Glossary of Terms: M

mA Milliamp
measurement of current.
MAC
Maximum Allowable Concentrations (replaced by TLVs) - toxic gas levels described by ACGIH
Malignant
As applied to a tumor. Cancerous and capable of undergoing metastasis, or invasion of surrounding tissue.
Manometer
An instrument for measuring air pressure
Maximum Level (Lmax)
The highest sampled sound level during the instrument’s run time allowing for the unit’s Response Time setting (Fast or Slow).
Medical surveillance
The systematic approach to monitoring health changes in workers to identify and determine which effects may be work-related.
MEL
Maximum exposure limit.
Melting point
The temperature at which a solid changes to a liquid. For mixtures, a range of temperatures may be given.
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.
Microphone polarization
Microphones require an electrical charge (polarization) to operate. Some microphones carry their own charge (have an internal polarization), while others require a charge (polarization) provided by the sound level meter. a. Condenser microphones require 200V polarization to be provided by the meter. b. Prepolarized (electret) microphones do not require the 200V polarization provided by the meter.
Microphone sensitivity
Different types of microphones may respond slightly different from one another. Check your instrument’s user’s manual before using a microphone that was not supplied with the meter. If a microphone with different sensitivity is used on a meter, internal meter settings may need to be changed and the meter should be calibrated with the new microphone.
Microphone Types
a. Free Field: When using this type of microphone, point the sound level meter directly at the sound source (0 degrees). b. Pressure: When using this type of microphone, point the sound level meter approximately 90 degrees to the direction of the sound. c. Random Incidence: When using this type of microphone, point the sound level meter approximately 70 degrees to the direction of the sound. The type of microphone determines the angle at which the SLM should be held. This angle only really matters at higher frequency noises.
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.
Minimum level (Lmin)
The lowest sampled sound level during the instrument’s run time allowing for the unit’s Response Time setting (Fast or Slow).
Mist
Small droplets of a liquid that can remain suspended in air. Mists can form when a vapor condenses back to its liquid state, or when a liquid breaks up (for example, by splashing or atomizing).
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.
Modbus
Digital communication protocol.
Monitoring
The systematic measurement of health hazards to which workers are exposed. There are two types of measurements that can be taken: biological (worker) and environmental (workplace air).
MSDS
Material Safety Data Sheet.
MSHA
Mine Safety and Health Administration, a division of the 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.
Multichannel
More than one measurement channel or input
Multigas
Portable gas detector with typically up to 4 gas sensors fitted.
Musculoskeletal Injuries
Injuries to the system of muscles, tendons, ligaments, joints, bones and related structures of the human body. Also known as musculoskeletal disorders (MSDs).
Mutagen
An agent that causes sudden and permanent changes in one or more hereditary features, generally by modifying one or more genes (changes to genetic material). The changes may or may not be passed on to offspring.
mV Millivolt
measurement of Voltage.