298 JOURNAL OF COSMETIC SCIENCE Table III DRT Values of Different Concentrations of BZ-3 0/W Lotion Formulations BZ-3 (%) DRT (min) 1.36 2 3.12 3 6.14 6 6.30 Figure 4. Photomicroscopic images of test BZ-3 o/w lotion formulations containing 1 %, 3%, and 6% BZ-3, showing crystallization at 6% concentration (200x).

UV SCREENING EFFECT OF SUNSCREENS 299 the crystals are not evenly distributed, the protective effect is greatly reduced. These findings clearly add to the potential of the developed microbiological method to detect physicochemical interactions. The method also proved sensitive to differences in the photoprotective effect of different types of sunscreen agents in single sunscreen test formulations and blends thereof in multi-sunscreen formulations (Figure 5). For instance, the photoprotective power of three lotions of different sunscreen agents expressed as DR T was in the order PBSA, 2% BZ-3, 2% Ti0 2 , 5% (Table IV). Inclusion of either PBSA 2% or Ti0 2 5% in the BZ-3 2% test lotion brought about a synergistic photoprotective effect that was much more pronounced with the "BZ-3 2% and PBSA 2%" system. The DRT of this system was 11 times the sum of the DRT values of the component agents. The same blends of sunscreen agents were reported to produce a synergistic photoprotective effect (18-20). The data obtained point to the sensitivity of the method to the sunscreen composition of the test formulations. The suitability of the microbiological method under study for the prediction of the SPF value of sunscreen products or test formulations was assessed. This was based on a correlation of DR T data obtained from log survivor plots of sunscreen market products with their SPF label claims. In exploring the relation between the DRT and SPF, only 2.5 -,---------------------- 2 \ \ .., \ 1.5 ' \ \ \ 0.5 \ ' \ & \ \ \ \ , .. \ ♦ 2% BZ-3 & 2% PBSA & 2% BZ-3 & 5% Ti02 ■ 2%PBSA • 2%BZ-3 X 5%Ti02 0 -+---�-r-------.------.-------,---�--�-----, 0 20 30 40 50 60 70 Exposure time, min Figure 5. Log survivor curves of individual sunscreen test o/w lotion formulations and their blends.