Although the three methods are based on common procedures, a number of differences in them provide useful variations for specific types of analyses. In 1992 the U.S. Environmental Protection Agency (EPA) specified the 1983 EPA research report of Chatfield and Dillon as EPA Method 100.1. It differs from the other methods in that it contains information about filtering samples only through polycarbonate (PC) filters. A March 1993 EPA memorandum by Mr. Joseph Conlon provides information about how the EPA Method 100.1 can be used to analyze water samples to meet the EPA Drinking Water Standards for asbestos. EPA Method 100.2 was written in 1994 to be specific to the EPA Maximum Contaminant Level (MCL) for asbestos in drinking water - 7 million asbestos fibers over 10 μm in length per liter. It requires counting only those long fibers, provides information about preparing mixed cellulose ester (MCE) type filters, and includes the quality assurance information that had been developed under the Asbestos Hazard Emergency Response Act (AHERA) of 1987. The American Water Works Association Standard Method 2570 was approved in 1993. It contains information about preparing either MCE or PC filters and provides for counting asbestos fibers greater than 0.5 μm in length (the AHERA fiber definition). An interlaboratory comparison of a prepared sample grid showed good correlation between the laboratories for fibers greater than 10 μm.