How to measure the Vitamin-D-synthetic activity of UV lamps used in phototherapy?

Ultraviolet lamps are widely used in phototherapy, and the positive effect of ultraviolet radiation is mainly associated with the synthesis of vitamin D in human skin. Nevertheless, to avoid harmful effects the biological efficacy of UV lamps is still evaluated based on the erythema action spectrum using UV detectors with an output in erythema units. Evaluation of vitamin D synthesis on this basis is inadequate because of the difference between the erythema and the Vitamin D synthesis action spectra. Hence, direct measurement of the vitamin-D-synthetic activity is a missing link in the metrology of UV lamps that are used for medical and/or cosmetic purposes. This paper presents original methods based on the same photoreaction in vitro by which vitamin D is synthesized in human skin via photoand thermoinduced conversions of 7-Dehydrocholesterol (provitamin D3). The UV photons are absorbed by provitamin D molecules in solution or embedded in specially designed UV transparent and stable matrix mimicking biological samples. Three operation modes of varying complexity have been developed to follow the photoreaction course in real time, and the results are presented of measuring the vitamin D synthesizing activity of several UV lamps, as well as the first performed comparative studies on direct measurements of the vitamin D level in blood and in solution. Introduction The discovery of UV radiation in the 19th century, its properties and the connection with physiological and pathological changes in humans led to the conclusion that UV radiation has both beneficial and harmful effects, depending on the type of organism, wavelength region and the radiation dose. The varied effects derive, in part, from the differences among UV-A (320-400 nm), UV-B (280-320 nm) and UV-C (100-280 nm) photons energy and also due to the different structures that are capable of absorbing UV photons in living organisms. UV photons are absorbed by UV sensitive molecules in skin and initiate a variety of photochemical reactions resulting in structural changes that could lead to positive or negative biologic effects depending on the accepted UV dose. Deoxyribonucleic acid (DNA) is one of the most important target molecules for photobiological effects. A large number of different types of UV induced photoreactions are produced in DNA molecule that can result in mutations and even cell lethality. Formation of cyclobutylpyrimidine dimers in DNA contributes to the mechanism of erythema and sunburn formation [1]. Fortunately, a cell has an enormous capacity to repair all types of damage to its DNA. Another chromophore in the epidermis is 7-dehydrocholesterol (7DHC, provitamin D3), and the UVB photons penetrating into human skin are responsible for its conversion into the active form of vitamin D3, necessary for the normal calcium absorption and metabolism in the body. In recent years interest in Vitamin D has increased greatly in view of new data about its important role in reducing the risk of cancer, multiple sclerosis, and type 1 diabetes mellitus [2]. The vitamin D active metabolite 1,25(OH)2D3 is recognized as a critical hormone regulating cell growth and modulating the immune system [3]. The measurement of the vitamin-D-synthetic ability of UV lamps used for medical and/or cosmetic purposes is especially important given the essential role of vitamin D in maintaining health, as well as taking into account the observed pandemic of vitamin D deficiency among the world's population [2]. In this article we will focus on the calculation and measurement of the vitamin D effective irradiance of UV sources using original method based on an in vitro model of vitamin D synthesis. Action spectra and biologically effective irradiances Each specific biological effect of UV radiation is characterized by its own action spectrum (AS) defined as the spectral dependence of the value of the biological effect initiated by monochromatic radiation of different wavelength with the same dose. A biologically effective irradiance Eeff of a UV source can be calculated by weighting its spectral irradiance by the appropriate action spectrum and integrating over the wavelength interval for which the action spectrum is non-zero [4]. Here Eλ(λ) spectral irradiance of a UV lamp measured by a spectroradiometer [Wm-2nm-1], Sλ(λ) action spectrum [relative units], λ wavelength [nm]. Being mathematically determined, the CIE erythema action spectrum is widely used for estimation of the erythemally active irradiance of sunlight and artificial UV sources [5]. Correspondence to: Irina P Terenetskaya, Doctor of Phys.-Math. Sciences, Professor, Leading Scientist, Department of Optical Quantum Electronics, Institute of Physics NAS, Ukraine, E-mail: teren@iop.kiev. ua