COMEDOGENICITY 329 .,. ! &t, Cosoz ISOCETYL ALCOHOL .. ., , _ .'-". , r•tl'.,.,'/' Figure 3. The brsnched-½h•in •l½ohol is more ½ornedo•eni½ •nd more irrit•tin• th•n the •l½ohol. As for the miscellaneous items, the usual sunscreen active ingredients are noncomedo- genic. Among chemical solvents, acetone, ether, and EGME are not problems, but xylene is comedogenic and an irritant. When metallic bases, such as lithium, magne- sium, and zinc stearate, are added to the fatty acids, the metal appears to prevent the comedogenic reaction. Among bases, triethanolamine is more comedogenic than ami- nomethylpropylamine. The classic formulation of a cold cream often involves a salt bridge between stearic acid and triethanolamine. In testing different ratios [4:1, 1:1, 1:4] of stearic acid to triethanolamine (stearic acid:TEA) in a cold cream base, all com- binations were found to be comedogenic. The influence of the vehicle or solvent on the comedogenicity and irritancy of a chem- ical appears quite significant. For example, the use of rapidly evaporating vehicles such as acetone or ether reduces the comedogenicity of fatty acids when compared to the results obtained with sunflower oil, a nonvolatile vehicle (Table III). The effects on irritancy are reversed. Fatty acids are less irritating when delivered in a nonvolatile vehicle. As with the fatty acids, the vehicle or carrier for the D&C red pigment is extremely important. Whereas the D&C red color may be noncomedogenic in volatile propylene glycol, it may be more comedogenic in mineral oil. Possible alternatives for mineral oil, such as pentaerythrital tetra capra/caprylate and polyethylene glycol 400, also reduce the comedogenicity of the red color (Table II). We have chosen propylene glycol as the routine diluent for these studies, as it gradually evaporates and leaves a concentrate of the raw material to be tested. Also, lot after lot of propylene glycol has proven to be nonirritating and noncomedogenic.

330 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS OLETH-iO OLETH. 2 Figure 4. Oleth-3 compared to oleic acid. The initial additions of ethylene glycols to potentially comedo- genic and irritating ingredients appear to increase this propensity. Further additions of ethylene glycols, such as oleth-10 and oleth-20, tend to reduce reactions. Some ingredient combinations--for example, the combination of glyceryl stearate with potassium stearate (available commercially as glyceryl stearate S.E.) and also the combi- nation of D&C red #36 and mineral oil--appear more comedogenic than the indi- vidual compounds themselves. These synergistic reactions need to be studied further.