280 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 2. Continue to insist on adequate testing of his products before they are marketed. 3. Admit the limitations of his testing methods and strive to improve them. 4. Continue to base his conclusions on scientific facts, not speculations. 5. In connection with all issues of major social importance, maintain his perspective on the value of human experience as it relates to the interpretation of scientific evidence. All these things are his obligations in his role as a scientist, as a protector of the public health, and as a member of human society. REFERENCES (1) Wax, Murray, .4m. •. Sociology, 62, No. 6,588 (1957). (2) Isaiah, 3: 24. (3) Macdonald, Eleanor, •. Soc. Cosmetic Chemists, 10, 246 (1959). (4) Hartwell, X., Pub. HeaM, Serv. Pub. No. 149, 2nd Ed. (1951). EFFECT OF VEHICLE COMPONENTS ON THE ABSORPTION CHARACTERISTICS OF SUN SCREEN COMPOUNDS By SIDNEY RIEGELMAN* and RICHARD P. PENNA Presented May 12, 1960, New York City THERE ^RE many physical, chemical and biological factors which must be considered in the formulation of a suntan cosmetic product. Cer- tainly none is more important than the selection of an active sun screen compound. However, this selection cannot be properly made solely on the basis of the information contained in the manufacturer's technical brochure. While this information may be accurate, it does not consider the effects of the vehicle components on the ultraviolet absorption characteristics of the compound. This paper will attempt to clarify some of these interac- tions and point to their significance in the formulation of a suntan prepara- tion. The region of the sunlight which a compound must absorb in order to be an effective sun screen is dependent on the intensity and wavelength of the solar radiation and the body's erythemic response to the light. In- significant amounts of the solar radiation reach the earth's surface below 290 millimicrons (m/•) (1, 2). The shorter wavelengths are screened out by the atmosphere. On the other hand, studies (3, 4) on the erythema pro- duced by ultraviolet light on untanned human skin indicate a maximal erythemagenic effect at 254 m/• and at 287 m•, separated by a minimum at 280 mu. Only the second maximum is within the solar radiation region. * Associate Professor of Pharmacy and Pharmaceutical Chemistry. University of California School of Pharmacy, Medical Center, San Francisco 22, Calif.

EFFECTS OF CHARACTERISTICS OF SUN SCREEN COMPOUNDS 281 It is possible to use these facts to establish what has been called a "sun- burn curve." Plotting the solar radiation curve and the erythema curve on the same graph, a sunburn curve can be constructed by multiplying the ordinates of the erythema curve by those of the solar radiation curve for each wavelength. This was originally done by Crew and Whittle (5) in 1938 who reported a maximum in the sunburn curve at 304 mu. This was later reinvestigated by Kumler and Daniels (6) who concluded that the sunburn curve ranges from 290 mu to 326 mu with a maximum at 308 mu. Thus in order to be a good sun screen, a compound must possess high absorption properties at 308 mu while superposing on the entire sunburn curve. A "sun screen index" was later proposed by Kumler (7) as a simple method of defining the relative sun screen power of different active ingre- dients. The sun screen index was defined as the optical density of a 1 per cent solution at a path length of 0.1 min. While an extremely useful con- cept, this definition did not include the effect of solvent on the absorption characteristics (8, 9). The total energy of a molecule is made up of electronic binding energy and vibrational and rotational kinetic energy. Molecules in solution absorb energy of a particular ultraviolet wavelength. The energy of the mole- cule is raised from the ground state to the energy level of the excited state. The absorption curve we obtain from the spectrophotometer is a composite of all the permitted transitions from the vibrational and rotational levels of the upper electronic state to the vibrational and rotational levels of the ground electronic state. Some of these individual transitions appear as fine structures in the gaseous phase or when the molecule is dissolved in non- polar solvents. Each solvent has its own unique perturbing effect on the electron configuration of the molecule in its ground states and in its excited states. In addition to this dielectric effect of the solvent environment, there can be spectral changes due to solvent-solute interaction resulting in changes in the permanent or induced dipoles in one solvent to a greater extent than in another solvent. Other influences are possible, such as hy- drogen bonding effects, dimerization, and similar molecular interaction. Finally, many molecules undergo profound changes in their spectrum with variation in pH. When a sun screen is dissolved in water, its spectrum usually differs from when it is dissolved in other solvents such as glycerol, alcohol or propylene glycol. Even larger effects may be found with solvents of lower degrees of polar character such as with fatty alcohols, triglycerides, fatty acid esters, or mineral oil. While all of these constituents are important, one must remember that the preparation is applied to the skin surface and must act as a sun screen after the volatiles have evaporated and the residual con- stituents have mixed with sweat and sebum constituents. The molecules of solvent immediately surrounding the sun screen molecules under its con-