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The effect of UVA radiation (321-400 nm) on UVB photocarcinogenesis was examined in lightly pigmented hairless hr/hr C3H/Tif mice. Five groups of 22 mice were exposed to UVB radiation (281-320 nm) from one Philips 12 tube for 10 min per day and 4 days per week. Four of the groups were simultaneously exposed to UVA from two to six filtered Philips 09 tubes. The daily dose of UVB was 2.0 kJ m-2 in all five groups; the UVA2 (321-340 nm) dose varied from 0.7 to 4.5 kJ m-2 and the UVA1 (341-400 nm) dose from 0.3 to 45.6 kJ m-2. A sixth group was exclusively irradiated with the filtered UVA tubes and served as a control. Skin tumor development was not significantly different for the groups exposed to the UVB source alone or the UVB source in combination with the largest daily UVA dose (0.2 > p > 0.1). Skin tumor development was significantly delayed in the other groups irradiated with the UVB source and the lower doses of UVA (p < 0.001). No tumors were observed in the control group. This study suggests that UVA1 radiation delays UVB-induced skin tumor development. However, the delay cannot be expected to persist when UVA is administered in higher daily doses.

The carcinogenic effect of three UVA tanning sources was studied in lightly pigmented hairless mice. The three tanning sources (Bellarium-S SA-1-12, Philips TL 09R and Philips TL 10R) have different emission spectra, and emit different amounts of UVB. Radiation from the tanning sources was administered for 20 min/day, 5 day/week in daily doses equivalent to those used in suntan salons. The radiation was given alone or after 12 weeks of exposure to solar-simulated UV radiation (SOLAR UV) (10 min/day, 4 day/week; daily dose, 19.5 kJ/m2 UVA and 3.9 kJ/m2 UVB). Irradiation with Bellarium-S SA-1-12 for 47 weeks and Philips TL 09R for 74 weeks induced skin tumours in 20/20 and 13/20 of the animals, respectively. When irradiation with Bellarium-S SA-1-12 and PHilips TL 09R was administered after 12 weeks of SOLAR-UV exposure, a strong enhancement of SOLAR-UV-induced photocarcinogenesis was observed (p < 0.001). Irradiation with Philips TL 10R was only slightly carcinogenic, and during 85 weeks of irradiation only one skin tumor appeared in a group of 20 mice. However, when irradiation with Philips TL 10R was administered after 12 weeks of exposure to SOLAR UV, an enhancement of SOLAR-UV-induced carcinogenesis was observed (p < 0.001). Our results suggest that the hazards of exposure to commercial tanning devices are increased when they are used after a period of natural sun exposure. Even tanning sources with a low carcinogenic potential are able to increase SOLAR-UV-induced carcinogenesis significantly.

The carcinogenic effect of 3 commercially available ultraviolet A (UVA) tanning sources was studied in lightly pigmented hairless mice. The tanning sources (Bellarium-S SA-1-12 and Philips TL 09R and TL 10R) have different emission spectra and emit different quantities of UVB. The tanning sources were administered either alone, or before irradiation with solar-simulated UV (solar UV). All 3 UVA tanning sources were able to induce skin tumors when administered in daily doses resembling those used in tanning salons (20 min/d, 5 d/week). Irradiation with Bellarium-S during 32 weeks induced skin tumors in all mice; a similar response was seen after 66 weeks of irradiation with Philips TL 09R. Irradiation with Philips TL 10R during 98 weeks induced tumors in 6 of 20 mice. Nine groups of 20 mice were pretreated 20 min/d, 5 d/week during 13 weeks with one of the UVA tanning sources. Three groups were irradiated with Bellarium-S, 3 groups with Philips TL 09R and 3 groups with Philips TL 10R in daily doses ranging from 0.2 to 1.8 minimum erythema doses (MED). The highest daily doses were equivalent to the doses received during one session in a commercial solarium. Subsequently all 9 groups were irradiated with 3.1 MED/d solar UV 10 min/d, 4 d/week until all mice had died. Time to first tumor was compared. All groups pretreated with Bellarium-S and Philips TL 09R showed an enhanced tumor development compared with a group irradiated with solar UV only. Pretreatment with Philips TL 10R did not enhance the carcinogenic effect of solar UV.