STINGING CAPACITY OF APPLIED SUBSTANCES 207 damage skin, often lack the capacity to induce stinging. On the other hand, substances that are nonirritating may possess striking stinging capabilities. As a class, acids tend to cause strong stinging. The differences in stinging potentiality among acids are not dependent on pH, but are probably related to their diffusional characteristics, the more troublesome ones penetrating more rapidly. Likewise, strongly alkaline substances such as sodium carbonate, trisodium phosphate, and sodium hydroxide can induce marked stinging. Acids (below pH 2.0) and bases (above pH 11.0) probably excite nerve endings directly. The strong buffering capacity of the skin doubtlessly limits the damage from a single application. While there is generally no clear correlation between irritancy and stinging, their properties may parallel each other within a specific class of substances, viz., metallic antiperspirants and c•-hydroxy acids. That excitation of nerve endings is central to the stinging phenomenon is suggested by a number of observations. The intensity increases with each additional application of non-irritating materials. Hypertonic solutions do not cause stinging, but can cause its revocation after a stinging episode. For example, stinging will reappear when a satu- rated solution of sodium chloride is applied after the stinging induced by ADP or lactic acid has abated. Moreover, inflamed skin is far more prone toward stinging, even when the horny layer barrier is intact as in freshly sunburned skin. Of course, in chemically damaged skin the nerve endings are not only more excitable but increased permeability enables increased diffusion. Stinging is perceived with 5 per cent ADP and 5 per cent lactic acid only when the sub- ject is sweating the intensity of the sensation is proportional to the amount and dura- tion of sweating. The longer a person sweats, the easier and faster stinging can be elicited. In all probability, the principal role of sweating is to hydrate the skin, thus greatly increasing its permeability. It is well known that the flux of substances increases in moistened '3kin. Another is the associated increase in skin temperature, heightening the sensitivity of nerve endings. We found that stinging could be induced even at room temperature if a wet compress was applied to the cheek for 20 to 30 min before ex- posure to 5 per cent lactic acid. However, the intensity and duration of the stinging was always much less than with induced sweating. Then, too, diffusion through sweat-filled ducts promotes transport to derreal nerves. The sharp decrease in stinging on cheeks rendered anhidrotic by aluminum chloride is further evidence of the role of hydration. Although, we have emphasized preselection of subjects, it is perfectly clear that sting- ing can be elicited in anyone by assuring access to nerve endings. Persons who do not sting, when lactic acid is applied to the normal cheek, will do so if the horny layer bar- rier is removed, partially or completely by scotch-tape stripping. Apparently, the pecu- liarity of "stingers" is more permeable skin. As a practical point, persons who are not stinging-prone may experience appreciable stinging if the skin has become inflamed through chemical or physical insults for example, sunburned skin is considerably more reactive. We have preliminary evidence that the facial skin of stingers is more perme- able. We have already mentioned that DMSO produced more intense whealing and erythema in stingers. It is our impression that stingers have more permeable skin everywhere.

208 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS At first glance, stripping might seem to offer the alternative of screening substances on areas other than the face and without the requirement of sweating. Unfortunately, this is not a feasible alternative. ADP, for example, did not elicit typical stinging in stripped skin of the back and was not different from its vehicle ethanol. Moreover, stinging in stripped skin was evanescent and not at all discriminating. Many irritating substances, which do not cause facial stinging, will produce pain and stinging on stripped skin. GUIDELINES FOR PERFORMING STINGING TESTS SUBJECTS Stingers are likely to be white females with light complexions who give histories of easy sunburning. They frequently complain of having sensitive skin and of having trou- ble with cosmetics and soaps. The lesser susceptibility of blacks is doubtless due to their possession of a more effective barrier, the horny layer being both more dense and with a greater number of cell layers (7). •.:.:•.• THERMAL STIMULATION The facial sauna* is a useful substitute for an environmental chamber the latter can hardly be viewed as standard equipment. The subject places the face directly into the steam stream for at least 15 min or until sweating is brisk. The steam exposure must be continued for another 10 min after plication of the test agent. We obtained comparable results with a variety of stinging formulations in a panel tested with both the chamber and the sauna. The intensity of stinging was a little less with the sauna. REPEATED TESTING If no stinging develops in 5 min, one can wipe off the substance with a wet towel and apply a new test material. This can be repeated at least twice on each cheek, allowing for the evaluation of 6 agents in a single session. The limiting factor in "no response":-:i.(: testing is the time that subjects can stay in the thermal chamber. They are generally i•?• wrung out by 45 min. On the other hand, a positive response increases susceptibility to stinging. To avoid i71. false positive readings, no further testing should be conducted on that cheek. We have never observed tachyphylaxis over the short term, that is, unresponsiveness induced by repeated testing. However, we have witnessed the development considerable resistance in 2 of 5 stingers, who were regularly used twice weekly for several months. One subject became virtually a nonstinger after 3 months of testing'":11i with stinging materials though irritation was never observed. This induced refratori- •i!i ness may be a subliminal form of "hardening" that occurs with prolonged exposure to irritants. In any event, subjects who are used repeatedly, should be monitored from !•:-:i *Facial Beautifying Mist, Model 60. Schick Electric, Inc., Lancaster, PA.