use a calibrated
survey instrument.
You said that UV is created by the sun. Is the intensity of radiation
the same from one city to the next?
Sunlight is definitely the greatest source of human UV radiation exposure,
affecting pretty much everyone. The extent of an individual's exposure, however,
varies widely depending on a multiplicity of factors such as clothing,
occupation, lifestyle, age, and geographic factors such as altitude and latitude.
There is greater UV exposure with decreasing latitude. Residing at higher
altitude results in a greater UV exposure such that for every 1,000 feet
above sea level, there is a compounded four (4) percent increase in UV exposure.
However, as I said earlier, UV exposure increases with decreased stratospheric
ozone.
What else influences the magnitude of UV exposure?
A number of things. Some examples are heat, wind, humidity, pollutants, cloud
cover, snow, season, and time of day.
Let me go back to ozone for a second. It seems that I hear a lot
in the news about the depleted ozone layer. Is ozone really necessary
to protect people on earth?
If solar radiation reached the surface of the earth unscreened, it would
be lethal to exposed living organisms. In practice, the shielding provided
by the atmosphere results in substantial attenuation of the UV light, with
ozone providing a major contribution.
Does the amount of ultraviolet radiation stay the same each day?
No. Solar flares, known as "sunspots", may change the amount of UV radiation
reaching the earth on any given day or from one season to another. Solar
flares increase ozone concentration in the stratosphere (above 50 km) thereby
reducing the amount of surface UVB. When solar flares are inactive, there
is a decrease in the ozone concentration, allowing increased UVB to penetrate
to the Earth's surface. Ozone is the main defense to manage our everyday
exposure to ultraviolet radiation. Changes in the concentration of the ozone
in the atmosphere may change the intensity of ultraviolet radiation on the
surface of the earth.
You said there were some artificial sources of ultraviolet radiation,
right?
Yes I did. There are many artificial sources which may result in an exposure
at work or at home. some examples are those used for medical therapy or a
tanning booth, or tungsten halogen lamps, and even arcs from welding operations.
However, there are few artificial sources that result in human exposure greater
than from solar radiation.
You also said they use ultraviolet radiation to sterilize things and kill
bacteria. How does that work?
Disinfection of water generally uses low pressure mercury vapor lamps to
generate ultraviolet radiation. These lamps generate short wave ultraviolet
in the electromagnetic spectrum region of about 254 nanometers. Light in
this region is lethal to micro-organisms including bacteria, protozoa, viruses,
molds, yeasts, nematode eggs and algae.
How much ultraviolet radiation does it take to kill bacteria?
Ultraviolet dosage is measured in "microwatt seconds per square centimeter".
In essence those units mean that the higher the microwatt level, the
higher the dosage. Or the longer the exposure time, the higher the dosage.
Finally, the greater the exposed area, the higher the dosage. For your
information, the U.S. Public Health Service published a policy stating that
the amount of UV necessary to sterilize drinking water is at least 16,000
microwatt seconds per square centimeter.
I thought most drinking water was sterilized with chlorine. Why use
ultraviolet radiation?
You're right. There are indeed two common methods of disinfecting water:
chlorine and ultraviolet (UV) treatment. Both have long been used to destroy
bacteria. However, UV is very effective in killing virtually all disease-causing
viruses and bacteria, and it does it in a very few seconds.
What about chlorine?
Chlorine, in the past, was the sterilizing method of choice, and was more
widely accepted than UV. Lately, however, chlorine has been strongly criticized
because it not only affects the taste and odor of the water it disinfects,
chlorine also produces cancer-causing by-products in some waters. It has
also become a much more expensive treatment option than UV. So, we now see
many more water treatment facilities converting their treatment operations
over to the use of UV to kill bacteria and mold.
What about light bulbs? I heard somewhere that flourescent lights use
ultraviolet radiation to make visible light. I thought you told me that
ultraviolet light is invisible to the eye.
A flourescent light fixture uses ultraviolet radiation to "create" visible
light.
How do they do that?
Electronic circuits in the base of the bulb energize a magnetic coil, creating
a magnetic field. The field then activates the mercury gas in the bulb, which
emits ultraviolet radiation. Visible light is released as the ultraviolet
light passes through a phosphor coating on the inside surface of the bulb.
The color of the visible light can be changed by changing the phosphor coating
and the makeup and concentration of the gas inside the tube.
What kind of bulbs do they use in a tanning booth? Are those flourescent
lights?
Yes they are. Fluorescent lamps can be designed for special applications
like tanning booths. These emit levels of UV sufficient to cause injury to
the skin and eye. The most potent artificial sources of UV, and particularly
of UVB and UVC, are those characterized as high intensity discharge (HID)
lamps. These include high pressure mercury, mercury metal halide and neon
lamps. High pressure mercury vapor lamps with little or no filtration for
ultraviolet radiation were once commonly used for cosmetic tanning. These
lamps are now little used, having been largely replaced by UVA fluorescent
and filtered HID UVA cosmetic tanning systems.
Are the tanning booths safe to use? Is there too much ultraviolet
radiation?
The tanning industry is rapidly growing in the United States. Currently,
more than a million Americans use commercial tanning facilities every day.
The biggest categories of users are adolescents and young adults, especially
women. While short (less than 20 minutes) exposure to UV radiation may, indeed
be beneficial to health since it causes the exposed skin to generate vitamin
D, overexposure may result in the burning of exposed skin and serious eye
effects.
Eye effects?
Yes. Eye exposure is especially dangerous because the results of over
exposure are not immediately evident. There is a potential for skin cancer
with constant overexposure. I'll tell you more about that in just a minute.
Any other common sources of ultraviolet radiation?
Gas welding, brazing and cutting processes operate at temperatures insufficiently
high to cause the emission of intense UV radiation. Arc welding processes
are particularly potent sources of UV radiation and even very short exposures
may be hazardous to the eyes and skin. Both gas and arc welding also emit
visible and infrared radiations which may be hazardous to the retina. Appropriate
personal protection always needs to be worn during welding.
Let's go back to effects of UV radiation. Can you tell me more about
them?
UV interacts with tissue predominantly by photochemical effects. Those organs
that receive the greatest damage are the skin, the immune system, and the
eyes. The extent of interaction with the skin depends on the wavelength,
skin pigmentation, and tissue thickness. Typically, the shorter UVB wavelengths
are absorbed by the outer layers of skin and, as the wavelength increases,
the depth of penetration increases. Short-term effects on the skin may be
seen as sunburn, principally consisting of erythema (skin reddening resulting
from expansion of blood vessels) and edema (swelling), both of which may
be very severe. In some people this sunburn is followed by increased production
of melanin and is recognized as a suntan.
So that's it. All I need is a good tan and I'm set.
Well, it is not that simple. A suntan is not an indication of good health
and only offers minimal protection against further exposure. It is a sign
that damaged skin is attempting to protect itself from further harm.
I can tolerate a sunburn every now and then. What's the problem?
The most serious health effects, for which exposure to UV radiation is a
recognized risk factor, are the skin cancers. UVB has been recognized for
some time as carcinogenic in experimental animals, and there is increasing
evidence that UVA, which penetrates more deeply into the skin, also contributes
to the induction of cancer. Basal cell cancer, the most common skin cancers
in Caucasians, are found primarily on sun-exposed areas such as the head
and neck, and hands. People with light complexion and who sunburn easily
have a much greater likelihood of tumors.
Are there lots of UV-induced skin cancers?
Yes. Skin cancers in which UV radiation exposure plays an important role
are now the most common form of cancer. In 1978, there were more than 500,000
new cases of UV-induced skin cancer.
Does skin cancer spread throughout the body like other soft tissue
tumors?
No. The non-melanoma skin cancers are mainly basal cell carcinomas and squamous
cell carcinomas. They are relatively common in Caucasian populations, although
they are rarely fatal. Unfortunately, the incidence of skin cancer is increasing
year after year. As I said before, they occur most frequently on sun-exposed
areas of the body such as the face and hands and show an increasing incidence
with increasing age.
Does every person have the same chance of getting skin cancer from a
sunburn?
No. Some people are more likely to have skin cancer than others. For example,
people reporting skin cancers have light skin (melanin absorbs UV radiation),
red or blond hair, blue eyes, or a tendency to burn and not tan on sun exposure.
There is extensive epidemiological evidence supporting the direct role sunlight
plays in human skin cancer.
How do I know if I have skin cancer?
The major signs of suspected malignant melanoma are: an existing mole getting
larger, developing an irregular outline, or showing shades of brown and black;
or a new mole growing quickly (in months). Minor signs include: a mole becoming
bigger than the blunt end of a pencil (around 5 mm), or becoming inflamed
or developing a reddish edge, or bleeding, oozing or crusting; or a mole
starting to feel different (e.g. itching or painful). If you have any questions
or suspicions, contact your physician for specific information.
Can UV damage other parts of the body?
Absolutely. Too much UV may damage your eyes; photokeratitis and
photoonjunctivitis (inflammation of the cornea and the conjunctiva,
respectively). The damage is most likely reversible but painful for several
days.
Are there rules, laws or standards designed to limit exposures to UV or
eliminate damage to the skin and eyes?
There is no OSHA standard for UV radiation exposure. However, the National
Institute of Occupational Safety and Health (NIOSH) recommends exposures
to the skin or eyes be less than 1,000 microwatt per square centimeter for
periods of greater than 1000 seconds. This NIOSH limit, however, changes
depending on the specific frequency that is present.
Are there any simple measures that I can take now to reduce my chances
of skin damage?
Yes. First, damage can be minimized by using clothing made of tightly woven
fabrics with long sleeves, long pants, wide-brimmed hats, etc.
Okay, I can do that. Anything else?
Yes. Sun screen and certain suntan lotions are useful. Minimum protection
is afforded by chemical sun screens with protection factor ratings of 15
or higher. Although most sun screens on the market today are appropriate
for UVB protection, combination sun screens that are effective against UVB
and at least part of the UVA spectrum are preferable. Waterproof sun screens
should be selected by swimmers and those who perspire sufficiently to wash
off non- waterproof products.
I can do that too. Anything else?
Most importantly, limit the amount of time that you are exposed. Research
data suggest that 50 percent of an individual's total lifetime UV radiation
exposure occurs by 18 years of age. Parents can play an important role in
protecting their children by managing outdoor activities at school, camp,
daycare centers, or the beach. Remember that more than 60 percent of the
daily UVB radiation reaching the Earth's surface arrives during a sunny day
in June between 10:00 a.m. and 3:00 p.m.
Where can I get more information about exposures to ultraviolet
radiation?
The "Links" section of the IEM web page is a good place to start. If you
go to the category of "Nonionizing Radiation", you will see several references
to UV light and UV exposure.
Any final words?
Yes, just a final note on the issue of UV exposure and skin cancer. Because
so many of these cancers can be prevented by reducing exposure to UV radiation,
the "Health Physics Society", a distinguished professional organization,
has issued a position statement that advises the public, health officials
and the news media on ways of reducing the risk of UV-induced cancer.
What are the Society's recommendations?
Pretty much the same as I told you about earlier. However, they bear
repeating. First, they advise people to avoid the use of tanning beds and
sun lamps. In fact, they state that unless directed by a physician, people
should not use this equipment because it offers very little health benefit
but significantly increases the risk of cancer.
Do they also offer protection recommendations?
Yes they do. To protect yourself from the sun, the Society advises you to
minimize exposure when the sun's rays are the strongest. (They even tell
you that if your shadow is shorter than you are, you should seek the shade.)
You are also advised to apply a broad-spectrum sun screen that protects against
both UVA and UVB, and that has an SPF of at least 15. Finally, they tell
you to reapply sun screen every two hours, even on cloudy days, to wear a
wide-brimmed hat and sunglasses, and to avoid reflective surfaces.
Sounds like good advice.
It is.
Copyright © 1999 Integrated Environmental Management, Inc.