This glossary contains terms you may run across on this site or in some other EHS context.
Major Stationary Sources: Used to determine applicability of Prevention of Significant Deterioration (PSD) new source review and Title V permitting programs. Emission thresholds for determining major stationary source status vary on the basis of the permitting program and the attainment status in which the source is located.
Manifest System: Tracking of hazardous waste from “cradle to grave” (generation through disposal) with accompanying documents known as manifests.
Markings: Written description of hazards associated with chemicals, including hazardous materials and hazardous wastes. The U.S. Department of Transportation requires all packages of hazardous materials and hazardous wastes to be properly marked before transportation. EPA requires all hazardous waste containers and tanks to be properly marked to identify their contents when used for storage. Note: A marking may be directly written on the package, container, or tank, but most markings are affixed as labels.
Marsh: A type of wetland that does not accumulate appreciable peat deposits and is dominated by herbaceous vegetation. Marshes may be either fresh or saltwater, and tidal or nontidal.
Material Safety Data Sheet (MSDS): Information required under the OSHA Hazard Communication Standard on the identity of hazardous chemicals, health and physical hazards, exposure limits, and precautions. Section 311 of SARA Title III requires facilities to submit MSDSs under certain circumstances. For more information, see the MSDS topic page.
Maximum Achievable Control Technology (MACT): Under the Clean Air Act, MACT is a standard that is determined by averaging the emission limitations achieved by the best performing 12 percent of existing sources of a particular hazardous air pollutant, assuming there are at least 30 sources emitting that pollutant. If there are less than 30 sources emitting that pollutant, then MACT is determined by averaging the 5 best performing sources.
Maximum Contamination Level (MCL): The federal drinking water standards; public water supplies must meet these levels. MCLs represent the highest allowable concentration of a contaminant in drinking water and are set as close to MCLGs as possible.
Maximum Contamination Level Goal (MCLG): Non-enforceable federal goals for drinking water standards. MCLGs are set at a level at which no known or anticipated adverse health effects would occur.
Media: Specific environments, including air, water, or soil, that are subject to regulatory activities.
Microgram per gram (µg/g): One part per million (ppm).
Microgram per liter (µgl): One part per billion (ppb).
Milligram per liter (mg/l): One ppm.
Mitigation: Measures taken to reduce adverse impacts on the environment.
Mobile Sources: All on-road or highway vehicles, such as automobiles, trucks, and motorcycles, and nonroad vehicles, such as trains, airplanes, and agricultural equipment and marine vessels. For more information, see the Mobile Sources topic page.
Monitoring: Periodic or continuous surveillance or testing to determine the level of compliance with statutory requirements and/or pollutant levels in various media or in humans, animals, or other living things.
Monitoring Wells: Wells drilled at a hazardous waste management facility or Superfund site to collect groundwater samples for the purpose of physical, chemical, or biological analysis to determine the amounts, types, and distribution of contaminants in the groundwater beneath the site.
National Ambient Air Quality Standards (NAAQS): Air quality standards established by EPA that apply to outside air throughout the country.
National Emissions Standards for Hazardous Air Pollutants (NESHAP): Emissions standards set by EPA for sources emitting hazardous air pollutants. For more information, see the Hazardous Air Pollutants topic page.
National Pollutant Discharge Elimination System (NPDES): A provision of the Clean Water Act that prohibits discharge of pollutants into waters of the United States unless a special permit is issued by EPA, a state, or (if allowed) a tribal government on an Indian reservation. For more information, see the NPDES topic page.
National Priorities List (NPL): EPA’s list of the most serious uncontrolled or abandoned hazardous waste sites identified for possible long-term remedial action under Superfund. The NPL is updated periodically and published in the Federal Register. For more information, see the Superfund topic page.
National Response Center: The federal operations center that receives notifications of all releases of oil and hazardous substances into the environment. The Center is operated by the U.S. Coast Guard.
Neutralization: Decreasing the acidity or alkalinity of a substance by adding to it alkaline or acidic materials.
New Source: Any stationary source that is built or modified after a final or proposed regulation has been published that prescribes a standard of performance intended to apply to that type of emissions source.
New Source Performance Standards (NSPS): Uniform national EPA air emission and water effluent standards that limit the amount of pollution allowed from new sources or from existing sources that have been modified. For more information, see the Air - NSPS topic page.
Nonattainment Area: Geographic area that does not meet one or more of the National Ambient Air Quality Standards for the criteria pollutants designated in the Clean Air Act.
Nonconventional Pollutant: Any pollutant that is not statutorily listed.
Nonpoint Source: Pollution sources that do not have a single point of origin, or are not introduced into a receiving stream from a specific outlet. Common nonpoint sources are agriculture, forestry, urban runoff, mining, construction, dams and channels, land disposal, and saltwater intrusion.
Enviro Glossary Terms A
Enviro Glossary Terms B and C
Enviro Glossary Terms D and E
Enviro Glossary Terms F through H
Enviro Glossary Terms I through L
Enviro Glossary Terms M and N
Enviro Glossary Terms O and P
Enviro Glossary Terms Q and R
Enviro Glossary Terms S and T
Enviro Glossary Terms U through Z