millimeters of mercury; a measure of vapor pressure (see below for explanation of "vapor pressure"). Solvents that evaporate slowly have a low vapor pressure, with mmHg of approximately 0 to 5. Solvents that evaporate more quickly have higher vapor pressures, ranging from 10 up to several hundred mmHg.
Abbreviation for millimeters of mercury and as it relates to breast pumps it means strength of vacuum or suction. Sufficient pressure is needed to stimulate let-down, not to suction the milk from the breasts as is sometimes erroneously believed. Hospital grade breast pumps have the highest mmHg numbers and are the best for establishing a milk supply in mothers who need help in that area because of low supply issues or a hospitalized newborn. After a supply is well-established some users have found that "less is more" and adjusting the suction lower has enabled better let-down. Fortunately most electric pumps have adjustable vacuum strengths.