Science Terms Definitions - M

Mass Number (symbolized A) - The number of nucleons (neutrons and protons) in the nucleus of an atom. Also known as the atomic weight of an atom.

Material Safety Data Sheet (MSDS) - A compilation of information required under the OSHA Communication Standard on the identity of hazardous chemicals, health, and physical hazards, exposure limits, and precautions. Section 311 of SARA requires facilities to submit MSDSs under certain circumstances.

Maximum Contaminant Level Goal (MCLG) - Under the Safe Drinking Water Act, a non-enforceable concentration of a drinking water contaminant, set at the level at which no known or anticipated adverse effects on human health occur and which allows an adequate safety margin. The MCLG is usually the starting point for determining the regulated Maximum Contaminant Level.

Maximum Contaminant Level (MCL) - The maximum permissible level of a contaminant in water delivered to any user of a public system. MCLs are enforceable standards.

Maximum Tolerated Dose - The maximum doses that an animal species can tolerate for a major portion of its lifetime without significant impairment or toxic effect other than carcinogenicity.

Media - Specific environments-air, water, soil-which are the subject of regulatory concern and activities.

Melanoma - A pigmented, highly malignant form of cancer of the skin.

MEM - A management tool to evaluate and categorize EM activities based on the risks associated with the activities of each project.

Metastasis - The transfer of disease from one organ or part to another not directly connected with it.

Metric Ton - 1,000 kilograms or about 2,204 pounds. The usual U.S. ton measurement, called a short ton, is 2,000 pounds.

Micron - One one-millionth of a meter.

Mill Tailings - A slurry of about 40 percent solids (including radioactive particles and chemically hazardous metals) and 60 percent liquid, primarily water.

Mixed Waste - Any radioactive waste that meets the requirements of LLW that contains a RCRA hazardous component. Also, mixed waste is waste that contains both hazardous waste and radioactive material (source, special nuclear, or by-product material as regulated by the Atomic Energy Act of 1954 [42 U.S.C. 2011 et seq.]). Mixed waste is classified by DOE according to the type of radioactive waste that it contains as either mixed low-level waste (MLLW), or mixed transuranic waste (MTRU). DOE's high-level waste (HLW) is assumed to be mixed waste because it contains hazardous components or exhibits the characteristic of corrosivity. If the radionuclides are those that have an atomic number greater than 92, have a half life longer than 20 years, and are present in concentrations greater than 100 nanocuries/gram the waste is termed Mixed TRU waste. The waste in Oak Ridge is predominantly Mixed Low Level Waste.

Mobility - The ability of a chemical element or a pollutant to move into and through the environment (e.g., the mobilization of an element from a water column to sediment).

Molecule - A group of atoms held together by chemical forces. A molecule is the smallest unit of a compound that can exist by itself and retain all of its chemical properties.

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, plants, and animals.

Monitoring Well - A well used to obtain water quality samples or measure groundwater levels. Well 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.

Morbidity - A departure from a state of physical or mental well-being, resulting from disease or injury. Frequently used only if the affected individual is aware of the condition. Awareness itself connotes a degree of measurable impact. Frequently, but not always, there is a further restriction that some action has been taken such as restriction of activity, loss of work, seeking of medical advice, etc.

Mortality - Death; the death rate; ratio of number of deaths to a given population.

Mortality Rate - The number of deaths that occur in a given population during a given time interval; usually deaths per 103 or 105 people per year. Can be age, sex, race, and cause specific.

MOX - A fuel composed of a mixture of plutonium dioxide and uranium dioxide.

MSHA - The Department of Labor (DOL) Mine Safety and Health Administration (MSHA) is the agency directly responsible for the nation's mine safety and health programs.

Multistage Model - A carcinogenesis dose-response model where it is assumed that cancer originates as a "malignant" cell, which is initiated by a series of somatic-like mutations occurring in finite steps. It is also assumed that each mutational stage can be depicted as a Poisson process in which the transition rate is approximately linear in dose rate.

Mutagen/Mutagenicity - An agent that causes a permanent genetic change in a cell other than that which occurs during normal genetic recombination. Mutagenicity is the capacity of a chemical or physical agent to cause such permanent alternation.