Malignant
As applied to a tumor. Cancerous and capable of undergoing metastasis, or invasion of surrounding tissue.

Metastasis
Transfer of the causal agent (cell or microorganism) of a disease from a primary focus to a distant one through the blood or lymphatic vessels. Also, spread of malignancy from site of primary cancer to secondary sites.

Meter
A metric unit of length, equal to about 39 inches.

Micron (micrometer, m)
A unit of length equal to one millionth of a meter, approximately 1/25,000 of an inch.

Milligram (mg)
A unit of weight in the metric system. One thousand milligrams equals one gram.

Milligrams per cubic meter (mg/m3)
Unit used to measure air concentrations of dusts, gases, mists, and fumes.

Milliliter (mL)
A metric unit used to measure volume. One milliliter equals one cubic centimeter.

Millimeter of mercury (mmHg)
The unit of pressure equal to the pressure exerted by a column of liquid mercury one millimeter high at a standard temperature.

Mists
Suspended liquid droplets generated by condensation from the gaseous to the liquid state or by breaking up a liquid into a dispersed state, such as by splashing, foaming, or atomizing. Mist is formed when a finely divided liquid is suspended in air.

MSDS
Material Safety Data Sheet.

MSHA
Mine Safety and Health Administration, US Department of Labor.

Mucous membranes
Lining of the hollow organs of the body, notably the nose, mouth, stomach, intestines, bronchial tubes, and urinary tract.