inside the ears, but they also have only atmospheric pressure in the outer ear canal. The PADI (Professional Association of Diving Instructors) advises in the "Open Water Diver Manual" that only vented earplugs designed for diving should be used in diving. Flight ear protection Earplugs are also available which help to protect ears from the pain caused by airplane cabin pressure changes. Some products contain a porous ceramic insert which reportedly aids equalization of air pressure between the middle and outer ear thereby preventing pain during landings and take-offs. Some airlines distribute regular foam earplugs as part of their amenity kits for passengers to aid their comfort during landings and takeoffs as well as to reduce exposure to the aircraft's noise during the flight. These can help passengers get to sleep during the flight if desired. Rating The United States Environmental Protection Agency (EPA) mandates that hearing protection is rated and labeled. To be rated, hearing protection is tested under ANSI S3.19-1974 to provide a range of attenuation values at each frequency that can then be used to calculate a Noise Reduction Rating (NRR). Under this standard a panel of ten subjects are tested three times each in a laboratory to determine the attenuation over a range of 9 frequencies. In the European Union, hearing protectors are required to be tested according to the International Organization for Standardization (ISO) acoustical testing standard, ISO 4869 Part 1 and the Single Number Rating (SNR) or High/Middle/Low (HML) ratings are calculated according to ISO 4869 Part 2. In Brazil, hearing protectors are tested according to the American National Standards Institute ANSI S12.6-1997 and are rated using the Noise Reduction Rating Subject Fit NRR(SF). Australia and New Zealand have different standards for protector ratings yielding a quantity SLC80 (Sound Level Class for the 80th percentile). Canada implements a class system for rating the performance of protectors. Gauger and Berger have reviewed the merits of several different rating methods and developed a rating system that is the basis of a new American National Standard, ANSI S12.68-2007 The various methods have slightly different interpretations, but each method has a percentile associated with the rating. That percent of the users should be able to achieve the rated attenuation. For instance, the NRR is determined by the mean attenuation minus two standard deviations. Thus, it translates to a 98% statistic. That is, at least 98 percent of users