The idea antisensory anti-irritant would effectively inhibit stinging, burning, and itching caused by a broad range of acidic, neutral, and basic chemical irritants by reducing the sensitivity of type-C nociceptors. In contrast, it would not inhibit the warning symptom of pain mediated by A-delta nerves, nor would it affect other nerve sensors that mediate tactile, temperature, or vibratory sensations. Since most cosmetic-induced sensory irritation occurs within several minutes after application, the ideal anti-irritant should work within seconds when formulated with the irritant. For broad product use, it should also work when applied as a pretreatment before the irritating formulation and it should work when applied after irritation has occurred. Because cosmetics use a wide range of chemicals, the anti-irritant should be stable in many chemical environments and inexpensive enough to be used in low-cost products. With repeated daily use, the ideal anti-irritant should provide the same effective level of anti-irritant protection (no tachyphylaxis) and, most importantly, it must be safe for broad, unsupervised use.
With the exception of local anesthetics that are regulated as drugs in most countries and may have undesirable side effects and safety concerns, no compounds have been described that are able to broadly inhibit sensory irritation from cosmetics and pharmaceuticals. Because a safe compound capable of blocking sensory irritation and inflammation would provide considerable benefit, I sought to identify compounds that could effectively block sensory irritant reactions. Simple water-soluble strontium salts have proved to be potent and selective inhibitors of chemically induced sensory irritation and neurogenic inflammation in humans and do not produce numbness or loss of other tactile sensations [7-10].
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