SUMMARY Background/PurposeThe inorganic metal oxide sunscreens titanium dioxide and zinc oxide have been considered to protect against sunburning ultraviolet radiation by physically reflecting/scattering the incident photons and thus protecting the skin. Earlier publications suggested, however, that the primary action of UV protection by these sunscreen agents is through absorption and not by reflection. The purpose of this work was to quantitate the contributions of each of these modes of action to the protection provided by inorganic UV sunscreen filters.

The hairless mouse has been used as an experimental model for photocarcinogenesis for about 20 years. Although the carcinogenesis action spectra for mice and man are not known, acute responses to ultraviolet radiation (UVR) in the biologically active UVB and UVC region (wavelengths below 320 nm) can be compared. Vascular response (predominantly edema) action spectra for monochromatic radiation in the Skh:HR‐l (albino hairless) male mouse were determined. These action spectra were found to be very similar to the human erythema action spectrum that had been developed using the same monochromator. The accuracy of this experimentally derived action spectrum was tested with a series of polychromatic source spectra. The mice were exposed to radiation from a long arc Xe lamp filtered by varying thicknesses of Schott WG320 filters, which yielded a wide range of biologically effective spectra. Spectral irradiance measurements, when weighted with the mouse edema and human erythema action spectra and multiplied by the irradiation time required to elicit a threshold response (edema), yielded a constant weighted dose regardless of irradiation spectral quality. The integrated effective dose was approximately 200 J/m2 of 297 nm equivalent energy, agreeing with requirements for the monochromatic 297 nm dose in the mice as well as for minimal human erythema. These data suggest a commonality in the UVR chromophores of mice and men as they relate to the acute responses described, and a direct additivity of effectiveness from the UVR components in a polychromatic beam, at least over the portion of the UVR spectrum tested (λ > 295 nm).

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Products containing ultraviolet (UV) radiation absorbing or scattering ingredients provide varying degrees of protection from sunlight (or other UV sources), thus minimizing the deleterious effects on the skin. The ''sun protection factor'' (SPF) of sunscreen products has become a well recognized indicator of protection against sunburn induced predominantly by ultraviolet B radiation (UVB: 290-320 nm). A similar system of denoting sunscreen protection from ultraviolet A (UVA: 320-400 nm) radiation has not been universally recognized. A variety of test methods have been proposed, both in vitro and in vivo, each with specific virtues and shortcomings. RegulaThe major source of UVA damage -sunlight: its spectra and variation with season There are many misconceptions about the intensity and doses of UV radiation that constitute our daily exposure risk. While there is a greater prevalence of UVA radiation in the sun's spectrum compared with UVB, the intensity is not constant during the day or during the year. Table 1 and Fig. 1 shows the variation of both UVB and UVA during the day and over the seasons as measured in Central New Jersey (40ae N. Lat.) on clear days on or about the summer solstice, the vernal and autumnal equinoxes, and the winter solstice. They show that the UVA daily doses in the winter are only half that of a summer, spring, or fall day. The intensity of UVA is similarly only about half that experienced on a summer day. This is due to the lower sun angle during the winter as well as the shorter daylight hours from the earth's tilt on its axis during the winter.The UVB radiation changes more dramatically than the UVA radiation during the daytime as well as over the seasons as it is absorbed preferentially by the ozone in the stratosphere (longer pathlength during the winter and at low sun angles at the beginning and end of the day, resulting in lower UVB dose reaching the earth's surface).2 tory agencies and industry have been reviewing the available methods over the past decade in an effort to develop consumer meaningful claims and appropriate substantiation methods. This article reviews these test methodologies, in vitro and in vivo, as well as the biological background that establishes the need for UVA protection, and the UVA content of solar radiation and its variability.

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