Ultraviolet UV light lies between visible light and X-rays on the light spectrum. It is invisible to the human eye because our eyes cannot detect shorter wavelengths. For individuals with XP, UV exposure must be avoided. However, sunlight’s color and warmth when properly filtered can be low risk. The challenge is to effectively filter or eliminate to make a safe environment for someone with XP.
Ultraviolet UV light lies between visible light and X-rays on the light spectrum. It is invisible to the human eye because our eyes cannot detect shorter wavelengths. For individuals with XP, UV exposure must be avoided. However, sunlight’s color and warmth when properly filtered can be low risk. The challenge is to effectively filter or eliminate to make a safe environment for someone with XP.
Although some people with XP are very sensitive to bright lights, the visible light region and infrared (i.e., heat) are not considered carcinogenic because their energies are too low (i.e., wavelengths too long) to cause DNA damage.
What are the main sources of everyday UV? The sun, shade, sun with cloud cover, sun through unfiltered glass windows, tanning beds, and many common artificial light sources such as fluorescent, halogen, and metal-halide lamps are all sources of UV harmful for XP. Halogen lights are commonly used in medical and dental offices and in headlights. Metal-halide lamps are used in stadiums and gymnasiums and represent a dangerous source of UV for even the general public if the outer bulb or fixture is broken. Blacklight is a strong UVA source and is commonly found in some museums or amusement park rides. See page on artificial light because there are ways to deal with many of these artificial light sources.
What are not UV sources in everyday life? The moon and stars, sun through glass windows with UV blocking film, low wattage incandescent light bulbs, filtered fluorescent tubes, blacklight created only from purple paint on incandescent bulbs, televisions, plasma screens, computer screens, candle light, and camp fires are all not considered harmful for XP patients. For questionable sources, UV meters are used to determine if a light source is a significant risk when weighing quality of life.
Are Xrays as harmful as UV? No. Wavelengths below 100nm are not considered especially harmful for XP patients. The type of DNA damage that can result from Xrays are of a type that can be repaired as efficiently in XP cells as normal cells (P.R. Cook, 1978).
UV is defined as the wavelengths between 100nm and 400nm. It is broken down further into three bands called UVC, UVB, and UVA, listed in order from strongest to weakest assaults on DNA.
UVC is the region below 280nm. Fortunately, UVC from the sun is absorbed completely by a few hundred meters of air. UVC from the sun is involved in the chemical process that creates ozone in our upper atmosphere. Accoding to many sources it is almost never found in nature. UVC is used in germicidal lamps to kill bacteria in water and air, but it is not a general lighting artifact.
UVB is the region between 280 and 320nm and is referred to as the erythemal UV band, also called the sunburning region. Measurements have been made of sunburn intensity on human skin that show the greatest sunburn response in the UVB region and then declines towards UVA wavelengths Measurements on DNA have also been made on cells and show a similar trend, that UVB causes more DNA damage per dose than UVA. The response of XP skin is the same, but because there is no repair mechanism the damage is cumulative and irreversible. Even in low doses, UVB presents the greatest risk to XP patients and should be filtered. The stratospheric ozone in our atmosphere greatly reduces the levels of UVB especially below 300nm, but the residual UVB is still very harmful. That the ozone layer depletion and global warming trends may worsen the filtering of our atmosphere should be of concern to all of us, not just XP.
UVA is defined as the waves between 320 and 400 nm. The region just greater than 400 nm is where our eyes first detect the color purple. In some literature, UVA is further divided into two regions, UVA1 (340 to 400nm) and UVA2 (320 to 340nm), in order to distinguish their potentially different biological effects. UVA2 presents the next greatest risk to XP patients, followed by UVA1. While UVA is overall less efficient at causing direct DNA damage than UVB, research is ongoing to determine its exact role in causing skin cancer through indirect paths. For example, it is thought that UVA reduces one’s immunity to skin cancer by lessening the expression of the cancer fighting gene, p53. UVA also penetrates deeper than UVB and is thought to generate excess free oxygen radicals that can lead to photoaged and wrinkled skin.
The biggest source of UV in the lives of people with XP is the sun. The sun emits its energy over all wavelength bands, from below x-rays, to the ultraviolet and visible regions, to beyond infrared (heat). It is the ultraviolet in the region of 280nm to 400nm that is of importance to managing XP.
The sun does not emit energy equally over the entire spectrum, but rather has what is called a blackbody spectrum. A blackbody radiator is a term that describes how energy is emitted from objects that are heated to very high temperatures. Any object that is extremely hot will emit visible light as well as ultraviolet.
SOHO spacecraft image of the sun’s extreme ultraviolet rays,
the UV band closest to X-rays, taken with a special camera
This is why even very bright incandescent bulbs will produce a reading on an UV light meter, however it is usually small. It takes temperatures in excess of 2,000 C, or 3,632 F, to produce a detectable amount of UV.
Sunlight at sea level has a temperature of about 5,500C. A 100 Watt incandescent light bulb will reach a temperature of 2,600 C and has measureable UV, but a hot camp fire will only get to 1,700 C and does not emit UV.
A graph of the sun’s energy output is shown above. The vertical axis represents the amount of energy, specified as Watts per square centimeter for each wavelength on the horizontal axis. This is called irradiance. The graph called “Extraterrestrial” illustrates the output as measured in space, outside our atmosphere. Fortunately, the Earth’s ozone layer filters much of the sun’s energy as can be seen in the “Sea Level” graph.
There is very little detectable UV on the Earth’s surface at 300nm, and it is virtually zero below 280nm. The UVB region shown in red is really only a small fraction of the total UV at sea level. There is typically about 20 times more UVA than UVB reaching the Earth. However, keep in mind that while UVB is relatively small in intensity, it is the most efficient at causing DNA damage. On the other hand, while the UVA region shown in yellow is less able to cause DNA damage, there is more of it so it can be equally harmful.
To understand how to compare the effects of UVA and UVB, one has to become familiar with biological effective UV measurements. This is a mathematical technique that weights the UV irradiance of each wavelength with its effectiveness at causing DNA damage and then adds them all up to arrive at a single number. One such measure of biologically effective UV is presented in the form of the UV Index.
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