I noticed that some of your products have the abbreviation NRR with a nunber of 25, 24, et cetera, next to it. What does NRR stand for?

The NRR abreviation stands for the Noise Reduction Rating of a product.
It is should also be noted that a NRR is required by law (via the United States Environmental Protection Agency) to be posted on all hearing protection items sold in the United States. With this Noise Reduction Rating we are able to determine which hearing protection products suit our needs, according to the different standards and recommendations emplaced by such organizations as NIOSH , OSHA and ANSI .

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Reference:
Rosenstock, Linda, M.D., M.P.H. (1998, June). Criteria for a Recommended Standard: Occupational Noise Exposure. Retrieved on June 1, 2007,  from
http://www.cdc.gov/niosh /docs/98-126/