MAC Maximum Allowable Concentration.  A recommendation for the highest level for long-term exposure to a chemical which will not produce adverse effect. Malignant Cell A cancer cell that has the potential to invade surrounding tissues or spread to other areas of the body (metastasis). Malignant Tumor A tumor that can invade surrounding tissues or metastasize to distant sites resulting in life-threatening consequences. Margin of Safety (MOS) The ratio of the dose that is just within the lethal range (LD01) to the dose that is 99% effective (ED99), LD01/ED99.  A ratio of greater than 1 gives comfort to the physician whereas an a ratio of less than 1 denotes caution. Mechanism of Action The specific manner by which a substance causes a particular effect. Median Toxic Dose The dose level at which 50% of the population will experience toxic effects. Metabolism The conversion of a chemical from one form to another.  same as Biotransformation. Metabolite A chemical produced when a substance is metabolized by a biological organism. Metastasis The movement of diseased cells, in particular cancer cells, from the site of origin to another location in the body. Mg/kg A commonly used dose that stands for mg of a substance per kg of body weight. Mg/kg/day A commonly used dosage that stands for mg of a substance per kg of body weight on a daily basis. Mg/M3 An exposure unit used to express concentrations of particulates in the air, standing for milligrams of compound per cubic meter of air. Microgram (µg) A commonly used unit of weight consisting of one millionth (1 x 10-6) of a gram. Micronucleus Test A test for mutagenicity in which bone marrow or peripheral blood cells are examined for the presence of micronuclei (broken pieces of chromosomes surrounded by a nuclear membrane). Milligram (mg) The most commonly used unit of measure in medicine and toxicity consisting of one thousandth of a gram (1x10-3 g). Minimal Risk Levels (MRL's) A risk level calculated by the ATSDR for noncancer end points.  The MRL is an estimate of daily human exposure to a substance that is likely to be without an appreciable risk of adverse effects over a specified duration of exposure.  MRL's are derived for acute (14 days or less), intermediate (15-364 days), and chronic (365 days or more) duration exposures for either inhalation or oral routes.. Mouse Specific Locus Test The major test for gene mutation in whole animals. Exposed mice are bred and observed for induced hereditary changes. MTD Maximum Tolerated Dose.  The highest dose used in an animal cancer test which can be tolerated without serious weight loss or other toxic effects. Multi Hit Model The least conservative quantitative risk assessment model.  It assumes that several interactions are needed before a cell can be killed, damaged, or transformed into a cancerous cell. Mutagen A substance that causes mutations (genetic damage). Mutagenesis The process whereby a substance damages DNA and produces alterations in or loss of genes or chromosomes. Mutation DNA damage resulting in genetic alterations ranging from changes in one or a few DNA base pairs (gene mutations) to gross changes in chromosomal structures (chromosome aberrations) or in chromosome number.