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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 Plexus-NSD 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.