The EPA has determined that certain solid wastes are hazardous wastes if they may cause an increase in mortality or illness or pose substantial hazard to human health or the environment when improperly managed. A waste is considered hazardous if it: 1.) is listed on one of the lists of hazardous wastes published by EPA in 40 CFR Part 261, Subpart D; or 2.) exhibits one or more of the four hazardous characteristics defined in 40 CFR Part 261, Subpart C. Listed and characteristic hazardous wastes are identified using EPA codes consisting of one letter followed by three digits. Some states may use additional codes for state-unique hazardous waste streams. For example, the hazardous waste code for used oil in Massachusetts is MA98. For listed wastes, the first letter in the code corresponds to the common list name (e.g., "F-List"). The first letter in the codes of characteristic wastes is D. Generators are required by law to identify their hazardous wastes with all applicable codes.
To determine if a waste is a regulated hazardous waste, the generator needs to accomplish three things. First, determine whether the waste fits within the definition of "solid waste" and, if so, whether the waste is excluded from regulation under 40 CFR § 261.4. Next, they need to examine whether the waste is listed as a hazardous waste in 40 CFR Part 261, Subpart D. Finally, the generator needs to consider whether the waste exhibits one or more of the hazardous characteristics identified in 40 CFR Part 261, Subpart C. In making these determinations, generators may use either analytical testing or their knowledge regarding the process that generates the waste. Wastes such as outdated raw materials may not require testing since knowledge of the chemicals' origins and characteristics may be applied in the determination. Material safety data sheets (MSDSs) are often useful for this purpose.