The EPA uses human subjects for testing and uses low level sound. They make an arithmetic assumption that the same noise reduction will be achieved in high level environments as in low. This may be true for earplugs and earmuffs that attempt to block out all sound. Since the test uses low level sound, these units score low NRR's even though they may provide very good protection against harmful sound. The liquid used in our ear cushions is a 50-50 mixture of water and propylene glycol.
Propylene glycol is used in food additives.
It is also the chemical added to RV water systems to keep them from freezing in the winter. Occasionally a liquid cushion will develop a leak, especially if the edge is abraded or if it is left in the sun often. This number gives the consumer a method to judge performance among different models. Although hearing protection can be recommended for protection against impulse noise, the rating is applicable to continuous noise such as in factories, and it may not be an accurate indicator of protection against impulse noise such as gunfire. Comfort and cost are two other important factors to consider when purchasing hearing protection.