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| MAC |
| | Maximum Allowable Concentration. A recommendation for the highest level for long-term exposure to a chemical which will not produce adverse effect. |
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| Malignant Cell |
| | A cancer cell that has the potential to invade surrounding tissues or spread to other areas of the body (metastasis). |
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| Malignant Tumor |
| | A tumor that can invade surrounding tissues or metastasize to distant sites resulting in life-threatening consequences. |
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| 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. |
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| Mechanism of Action |
| | The specific manner by which a substance causes a particular effect. |
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| Median Toxic Dose |
| | The dose level at which 50% of the population will experience toxic effects. |
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| Metabolism |
| | The conversion of a chemical from one form to another. same as Biotransformation. |
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| Metabolite |
| | A chemical produced when a substance is metabolized by a biological organism. |
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| Metastasis |
| | The movement of diseased cells, in particular cancer cells, from the site of origin to another location in the body. |
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| Mg/kg |
| | A commonly used dose that stands for mg of a substance per kg of body weight. |
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| Mg/kg/day |
| | A commonly used dosage that stands for mg of a substance per kg of body weight on a daily basis. |
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| Mg/M3 |
| | An exposure unit used to express concentrations of particulates in the air, standing for milligrams of compound per cubic meter of air. |
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| Microgram (µg) |
| | A commonly used unit of weight consisting of one millionth (1 x 10-6) of a gram. |
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| 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). |
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| Milligram (mg) |
| | The most commonly used unit of measure in medicine and toxicity consisting of one thousandth of a gram (1x10-3 g). |
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| 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.. |
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| Mouse Specific Locus Test |
| | The major test for gene mutation in whole animals. Exposed mice are bred and observed for induced hereditary changes. |
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| 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. |
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| 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. |
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| Mutagen |
| | A substance that causes mutations (genetic damage). |
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| Mutagenesis |
| | The process whereby a substance damages DNA and produces alterations in or loss of genes or chromosomes. |
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| 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. |
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