Construction of spectral sensitivity function using polychromatic UV sources

Journal

of

Photochemistry Photobiology and

www.elsevier.nl/locate/jphotobiol

ELSEVIER

B:Biology

J. Photochem. Photobiol. B: Biol. 49 (1999) 171-176

Construction of spectral sensitivity function using polychromatic UV sources K. Modos a.,, S. Gaspar a, p. Kirsch b, M. Gay b, Gy. Ronto a Semmelweis University of Medicine, (/ll6i u. 26, H-1085 Budapest VIII, Hungao' h University of Manchester Institute of Science and Technology, PO Box 88, Manchester M60 1QD, UK Received 22 July 1998: accepted 10 March 1999

Abstract

A procedure is presented for constructing the spectral sensitivity functions of biological dosimeters, using five polychromatic UV sources possessing different emission spectra. Phage T7 and uracil biological dosimeters have been used for measuring the dose rates of the lamps. Their spectral sensitivity functions consisting of two exponential terms have been constructed. The parameters of the spectral sensitivity function:~ have been determined by comparing the directly measured and calculated dose-rate values. The parameters of the sensitivity function are accepted as correct values when the deviation of the measured and calculated values is a minimum. Based on the deviations between the constructed and the experimentally determined spectral sensitivities with monochromatic sources, the differences between the measured and calculated results are interpreted. The importance of the correct spectral sensitivity data is demonstrated through the effectiveness spectra of a TL 01 amp for phage T7 killing, uracil dimerization and erythema induction. © 1999 Elsevier Science S.A. All rights reserved. Keyworda: Polychromatic UV sources; Spectral sensitivity of biological dosimeters: Effectiveness spectrum; Construction of sensitivity function

1. Introduction

The measurement of the biologically effective dose (BED) is becoming more and more essential because of the growing biological hazard due to increasing UV-B radiation connected with stratospheric ozone depletion [ 1-3]. This tendency, induced by man-made atmospheric changes, has been a potential global problem for several decades. Deviations from the average ozone level, obtained in the years 1964-1980 for the middle and the polar latitudes, show a dramatic decrease over the last 20 years [4,5]. The UV data are collected by continuously improved physical methods/equipment allowing more and more precise measurement of the UV-B radiation. Biological consequences of increasing UV-B radiation are relevant at both global and local scales: the tendency of the biological risk is a global problem, while the local and temporary ozone reductions can be of local importance [6,7]. Recently different biological dosimeters have been developed, suitable for monitoring the (BED) essential for assessing the biological risk. Among these dosimeters several measure the DNA damage induced by UV-B radiation [ 8 * Corresponding author. Tel./Fax: +36-1-266-6656; E-mail: modos@ puskin.sote.hu

12]. Field intercomparison campaigns have been organized with the aim of comparing the reading of the biological dosimeters with the spectroradiometric data weighted by the corresponding spectral sensitivities and, in addition, of determining precisely the possibilities of transformation of the different readings obtained by different biological dosimeters. These comparisons/transformations have been performed by simultaneous measurements at the same places, i.e., the spectral distribution of the environmental radiation can be considered as identical for all measurements [ 13]. In laboratories studying the biological effects of artificial UV sources, the aforementioned comparability would also be essential for the interpretation and generalization of the experimental data. However, in the case of radiation from the mostly polychromatic artificial UV sources with different spectral distributions, this requires accurate knowledge of the spectral sensitivity functions of the biological effects and dosimeters. This problem is of special importance in complex biological responses, like the development of skin cancer or immunosupressive reaction. In addition, for polychromatic radiation the wavelength interaction (synergism or antagonism) can influence the biological effect to be measured as well. For validation of the biological dosimeters, the BIODOS group recommended the determination of the spectral

1011-1344/99/$ - see front matter © 1999 Elsevier Science S,A. All rights reserved. PHS101 !-1344(99)00052-4

172

K. Modos et al. / J. Photochem. Photobiol. B: Biol. 49 (1999) 171-176

sensitivities with polychromatic UV sources as well as the normal monochromatic studies [ 14]. The calculated dose based on an action spectrum measured with a monochromatic light source was different from the measured dose using a polychromatic source. This fact indicates the problem of accuracy of the monochromatic action spectrum. In this study a relatively simple method resulting from a simple mathematical description is presented for the construction of the wavelength dependence of the sensitivity of any UV effect using conveniently available polychromatic UV sources of known spectral irradiances for the calibration. Bioiogical dosimeters based on simple systems (phage T7 and uracil thin layers) are used to demonstrate the procedure and a comparison with the commonly used RB meter is included as well. The possibilities and limits of the method are discussed.

2. Materials and method

2.1. Radiation sources For irradiation the following lamps have been used: TL 01 (Philips) without filtering and with a phthalic acid filter, FS 20 (Westinghouse) without filtering and filtered with phthalic acid solution, and a solar simulator (Xe lamp) with WG 305 filter. The spectral irradiances have been determined in the 270340 nm wavelength range at 1 nm steps with a spectroradiometer (Optronic 742). Fig. 1 presents all five spectra. The absolute irradiance values, indicated on the vertical axis, only show values above the noise level of the instrument. The filtering effect of phthalic acid (0.055%, 1 cm thick) is well observable: a decrease of about two orders of magnitude is found at the wavelength 300 nm. Irradiation of phage T7 suspension and uracil thin layers has been performed at the same position for which the irradiance data are given. The mea~,+uring head of the RB meter (YES UV-1 ) also had the same position.

2.2. Measurement of the biological dose rates The phage T7 solution was irradiated in an open glass Petri dish, and samples were taken at appropriate exposures. The number of survivors has been determined on E. colt B/r host cells. Using these values, the H T7 dose (the dose of radiation that decreases the number of active phages by a factor of e, so that the term ln(n/no) is equal to - 1 ), related to 1 h, has been determined according to the method published earlier [9]. Irradiation of the uracil thin layers was performed at the same place as for the phage T7, but for longer exposures because of the lower sensitivity of the evaluation method. Determination of the H v dose related to I h (the dose of the radiation when the absorbance of the uracil layer decreases

10-3G

1

--

-, ~"~

. . . . FS20

10-5

~

solar simulator --TLOI

- - -TL Ol + filter . . . . FS 20 + filter

.- ~

.~''

,."p

l

/ /

£ .JI. ~ . . . " ~ '.~" . / " .-.~ . ~ : ~ - J

10"7

.¢,,," ~ ,

10-9

10-1~_ 260

280

......;'~' 300 wavelength (rim)

320

Fig. 1. Emission spectra of UV sources, measured with Optronic 742 spectroradiometer, corrected for light scattering.

by a factor of e) has been performed by measuring the decrease of uracil absorbance with a UV-Vis JASCO 7800 spectrophotometer. The method has been described previously [ 12]. For the RB meter the direct readings of erythemally effective UV have been taken into account without further data processing. Phage T7 and uracil biological dosimeters weight directly the irradiances coming from the lamps [ 15,16], thus the possible inaccuracies in the spectral sensitivities do not influence the accuracy of the dose/dose-rate values.

2.3. Construction of the sensitivity spectra For the analytical description of the sensitivity spectra the evaluation method for the DNA damage action spectrum used by Setlow [17,18] was applied, i.e., in the 270-340 nm wavelength range the sensitivity spectra have been assumed to consist of the following two exponential functions:

S(h)=aexp[-bl (A-At)] if h
(la)

and

S ( A ) = a e x p [ - b z ( A - A o ) ] if A->At

(lb)

where At is the wavelength of the common point of the two exponential parts, while a, bl and be are constants. Using this constructed sensitivity curve, a dose value related to 1 h was calculated for each lamp. Eq. (2) gives an example for the uracil dosimeter: H i u alc ~---EE(A)S(A)At

(2)

where the index i denotes the lamps and At was taken as 1 h. A similar calculation was done for the T7 phage dosimeter. These calculated dose values depend on the unknown parameters of the sensitivity curve. Our aim was to find the best sensitivity curve when these calculated dose values are closest to the measured ones (Hi ..... ). Qe has been set up according to the following equation:

(H,c.,c-Hi....V-

Varying the parameters of the sensitivity curve in Eqs. ( 1 ), the value of Qe has been changed. The best sensitivity curve

K. Modos et al./ J. Photochem. Photobiol. B: Biol. 49 (1999) 171-176

was accepted, characterized by the parameters, a, b l , b2 and Ao, when Q, was the minimum. To determine the minimum of the quadratic sum, the non-linear Newton-Raphson approximation method [ 19] was applied. For phage T7 and uracil dosimeters, as well as for the RB meter, these bestfitting parameters of the sensitivity spectra are presented in Table l.

3. Results and discussion

173

Table 1 Parameters of spectral sensitivity functions calculated for different dosimeters

Ao (nm) a (m2/J) bl (l/rim) be (1/nm)

Dosimeter T7 phage

Uracil layer

RB meter

286.0 0.313 0.0 0.283

287.9 0.00319 0.0 0.4

302.9 0.00131 0.0 0.152

3.1. Results of the calculations 1-

The results of the calculations are presented in both Fig. 2 and Table 1. In the latter the parameters are listed which satisfy Eqs. (1) and give the best fit to the measured dose rates with the five various UV sources. In Table 1 for all three dosimeters one common point ()to) of the ~alrves is given, i.e., the best fit of the spectral sensitivity curves found from the two exponential terms. The wavelength~ of the common points for phage T7 and uracil dosimeters are very close to each other at 286 and 287.9 nm, respectively, while the spectral sensitivity curve of the RB meter shows a common point at 302.9 nm. For each dosimeter three t~xponential terms (two breaking points) have been used ar~ well, but the fit was not better. Parameter a gives at )t = Ao the intercept of the vertical axis in m2/j. For phage T7 this value is 0.313 me/J, which is higher by about two orders of magnitude than for the uracil dosimt:ter and RB meter, indicating the higher sensitivity of the phage T7 dosimeter because of the 'amplifying ability' of the biological system. This is based on the specificity of the ph~ge, which contains a small DNA molecule comprising only w)out 10% non-coding sequences, in which a single photoproduct (e.g., pyrimidine adduct) can induce phage inactivation [20-23]. In the uracil dosimeter UV photons induce similar photoproducts (mainly dimers); however, their II~easurement is less sensitive due to the spectrophotometric method used in the evaluation. For the RB meter a similar .,ensitivity was found to that of the uracil dosimeter. Parameter b~ corresponds to the slope of the first exponential teml: this value was found to be zero by all the three dosimeters, indicating a constant sensitivity for the wavelength range 250 nm-)to. Similarly, a zero slope value has been fi)und for the analytical description of the erythema action spectrum, and is generally applied according to CIE recommendations [24]. This is acceptable taking into account that there is virtually no solar radiation at A < 295 nm. Our result concerning the value of bj shows that the contribution of this wavelength range to the total dose is negligible, so the fitting method is not sensitive to the value ofb~. Considering the spectra of the lamps used for the determination of the action spectra of phage T7 and uracil dosimeters, in the case of filtered TL 01 and FS 20 lamps the contribution of the wavelengths under )to is completely negligible. In the

..... I......I'.....I. _~

..... ~ ! . . ~ .r

,0 4

\

<.:?.

~

\'.

,0

.7 ...... "

m

10-8 -

--

T7 (measured) -

.....

'...

0a,oolate.) \

uracil ( c a l c u l a t e d )

. .'-. \

uracil ( m e a s u r e d )

"

\

"" ~ .

i

\ \ \ \

250

270

290

310

330

wavelength(nm) Fig. 2. Spectral

sensitivities

of T7 and

uracil

dosimeters.

case of the other three lamps, the UV-C components do not play any definitive role in the measured dose rates. Therefore a more precise estimation of parameter b] (and the value of intercept a) is possible using UV sources with a higher contribution of shorter-wavelength components. Hence these short-wavelength parts of the action spectra have only limited application possibilities, i.e., only with sources that have negligible output at h < Ao. Parameter be describes the slope of the second exponential term, from the common point, ho, to 320 nm. As in this term the exponent has a negative sign, this value characterizes the rate of decrease of the spectral sensitivity in the wavelength range higher than the common point. The most rapid decrease has been found for the uracil dosimeter.

3.2. Spectral sensitivities In Fig. 2 the spectral sensitivities (action spectra) are presented both for phage T7 and uracil dosimeters, based on the dose data obtained with polychromatic UV sources and constructed according to the calculations described above. For comparison the spectral sensitivities, determined earlier [ 9,12] with monochromatic radiation, are presented with the measuring errors. For phage T7 the two spectral sensitivities deviate at shorter wavelengths (A < 295 nm), while for uracil they deviate at longer ones (A > 295 nm). Table 2 shows the measured and calculated doses related to 1 h. The numbers in parentheses were calculated using the spectral sensitivity values measured earlier [9,12] with

K. Modos et al. / J. Photochem. Photobiol. B." Biol. 49 (1999) 171-176

174

Table 2 Comparison of measured (He...... ) and calculated (H~,c~) doses related to 1 h. (The calculated terms are based on the spectral sensitivity for polychromatic light. The calculated doses based on the experimentally determined sensitivity curves with monochromatic UV source are indicated in parentheses.)

H ~ 7 measured H3" calculated H t measured H t calculated

TL 01

TL 01 + F

FS 20

FS 2 0 + F

Solar sire.

20.00 25.03 (12.41) 0.170 0.197 (0.320)

4.00 3.85 (3.94) 0.00600 0.00533 (0.02600)

91.00 79.84 (19.92) 1.000 0.885 (1.290)

0.90 0.94 (0.81) 0.00150 0.00194 (0.00670)

2.60 2.36 (1.26) 0.0190 0.0189 (0.0380)

m~mochromatic UV light. For the unfiltered sources, containing significant short-wavelength components in their spectra, the T7 dose values related to 1 h, calculated according to the spectral sensitivities measured with monochromatic light, are faz (two to three times) below the measured as well as the ca)culated values, indicating that the phage inactivation effectiveness of the shorter wavelengths (A < 295 nm) was underestimated. Calculated n T 7 / h data, based on the spectral ser~sitivity data presented here, fit quite well to the measured onc'~.

'['he dose-rate data, HU/h, calculated for the uracil dosimer~er according to the spectral sensitivity obtained with monochromatic light, exceed by two to five times the measured values at both types of TL 01 lamp and the filtered FS 20 h~mp. Taking into account the emission spectrum of the TL (~1 lamp, dominated by the wavelength 312 nm, and that of 1he filtered FS 20 lamp having mostly long-wavelength components, the observed deviations indicate the overestimation of the uracil spectral sensitivity at A > 290 nm. For comparison, the RB meter spectral sensitivity data determined by the manufacturer have been used. Performing the same calibration procedure as for phage T7 and uracil dosimeters, no significant difference can be found either in spe¢ lral sensitivity or in calculated dose-rate values.

3.3. ~Spectralsensitivities for biological UV effects T~e spectral sensitivity for various biological UV effects, like killing efficiency of simple organisms, production of i*

erythema, DNA damage, skin cancer induction, immunosuppression etc., has been determined mostly with monochromatic light [ 25-30]. Efforts have also been made for creating appropriate functions (two or more exponential terms) to describe mathematically the spectral sensitivity data [ 18,34]. However, solar radiation is a polychromatic source, thus, for complex biological systems/effects the wavelength interaction (antagonism or synergism) can modify the action spectrum determined with monochromatic sources [ 31 ]. The sensitivity of our dosimeters at longer wavelength is very small, so the determination of the correct sensitivity value in this range is very difficult. We had to use a wide monochromator slit (in an extreme case it was about 10 mm) in order to get enough light intensity and we had to irradiate the samples for a long time to get a measurable effect, both of which caused many difficulties. A wider slit transmits a higher amount of shorter-wavelength light. After the monochromator a small amount of shorter wavelengths is always present, which multiplied by the high sensitivity of the dosimeter in this region distorts the result. The WG 305 cut-off filter helps to remove this part of the radiation, but according to our measurements after a glass filter there is an effect of the shorter wavelength on the uracil layer that is not negligible. In Fig. 3 the dose contribution of the shorter-wavelength range on the uracil layer or T7 phage is significantly higher than that of the longer one, while the emission of the radiation source in this range is practically negligible. There

S

0.3E 0.25 o,2

'~ o.15

0.0: TL 01 lamp ~B meter 1 layer

wavelength(nrn)

o'-

Jl~

324 329

Fig. 3. Normalized emission spectrum of TL 01 lamp and its normalized effectiveness spectra for the RB meter, phage T7 and uracil dosimeters.

K. Modos et al. /J. Photochem. Photobiol. B: Biol. 49 (1999) 171-176

are filters, e.g., phthalic acid, that are better but they cannot be used for a long time without their parameters changing. Due to the very small sensitivity of the dosimeter, especially for the uracil layer, the duration of irradiation was hours or days. This observation is a consequence of the fact that the sensitivity of the layer decreases very rapidly with increasing wavelength in this range. We cannot avoid this phenomenon due to the very steep slope of the sensitivity curve. Hence the sensitivity curve in this wavelength range determined with a monochromatic source contains a systematic error. The calculation of the precise sensitivity value with a monochromatic source is obscure due to the difficulty in determining the accurate light dose. This fact appears as the small accuracy of these values or the discrepancy between sensitivity curves measured with a monochromatic source and the calculated one. In this wavelength range the presence of photodamage and photorepair should be considered as well. A similar difference between action spectra determined by monochromatic and polychromatic radiation was observed in normal human skin fibroblasts [ 32]. The result of these problems is the observed difference between the measured dose (with a polychromatic source) and the calculated dose (based on the measured irradiance of the lantp and the sensitivity curve determined with a monochrom;aic source). In the method presented here the sensitivity curve was constructed according to the measured doses (determined with polychromatic light sources), thus the fitting procedure resulted in the best curve when these differences were minimal. Ho~ ever, the use of a polychromatic source to determine spectral sensitivities has been suggested recently [33,34], aiming towards the validation of biological dosimeters. In the calibra6 on procedure presented here, an effort has been made to con;~truct spectral sensitivity functions and find simple exponential terms for the description of the action spectra. A precise and comprehensive action spectrum determination ha~; been performed earlier [35,36] for a complex effect, for Utrecht-Philadelphia skin cancer on hairless mice using 14 different polychromatic UV sources, including a lowpressure mercury lamp. In the 250-400 nm wavelength range the approximation by a Lagrange polynomial of fourth order resulted in a satisfactory fitting of the calculated and measured data. According to the statistical analysis, at A < 280 nm and A > 340 nm very large errors have been found. This is connected on the one hand with the low contribution of the shorter-wavelength components in the emission spectra of the lamps used, and on the other hand with the low response of the biological system to the longer-wavelength components. The spectral sensitivity of the dosimeters can strongly influent;e the measured dose values of artificial UV sources, as well as of solar radiation at different latitudes and times of the year [ 37]. In addition, the readings of different dosimeters with different spectral sensitivities can indicate for the same UV source the contribution of different wavelengths in

175

the effect. Thus, the correct spectral sensitivity of a dosimeter is an essential requirement both for biological and physical dosimeters. Fig. 3 gives some supporting evidence for this opinion, using the frequently used TL 01 lamp. The emission spectrum of the lamp is represented in the Figure, with the area under the curve taken as unity. In the emission spectrum 311-313 nm are the dominating wavelengths. The three other curves are the effectiveness spectra of the lamp for RB meter response, uracil and phage T7 dosimeters, respectively. All the effectiveness spectra are normalized, i.e., the doses/dose rates determined at the lamp with all the three dosimeters correspond to unity. However, for the RB meter reading the contribution of the wavelengths 310-318 nm is decisive, while for the uracil dosimeter the response originates mostly from wavelengths lower than 300 nm and for the phage T7 dosimeter the wavelengths 310-318 nm and those under 300 nm contribute almost equally to the total. From this example one can conclude that slight deviations in the spectral sensitivity can lead to large differences in the calculated doses due to the weighting process.

4. Conclusions Finally, we suggest performing similar procedures to construct spectral sensitivity functions/action spectra for the induction of different complex but diverse biologically important end effects. The use of various polychromatic UV sources (as in Refs. [35,36] ) can be very useful, e.g., in the construction of spectral sensitivity functions of different types of photoproducts, like dimers, (6--4) adducts, crosslinks, etc. The selection of the sources with appropriate emission spectra for these measurements seems to be very important to get the best fit in the sensitivity functions.

Acknowledgements This work was supported by the EU: EV5V-CT91-0034, ENV4-CT95-0044, ENV4-CT95-0174; OMFB (National Committee for Technological Development): EU96-C6-005; the Ministry of Welfare: ETT 7-14; and the Ministry of Education: FKFP 1190/1997. The authors thank A. Webb for her valuable discussions in preparing the manuscript.

References [ 1 ] R.D. Bojkov, C.S. Zerefos, D.S. Balis, I.C. Zlomas, A.F. Bais, Record low total ozone during northern winters of 1992 and 1993, Geophys. Res. Lett. 20 (1993) 1351-1354. [2] UNEP, Environmental Effects of Ozone Depletion: 1989 Update, Panel report pursuant to Article 6 of Montreal Protocol on substances that deplete the ozone layer, UNEP, 1989. [3] UNEP, Environmental Effects of Ozone Depletion: 1991 Update, Panel report pursuant to Article 6 of Montreal Protocol on substances that deplete the ozone layer, UNEP, 1991.

176

K. Modos et al. /J. Photochem. PhotobioL B: BioL 49 (1999) 171-176

[4] R.D. Bojkov, V.E. Fioletov, A.M. Shalamjansky, Total ozone changes over Eurasia since 1973 based on re-evaluated filter ozoneometer data, J. Geophys. Res. 99 (1994) 985-999. [5] R.D. Bojkov, Review of long-term ozone changes, in: A. Ghazi, P. Mathy, C. Zerefos (Eds.), Eastern Europe and Global Change, EC DG XH, 1997, pp. 49-60. [61 R.L. McKenzie, A Southern Hemisphere Perspective on Global UV Radiation, Special lecture at 7th ESP Congress, Stresa, Italy, 1997. [ 71 A.GJ. Buma, W.W.C. Gieskes, UVB Radiation and Ecosystems in Polar Regions, Invited paper, EC Advanced Study Course, Bad Honnef, 1998. [ ~1 N. Mtmakata, IOlling and mutagenic action of sunlight upon B. subtilis spores: a dosimetric system, Mutat. Res. 82 ( 1981 ) 263-268. ['~] G. Ronto, S. Gasper, A. Berces, Phages T7 in biological UV measurement, J. Photochem. Photobiol. B. Biol. 12 (1992) 285-294. [ 1~11 L.E. Quintern, G. Horneck, U. Eschweiler, H. Bucker, A biolfilm used as ultraviolet dosimeter, Photochem. Photobiol. 55 (1992) 389-395. [1 1 J.D. Regan, W.L. Carrier, H. Gucinski, B.L. OUa, H. Yoshida, R. Fujimura, R.I. Wicklund, DNA as a solar dosimeter in the ocean, Photochem. Photobiol. 56 (1992) 35-42. [ 1~l P. Grof, S. Gaspar, Gy. Ronto, Use of Uracil thin layer for measuring biologically effective dose, Photochem. Photobiol. 64 (1996) 800806. [13] G. Horneck, Gy. Ronto, N, Munakata, A.F. Bais, P. Rettberg, S. Gaspar, P. Grof, S. Kazadzis, A. Kazandzidis, First field intercomparison of biological UV dosimeters and spectral UV measurements, Geophys. Res. Lett., submitted for publication. [141 BIODOS, Progress Report No. I. Contract No. ENU4-CT95-0044, 1977, Catalogue of criteria for reliable biological UV dosimeters. [ 151 G. Horneck, Quantification of the biological effectiveness of environmental UV radiation, J. Photochem. Photobiol. B: Biol. 31 (1995) 43-49. [ 1 6 Gy. Ronto, P. Grof, S. Gaspar, Biological UV dosimetry - - a comprehensive problem, J. Photochem. Photobiol. B: Biol. 31 (1995) 5156. [ 17 NSF UV Polar Spectroradiometer Monitoring Network: Standard CD ROM data release (1991). [18] R.B. Setlow, The wavelengths in sunlight effective in producing skin cancer: a theoretical analysis, Proc. Nat/. Acad. Sci. USA 71 (1974) ~363-3366. [ 19] &. Ralston, First Course in Numerical Analysis, McGraw-Hill, New York, 1965. [20] LJ. Dunn, F.W. Studier, Complete nucleotide sequence of bacteriophage T7 DNA and locations of T7 genetic elements, J. Mol. Biol. 166 (1983) 477-535. [211 F W. Studier, J.J. Dunn, Organization and expression bacteriophage T7 DNA, Cold Spring Harbor Syrup. Quant. Biol. 47 (1983) 999007. [22] Gy. Ronto, K. Sarkadi, I. Tarjan, Zur Analyse der UV-Dosiswirl,ungskurven der T7 Phagen, Strahlentherapie 134 (1967) 151-157.

[23] A. Fekete, A.A, Vink, S. G~isp/tr,A. B~rces, A. M6dos, Gy. Ronto, L. Roza, Assessment of various UV sources on inactivation and photoproduct induction in phage T7 dosimeter, Photochem. Photobiol. 68 (4) (1998) 527-531. [24] A.F. McKirday, B.L. Diffey, A reference action spectrum for ultraviolet induced erythema in human skin, CIE J. 6 (1987) 17-22. [ 25 ] N. Munakata, Ultraviolet sensitivity of Bacillus subtilis spores upon germination and outgrowth, J. Bacteriol. 120 (1972) 59-65. [26] R.M. Tyrrell, Solar dosimetry with repair deficient bacterial spores: action spectra, photoproduct measurements and a comparison with other biological systems, Photochem. Photobiol. 27 (1978) 571-579. [27] A. Emrick, J.C. Sutherland, Action spectrum for pyrimidine dimer formation in I"7 bacteriophage DNA, Photochem. Photobiol. 49 (1989) 35S. [28] G. Ronto, A. Fekete, S. Gaspar, K. Modos, Action spectra for photoinduced inactivation of bacteriophage T7 sensitized by 8-methoxypsoralen and angelicin, .l. Photochem. Photobiol. B: Biol. 3 ( 1989 ) 497-507. [29] C. Petit-Frere, P.H. Clingen, M. Grewe, J. Krutman, L. Roza, C.F. Arlett, M.H.L Green, Induction of interleukin-6 production by ultraviolet radiation in normal human epidermal keratinocytes, J. Invest. Dermatol,, submitted for publication. [30] R.B. Webb, M.S. Brown, R.M. Tyrrell, Synergism between 365 nm and 254 um radiations for inactivation of E. coli, Radiat. Res. 74 (1978) 298-311. [ 31 ] M.J. Peak, J.G. Peak, R.B. Webb, Synergism between different nearultraviolet wavelengths in the inactivation of transforming DNA, Photochem. Photobiol. 21 (1975) 129-131. [32] B.S. Rosenstein, D.L Mitchell, Action spectra for the induction of pyrimidine (6-4) pyrimidone photoproducts and cyclobutan pyrimidine dimers in normal human skin fibroblasts, Photochem. Photobiol. 45 (1987) 775-780. [ 33 ] G. Horneck, P. Rettberg, W. Strauch, D. Bolsee, D. Gillotay, P. Simon, Biologically weighted dosimetry of solar radiation with the biofilm: I. Characterisation of the biofilm, Lecture, 2rid Int. Workshop Biol. Dosimetry, Hungary, 15-30 Aug., 1996. [34] D. Bolsee, D. Gillotay, Catalogue of performances of the BIODOS characterization facility (polychrornatic radiation), personal communication. [35] F.R. de Gruijl, H.J.C.M. Sterenborg, P.D. Forbes, R.E. Davies, C. Cole, G. Kelfkens, H. van Weelden, H. Slaper, J. van der Leun, Wavelength dependence of skin cancer induction by ultraviolet irradiation of albino hairless mice, Cancer Res. 3 (1993) 53--60. [36] F.R. de Gmijl, J.C. van der Leun, Estimate of the wavelength dependency of ultraviolet carcinogenesis in humans and its relevance to the risk assessment of a stratospheric ozone depletion, Health Phys. 67 (1994) 319-325. [37] A.R. Webb, Solar ultraviolet radiation in southeast England: The case for spectral measurements, Photocbem. Photobiol. 54 (1991) 789794.