Discussion: The use of the "phototherapy" phosphor in reptile lamps

If you have not yet read the previous sections (in particular, the lamp test results) we suggest you may wish to do so before reading this section.

The "phototherapy phosphor"

As we described in the lamp test results, the spectra of the fluorescent lamps which have been connected in some way with cases of photo-kerato-conjunctivitis all appear to utilise a specific phosphor, which is generating low wavelength UVB from a threshold as low as 275 - 280 nm. (fig. 1)

This is the typical UV spectrum of the phosphor used in so-called "FS" lamps. FS lamps are used for testing the deterioration under UVB of resistant materials such as roofing and car bodywork, and in older-style human clinical phototherapy lamps.

Lamps which contain a high proportion of this "phototherapy" phosphor, and which have glass which does not block UVC, emit radiation as low as 270nm.

However, the lower wavelengths are apparently attenuated by reducing the proportion of phosphor in the blend(15) and by use of different glass. The Reptisun 10.0 lamp, for example, does not emit UVB below 280-285nm; the Big Apple Herpetological Mystic tube and the R-Zilla Desert 50 Series T8 lamp emit UVB own to 275nm.

The use of FS lamps and their spectra

FS lamps include old-fashioned broadband UVB phototherapy lamps such as the FS-T12/UVB series, also known as Philips TL-12 lamps (14) and UVB-313.(12)(13) The typical spectrum of one of these lamps may be viewed here (external link; opens in new window; pdf file - takes short time to load)

Other similar products are known as FS-40 and F40UVB lamps, and the phosphor which produces this typical UVB emission is also described as "Sylvania 2096"(15).

FS lamps were once widely used in broadband UVB (bUVB) phototherapy tubes for treatment of diseases such as psoriasis, atopic dermatitis and vitiligo, but recently, narrowband UVB phototherapy (nUVB), with fluorescent bulbs emitting near monochromatic UV around 311 nm, has become a preferred option.(16) Eye protection is always worn when any of these lamps are used in clinical work. These lamps are never used as tanning lamps or for cosmetic purposes.

FS lamps are also widely used for accelerated UV ageing and weathering tests for quality control applications, testing very durable materials such as roofing, exterior coatings for buildings, and automotive paintwork.(12)

Why would anyone put this phosphor in reptile lamps?

The lowest wavelengths ever found in natural sunlight at the earth's surface are 290 - 295nm; at higher latitudes little below 300nm reaches sea level. In contrast, the phosphor producing UV radiation in these lamps is emitting light from as low as 275 - 280nm (close to the UVB/UVC boundary). Its output increases sharply with increasing wavelength. Maximum output is between 310 - 315nm, contributing to the mercury peak at 313nm.

This phosphor is therefore producing almost all its output right in the middle of the "UVB" band. It is a very efficient producer of UVB, in the scientific sense. If a lamp specification required just "high UVB" then undoubtedly this would be the phosphor of choice, if one was not aware of the increasingly hazardous effect of the low wavelengths as they fall towards UVC.

This phosphor will also, undoubtedly, enable the production of vitamin D3 in the skin. The action spectrum for the conversion of 7DHC to previtamin D3 peaks at 298nm, which is therefore the most effective wavelength for vitamin D3 production.(4)

However, a significant proportion of the output is at wavelengths below those found in natural sunlight, where the benefits of high pre-vitamin D3 conversion may be outweighed by an increased risk of cell damage.

The presence of low wavelength UVB seems the most likely reason for the increased incidence of photo-kerato-conjunctivitis. It causes significant cell damage, particularly to DNA, causing cell death. Although the delicate cells of the surface of the eye would most likely be first to be noticeably affected, and hence photo-kerato-conjunctivitis would be a first sign of over-exposure to radiation, damage to other exposed areas cannot be ruled out.

Many species of reptile have very thick, armoured, protective skin which may be quite resistant to damage from UV light; our studies on UVB transmission through shed skin certainly show that many sun-worshipping reptiles allow very little UVB to reach deeper skin layers. Skin of some other species appears not so protective - which may account for the skin problems experienced by some chameleons under these lamps. The risk of skin cell death and possible development of skin cancer from damaged but surviving cells - which is well documented in humans - must be a possibility, however, even in sun-worshipping species exposed to very abnormal radiation for extended periods of time.

Scientific research using FS lamps

The effects of longterm exposure of reptiles to these wavelengths are not known. However, the effects of long-term exposure of various mammals to broadband UVB (bUVB) phototherapy tubes has been studied quite extensively, as lamps from the FS series have been widely used to study cell damage and immunosuppression, and induce skin tumours in laboratory animals.(17)(18)(19)

Relatively small amounts of non-solar spectrum UV energy can have significant effects. In one study, non-solar radiation (UV at wavelengths below those in sunlight - i.e. below 290nm) from FS lamps was shown to penetrate sunscreens, and cause immunosuppression, more than fifteen times as effectively as the solar equivalent.(20)

The very existence of these studies suggest that this spectrum may be problematical for use in lighting for animal enclosures.

Reptile studies

FS lamps, with their "phototherapy" phosphors have been used in at least two studies with reptiles which suffered detrimental effects apparently related to their exposure to particularly low wavelength UVB.

1. Harmful effects

In one study (21), Philips F40UVB tubes were used to supply UVB for up to an hour a day to groups of green iguanas at a minimum distance of just over 10cm. The tubes were mounted vertically in the enclosures, so they would have been in the reptiles' direct line of sight. The spectrum and UV Index gradient of these lamps was not recorded; the total UVB output was measured at 50µW/cm² at 30cm., with a Spectroline DM-300X broadband UVB meter. All the iguanas in the test groups with the F40UVB lamps apparently developed symptoms of photo-kerato-conjunctivitis (sitting motionless with their eyes closed all day) and developed darkened skin, and five of the sixteen exposed to the lamps died during the first month of the study. The researchers concluded this was "most likely due to over-exposure to ultraviolet light" although the total UVB levels were far less than found in natural sunlight, and the exposure times were extremely short.

In another study (unpublished), Oustalet's chameleons were observed to develop skin tumours when exposed to similar FS lamps.(22)

2. Altered vitamin D3 synthesis

The non-solar UV spectrum from these lamps may also have a detrimental effect upon the control of vitamin D3 synthesis in reptile skin, although whether this has clinical significance is unclear. Green iguanas (Iguana iguana) kept under lamps with the Sylvania 2096 phosphor developed abnormally high 25-hydroxyvitamin D blood levels, up to 1200 ng/ml.(15)

(Studies of normal blood levels do not seem to have been conducted on wild green iguanas, but blood tests have revealed circulating 25-hydroxyvitamin D blood levels of 100-448 ng/ml in wild Ricord's iguanas (Cyclura ricordii) and 104-148 ng/ml for wild rhinoceros iguanas (Cyclura cornuta cornuta)(23), and in captive green iguanas housed outdoors, figures around 400ng/ml have been cited.(15)) Although no ill effects were seen, the normal biological control mechanisms limiting the production of vitamin D3 in sunlit skin may have been bypassed in some way.

Blood tests on an apparently healthy pet iguana kept under a prototype high-UVB fluorescent tube with this phosphor also revealed a very high 25-hydroxyvitamin D value of 600ng/ml.(24)

Is the "phototherapy phosphor" harmful if it is used in reptile lamps?

It is important to note that although the spectrum of these lamps indicates that a "phototherapy" phosphor is being used, the R-Zilla Desert Series 50 lamps and the ZooMed Reptisun Compact Lamps are not FS lamps. They merely contain a similar phosphor. They do not produce UVC from 270 - 275nm, as FS lamps do. They contain other phosphors increasing the visible light, which the Big Apple Herpetological Mystic lamps and FS lamps do not, as can be seen from their full spectra. (Figs. 2, 3 and 4 (below)

It is also important to bear in mind that these lamps have been widely sold, yet only a comparatively small number of cases of photo-kerato-conjunctivitis, and an even smaller number of deaths, have been reported. All these have been clearly associated with accidental over-exposure of the animals to the radiation. Many experienced a UV Index far exceeding that of any sunlight on earth.

What, then, is the situation with these lamps at greater distances, where the UV Index falls to similar levels as natural light? We have observed that sunloving reptiles have been seen out when the UV Index is around 6 - 7; many other species are rarely seen out of the shade, or only bask at times of day when the UV Index is much lower. Are these lamps "safe" if they are producing a UV Index of 6? Or 4? Or 2?

The truth is, we do not know. We do know that regardless of the UV Index, a reptile sitting under one of these lamps will receive some UVB at non-solar wavelengths. If the lamp is far away, it may only be a very small dose... but it will be there. Will it cause long-term problems with the skin, or with immunosuppression, or with vitamin D3 synthesis? Could it increase the risk of cancer? We do not know.

It is important to remember, of course, that even natural sunlight is not actually "safe". Human tanning lamps, the output of which is very strictly controlled, and must only fall within the solar wavelengths, are never allowed to be described as "safe".....

There are no easy answers. However, the fact that low wavelength, non-solar UVB has been shown to have much more damaging effects upon living cells than solar UVB, coupled with the incidence of photo-kerato-conjunctivitis described in this report, suggests that lamps emitting wavelengths from 275 - 290nm may pose greater risks to health than those which do not.

Hence our main recommendation:

The production of non-solar UVB and high levels of low-wavelength solar UVB by the "phototherapy" phosphor used in these lamps may be a cause for particular concern.
Further studies are needed to ascertain whether long-term exposure to these wavelengths, at any intensity, is in any way detrimental to reptile health.
Until such studies have been carried out, and the risks to reptile health properly assessed, we recommend that alternative phosphors be used in these lamps.

Already, we are receiving very positive responses from the companies whose products are featured here. Please read their action statements in the next section.

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