UV radiation
Electromagnetic radiation or "light" is the collective name for all forms of energy that move with the speed of light. There are different "types of light" in the spectrum, depending on their energy, which is related to the wavelength (freqency) of the light: the lower the wavelength, the higher the energy.
The human eye is sensitive for only a part of the spectrum, referred to as "visible light": between 400 and 780 nm (1 nm = 10-9 meter). The wavelength of the light determines the colour: 400 nm is blue, 700 nm is red.
The part of the spectrum immediately to the left of blue, between 200 and 400 nm is the ultraviolet light (UV). The UV is usually divided into three components, with increasing energy:
- UV-A: 320-400 nm
- UV-B: 280-320 nm
- UV-C: 200-280 nm
Of these UV-B is the most dangerous form, since part of it reaches ground level, where it can affect human health (
e.g. cause certain forms of skin cancer) and damage the DNA of flora and fauna. Note that the wavelength of the division between the UV-A and UV-B bands is sometimes set at 315 nm (
see note below).
The UV-C energy is potentially more dangerous, but it decreases dramatically as ozone increases, because of the strong absorption in the 200-280 nm wavelength band. The UV-B is also strongly absorbed, but a small fraction reaches the surface. The UV-A is only weakly absorbed by ozone, with some scattering of radiation near the surface.
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The curve shows a typical vertical profile of ozone in the midlatitudes of the northern hemisphere: the concentration of ozone as function of altitude. Superimposed on the figure are plots of UV radiation as a function of altitude for UV-A, UV-B and UV-C. The width of the bar indicates the amount of energy as a function of altitude. UV-C is absorbed completely in the stratosphere. Of the global UV radiation at the ground, 94% is UV-A, 6% is UV-B.
figure adapted from
Stratospheric Ozone, An Electronic Textbook
Atmospheric ozone thus shields life at the surface from most of the harmful components of the solar UV radiation. Chemical processes in the atmosphere can effect the level of protection provided by the ozone in the upper atmosphere.
Ozone decline in the stratosphere can be caused by:
- Chemical processes resulting from the breakdown of CFCs and other ozone depleting gases.
- Changes in the stratospheric meteorology, e.g. due to changes in the climate and in trace gases such as nitrous oxide (N2O), water (H2O) and methane (CH4).
Thinning of the atmospheric ozone in the stratosphere leads to elevated levels of UV-B at ground level and increases the risks of DNA damage in living organisms. A 1% decrease in ozone, for example, will lead to an estimated increase of UV-B of about 2%.
It is therefore important to monitor the UV radiation that reaches the ground. One of the tools for this is the
UV index.
Note on the UV-A & UV-B wavelength ranges
The wavelength of the devision between UV-A and UV-B varies in the literature and this may lead to some confusion.
The
Commission Internationale de l'Éclairage (
International Commission on Illumination) uses 280-315 nm as UV-B and 315-400 nm as UV-A.
Other sources put the devision point at 320 nm, as in the above given definition. In particular this is done in medical (dermatological) applications, as well as in cosmetics. Also several text books on UV use 320 as devision point.
To avoid confusion, one could use the following short-hand notation: dUVB for 290-320 nm and dUVA for 320-400 nm, where "d" stands for dermatological, and use UVA and UVB for the 315-nm devision quantities. The use of the prefix "d" is not very common, though.
Within the TEMIS project, the data supplied are the
UV index and
UV dose, which cover (parts of) both UV-A and UV-B. The precise wavelength range that is relevant for these quantities depends on the
action spectrum applied:
- UV range relevant for erythema: 280 - 400 nm
- UV range relevant for general DNA damage: 256 - 370 nm
UV index
The erythemal UV index -- usually simply called the UV index (UVI) -- is an estimation of the UV levels that are important for the effects on the human skin, where 1 unit equals 25 mW/m2. It is usually given for local solar noon, when the Sun is highest in the sky, and it is valid for clear-sky conditions: effects of clouds shielding part of the UV radiation are
not taken into account.
The erythemal UV index is an artificial quantity derived from the erythemal irradiance, which is an integration of the UV irradiance at the ground weighted by the CIE spectral action function. The CIE action spectrum is a model for the susceptibility of the caucasian skin to sunburn (erythema). It is proposed by McKinlay & Diffey (1987) and adopted as a standard by the
Commission Internationale de l'Éclairage (
International Commission on Illumination).
Of the global UV radiation at the ground, 94% is UV-A, 6% is UV-B.
Of the erythemal UV irradiance, however, 17% is UV-A, 83% is UV-B.
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The CIE action spectrum is a model for the susceptibility of the caucasian skin to sunburn (reddening of the skin; erythema). It was proposed by McKinlay & Diffey (1987) and adopted as a standard by the Commission Internationale de l'Éclairage
(International Commission on Illumination).
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Ultraviolet spectrum measured with the Brewer Spectrophotometer at De Bilt (Netherlands) on 1 June 2002, a completely cloud-free day. Also drawn is the CIE erythemal action spectrum. The multiplication of these two gives the erythemal UV spectrum, and the surface below this graph (shown in yellow) is the UV index. The value of the UVI from this measurement is 6.3.
figure by Marc Allaart, KNMI, De Bilt
Ground-based measurements such as the one from the previous graph give UVI values for these specific sites at these specific moments in time only. In order to obtain the UVI for all locations, it must be computed from total ozone amounts, in combination with the solar zenith angle (SZA): the angle from which the Sun shines. The UV index is usually given at noon of the local solar time: the moment when the Sun is highest in the sky (
i.e. in the zenith); this moment is therefore
not noon of the local time zone.
Ground-based measurements of the UVI and simultatiously measured total ozone column (TOC) values have resulted in a parametrisation of the UVI as function of TOC and SZA, both at local solar noon. The TOC at local solar noon is determined from satellite observations in combination with data assimilation, which uses meteorological fields (wind, temperature, pressure) to obtain a global ozone field at local solar noon. The SZA at local solar noon depends on the latitude and the day of the year. The UV index thus is a measure for the amount of UV radiation valid for clear-sky conditions and at local solar noon. This does not say much about the
erythemal UV dose: the total amount of UV that actually can reach the human skin during a day.
ref:
http://www.temis.nl/uvradiation/info/index.html
So essentually its; UVB`s (280-300nm) is the spectrum that we are looking for........UVB in natural sunlight is 290-295nm.
UVB`s on REPTILES HAve BEEN TESTED!
[FONT=Verdana, Arial, Helvetica, sans-serif]REPTILE TEST FOR UVB - [/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]We tested samples of fluorescent lamps from each brand which has been alleged to have caused photo-kerato-conjunctivitis in reptiles, and compared the test results with those from other brands. [/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]Broadband UVB meters did not prove very helpful in determining the cause of the problem; they indicated that at the distances at which problems occurred, the total UVB (in microwatts per square centimetre) from these lamps was no higher than that found in natural sunlight. However, total UVB measurements give little indication of the photobiological activity of the lamp. [/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]
Measurements of the UV Index (which does provide a measure of this) revealed that whereas 100 µW/cm² total UVB from sunlight was recorded as yielding a UV Index of 1.6 – 2.0, these lamps were producing emissions in which 100µW/cm² total UVB yielded a UV Index of between 8.9 and 14.2. Light from these lamps would therefore appear to be between four and eight times as photobiologically active as light from the sun. At close range these lamps were all producing hazardous levels of UVB. [/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]Spectrograms indicated that all these lamps utilise a distinctive phosphor of a type used in lamps for testing the deterioration under UVB of resistant materials such as roofing and car bodywork, and in older-style human clinical phototherapy lamps (so-called "FS" lamps).
The lamps we tested from three different brands generate low wavelength UVB, some from as low as 275 - 280nm, whereas the lower limit of UVB in natural sunlight is 290-295nm. (The phosphor used in many
other brands of reptile UVB lamps is of a type used in some human tanning lamps, which mimics the UV in sunlight and produces no UVB at wavelengths below 290nm.) [/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]As well as a much higher proportion of more damaging non-solar UV energy at wavelengths below 295 nm, the lamps with the problem phosphor proved to have a higher
total UVB output than most other brands of fluorescent reptile UVB lamps. Because much of this is in the more photobiologically active wavelengths, the risk of reaching a threshold dose for photo-kerato-conjunctivitis, and possibly other forms of UV radiation damage, is much greater than with other lamps. [/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]A combination of
other factors apparently increased the risk of photo-kerato-conjunctivitis with these lamps even further: [/FONT]
- [FONT=Verdana, Arial, Helvetica, sans-serif]In some cases, product literature did not give adequate information. It is essential that lamps are not sold without clear recommendations regarding suitable basking distances and the hazards of over-exposure. Many reptile keepers are unaware that there are any risks associated with close contact with a fluorescent UVB source. The history of fluorescent UVB lamps is such that they are often perceived as "weak" sources of UVB and keepers are often advised to position them close to the reptile. [/FONT]
- [FONT=Verdana, Arial, Helvetica, sans-serif]When placed in aluminium reflectors, in some cases UVB beneath compact lamps was increased by more than 700%. The extreme increase in UVB underneath aluminium reflector domes has not been widely known, or the hazard recognised, either by manufacturers or hobbyists. [/FONT]
- [FONT=Verdana, Arial, Helvetica, sans-serif]Most of the lamps have a low visible light output. They are therefore less likely to induce an aversive reaction, or pupillary constriction, when in the reptiles' line of sight. They do not "look like" very intense, direct tropical sunlight.[/FONT]
- [FONT=Verdana, Arial, Helvetica, sans-serif]Most of the UVA output of these lamps is not in the visible UVA range for reptiles, since the threshold for vision is about 350nm. This reduces even further the visual impact of the lamp to the reptile. [/FONT]
- [FONT=Verdana, Arial, Helvetica, sans-serif]Fluorescent lamps produce a small amount of heat. This is insufficient to deter a reptile from a close approach, and in fact the gentle warmth may even prove an attraction. [/FONT]
- [FONT=Verdana, Arial, Helvetica, sans-serif]Broadband UVB meters such as the Solarmeter 6.2 provide only readings of the total UVB in microwatts per square centimetre. This gives little indication of the photobiological activity of the UVB radiation from this type of lamp. Misleading comparisons of lamp and solar readings taken with these meters may be made, possibly leading to incorrect placement of the lamp, if this is not understood. [/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]We believe the use of a “phototherapy” phosphor in these lamps is a cause for concern. The effects of long-term exposure to non-solar wavelengths on reptiles is unknown, but these wavelengths are far more damaging to living cells than solar wavelengths.[/FONT][FONT=Verdana, Arial, Helvetica, sans-serif] FS lamps have been used widely in research upon cell damage and immunosuppression, and to induce skin tumours in laboratory animals. FS lamps have been used in at least two studies with reptiles which suffered unexpected detrimental effects apparently related to their UV exposure, including photo-kerato-conjunctivitis, depression and death, and skin tumours. Although the compact lamps we tested from ZooMed, and the T5 and T8 tube we tested from R-Zilla are not FS lamps, they do appear to contain the same phosphor. The Big Apple Herpetological Mystic tube and compact lamp we tested, however, have spectra very similar indeed to FS lamps. [/FONT]