What does the UVB percentage
of sun lamps really mean ?

When choosing a particular make of sun lamp, many users, be they private or professional, seem to consider the UVB percentage to be one of the most important criteria, if not the most important. Ever since sun lamps came onto the market, the proportion of UVB rays has been a dominant theme when discussing the merits of a lamp. Evidently, not only have consumers got used to this, they have actually seized on this parameter as a sign of quality. And on the face of things, the simple conclusion that the more UVB rays there are, the more effective the lamp is, seems logical.
Recently, the range of sun lamps available has become so vast as to be confusing even for people "in the know": the only question that seems to count when it comes to quality is the percentage of UVB. In addition, it is becoming impossible to ignore the fashion favouring increasingly high UVB percentages. This begs the question of whether some manufacturers aren’t just oversimplifying things by staking everything on the percentage of UVB rays. Is this simplification of the complexity of sun lamps and how they work misleading, or even incorrect? On closer examination, the equation that more UVB = deeper tanning is at the least open to considerable doubt. There are three good arguments, among others, which corroborate this finding.

Argument 1: UVB percentage on what basis ?

The different sun lamp offers that are being mailed to prospective buyers today reveal that the price of lamps increases with the percentage of UVB rays. This gives the impression that the value of a lamp is determined by the amount of UVB, which, in the absence of any additional information, is unjustifiable. Such offers frequently fail to define the basis on which the percentage is calculated, i.e. the total UV radiation. The customer is therefore given no information whatsoever about the absolute amount of UVB radiation - in some cases, quite deliberately.
Two different lamps, both of which claim to have, say, 1.4 per cent of UVB, do not necessarily have the same UVB capacity:

the first sun lamp emits 20 per cent more UVB than the second. Differences of this order in the overall UV radiation are not unusual on today’s sunlamp market, as can be seen by the varying radiation intensities on offer in solaria, wholly dependent on the types of lamps used.
For example, a sun bed using a conventional lamp might achieve radiation equivalent to 20 mW/cm², whereas the same sun bed using a different lamp might achieve radiation of 25 mW/cm² or more.
This means that simply stating the percentage of UVB radiation is completely inadequate when assessing the actual amount of UVB radiation. It is therefore of very minor importance when judging the quality of a sun lamp, as this next example will also demonstrate.

this has the effect of lowering the total amount of UV radiation by around 15 per cent, which is bound to reduce the lamp’s intensity, in particular regarding the immediate effect of pigment darkening. Despite the higher percentage of UVB, this "improved" model is in fact no more effective; i.e. exposure times cannot be reduced.

It is therefore misleading to judge a lamp’s effectiveness on the strength of the UVB percentage.

It’s a well-known fact that in physical terms, the UVB range covers the wavelength spectrum from 280 to 315 nm. But this value, being a purely physical definition, gives only an approximate indication at best of the effects that can be expected. The individual UVB values have to be considered in the light of their corresponding function to understand more about their effectiveness. The effects of individual wavelengths are sufficiently known in the case of various photo- biological phenomena, such as sunburn or erythema, not to mention tanning (pigment formation and darkening); that is, there exist "action spectra" (relative spectral efficiency curves) from which the effectiveness of each individual wavelength can be deduced.
If one looks at the efficiency of the individual UVB wavelengths for pigment formation, for example, it can be seen that UVB at 300 nm trigger ten times more melanin production than radiation at 310 nm. This means that more than ten times the exposure to UVB radiation at 310 nm is required to achieve the same pigment-forming effects as with UVB radiation at 300 nm. UVB radiation at higher wave- lengths, such as 314 nm, is even less effective in terms of pigment formation – one-fortieth as effective as at 300 nm.

However, stated UVB percentages include all UVB wavelengths together, despite the fact that their effectiveness can vary by a factor of around 100 depending on the wavelength. It would therefore be much more informative to state a value that indicates the pigment-forming capacity rather than the percentage of UVB radiation. Only then would it enable customers or prospective buyers to judge the real tanning performance of a sunlamp.

Of course, to achieve a lasting tan it requires both pigment formation and pigment darkening. It is usually said that pigment formation is triggered by the UVB rays, which is largely true of natural sunlight, but is not true of sun lamps. Owing to the comparatively high radiation intensity of UVA, some of these rays also play an important role in pigment formation. Depending on lamp type, up to 70 per cent of the pigment-forming effects can come from the UVA rays; that is, the UVB often play only a minor role in forming melanin. Even in lamps with a high proportion of UVB, the UVA radiation is still responsible for around 40 per cent of pigment formation. The percentage of UVB radiation therefore by no means gives sufficient indication of the pigment-forming efficiency of a sun lamp.
   

This is not so in the case of the effects of UV radiation in pigment darkening. In this case, there is an almost direct correlation between the UVA percentage and the immediate tanning effect, whereas the effect of the UVB is virtually negligible. This means that the higher the proportion of UVA radiation, the faster and more effective is pigmentation. The required exposure times until tanning becomes visible are therefore shorter the more intensive the UVA radiation.

To achieve a beautiful, tanned skin it requires both kinds of UV radiation, UVA and UVB. The unique emphasis placed on UVB radiation and even more so on UVB percentages is of practically no use in judging the effectiveness of a sun lamp. Ultimately, both tanning and the length of exposure depend on the right mixture of UVA and UVB. In fact, the real key to the whole question is exposure time: after all, the aim is to trigger the skin's tanning mechanism during a defined period of use while avoiding reddening. Instead of inquiring about the proportion of UVB, solarium users should ask about the recommended exposure times for the different lamp types, since it is these values which indicate the intensity of radiation that will induce tanning without causing sunburn. This is the only way of ensuring user satisfaction in the long term.
A second point should also be taken into account when choosing sun lamps: the reduction over time in radiation efficiency. This again may vary widely from lamp to lamp. Some lamps show a reduction in UV efficiency of 20 per cent or more after only 100 hours of use, and others a reduction of only eight per cent or less. These quality-related characteristics greatly affect not only the useful life of lamps, but also the effective exposure time for individual session.

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