Manufacturers fully understand the necessity of thorough safety testing before introducing a topically applied product. A battery of standard safety tests to determine the irritancy potential of the product to contact sensitization and photosensitization are routinely conducted. However, often missing are tests to determine potential subjective discomfort to the product, such as stinging, burning, and itching. In 1977, Frosch acknowledged that products that meet standard safety parameters may still be rejected by the consumer if disagreeable subjective discomfort develops after application .
Early work in this area investigated subjective response to substances applied to skin that had been damaged by either blisters  or scotch tape-stripped skin . However, these were considered measurements of pain rather than measurements of more transient subjective discomfort. In response to concerns that some substances, such as sunscreens, may cause delayed stinging, Frosch and Kligman developed a method for identifying potential ''stingers.'' This method involved applying lactic acid to the nasolabial fold and cheek area of subjects brought to a profuse state of sweating. The intensity of stinging was then measured by the subject using a 4-point scale at 2.5, 5.0, and 8.0 minutes after application. It was also established that a stinging response could be induced in nonsweating subjects by increasing the concentration of lactic acid. An arbitrary method for classifying the irritancy potential of substances was also developed that identifies if the substance has a slight, moderate, or severe potential to cause stinging . Although this method was established over 20 years ago, modified versions of it remain the basis for identifying subjects that are unusually sensitivity to stinging.
Grove improved the method by defining the demographic profile of the subjects and recommending the exclusion of males and older individuals. He also established criteria limiting the frequency of use and determined that sensitive subjects often reported a history of problems with soaps, cosmetics, and other personal-care products. Subjects who repeatedly reported a stinging response to lactic acid applied under ambient conditions were also tested for a burning and itching response. A method for evaluating burning sensations using a 20:80 mixture of chloroform:methanol pipetted into a greased aluminum cylinder covered and placed against the skin was used. To elicit itching, a 4% histamine base was also loaded into a grease-ringed cylinder and placed against the skin. Results found good correlation between burning and stinging, but individual response variability was rather high. A distinct correlation between itching and stinging was not observed .
A new interest in subjective sensory responses was renewed with the impact of alpha-hydroxy acids (AHAs) in the marketplace. When applied to the skin, these acids often cause a burning, stinging, and/or itching response, often without a visible sign of typical irritation. Draelos states that users have been conditioned to believe that stinging or burning sensations are an indication that the product is working, whereas ''in fact, they feel pain because the acid has penetrated the dermis and is interacting with the dermal nerve endings .'' Manufacturers have responded to these concerns by introducing a second generation of AHAs that do not penetrate the skin to the same degree. Although the FDA has found AHAs safe at low concentrations, the need to routinely include independent sensory assessments in the standard battery of safety studies is apparent.
Obviously missing from the available literature is a clear understanding of the sensory principles that were followed, which opens the door to certain questions that need to be addressed. Were the testing environments controlled? How were the unknown materials presented to the subjects? Were different scales explored? How was the scale that was used presented to the subjects?
For example, it may be advantageous to screen individuals for current use of certain medications that may affect their response, such as cortisones and other anti-inflammatory medications. Subjects should be screened for obvious skin pathology or irritation as well
as a history of allergic reactions. When conducting the screening test or evaluating unknowns, the subjects should be preconditioned for several days. During this period, they should be provided with instructions informing them of, e.g., which cleanser to use, when males can or can't shave, what cosmetics are acceptable the day of the study, and when to cleanse the face. If the test is to be conducted at ambient conditions, the subjects should be preconditioned in a quiet, climate-controlled room. If the subjects will be brought to a ''profuse'' state of sweating, this state should be carefully defined to ensure that all are brought to the same level.
The actual screening probe should be administered in isolated areas to avoid subject interaction and influence. The subjects should not be told what the appropriate response is. For that reason, it would be wise to either eliminate the word ''stinging'' from the scale or not let the subject see the scale and ask them to verbalize their response. The ''purest'' way to approach it would be to administer the test and ask the subject to report any sensation they experienced. To increase the sensitivity of the results and decrease variability it may be worthwhile to explore a 0- to 7-point scale. Finally, the frequency of subject use should be limited with a distinct rest period (—48 hours) between evaluations. With these considerations incorporated with the methods previously developed, it would be possible to quantitatively assess the intensity of facial stinging. Once a reliable method is established, a database of responses to known ingredients can be collected that will allow unknown substances to be tested for subjective discomfort with confidence.
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