Millimeter of Mercury

Formal Definition

The millimeter of mercury (symbol: mmHg) is a unit of pressure defined as the pressure exerted at the base of a column of mercury exactly 1 millimeter high, at a temperature of 0°C, under standard gravitational acceleration of 9.80665 m/s². One mmHg equals approximately 133.322 387 415 pascals. Standard atmospheric pressure equals exactly 760 mmHg.

The mmHg is a manometric unit — its definition is based on the physical properties of a specific substance (mercury) under specified conditions. Mercury's high density (13,595.1 kg/m³ at 0°C) makes it practical for barometers and manometers because even moderate pressures produce measurable column heights. Water, being much less dense, would require columns roughly 13.6 times taller to measure the same pressures.

Relationship to the Torr

The mmHg and the torr are often used interchangeably. One torr is defined as exactly 1/760 of a standard atmosphere, which equals approximately 133.322 368 421 Pa. One mmHg equals approximately 133.322 387 415 Pa. The difference — about 0.000015% — arises because the torr is defined algebraically while the mmHg is defined physically. For all medical, meteorological, and engineering purposes, 1 mmHg = 1 Torr.

Origin of the Term

The name "millimeter of mercury" is literally descriptive: it refers to the height of a mercury column in millimeters. The Latin name for mercury is "hydrargyrum" (from the Greek "hydrargyros" — water-silver), which gives mercury its chemical symbol Hg. The abbreviation mmHg combines the metric length unit (mm) with the chemical symbol for mercury (Hg).

Historical Development

Mercury column measurements originated with Torricelli's barometer in 1643. For nearly 300 years, atmospheric pressure was reported as the height of the mercury column in a barometer — typically around 760 mm at sea level. Medical use of mmHg began in 1896 when Scipione Riva-Rocci introduced the mercury sphygmomanometer for measuring blood pressure. The mmHg became so firmly established in medicine that it persists to this day despite the availability of SI alternatives.

Torricelli's Mercury Barometer

In 1643, Evangelista Torricelli created the first mercury barometer by filling a glass tube with mercury, inverting it in a dish of mercury, and observing that the column fell to approximately 760 mm. This measurement directly gave atmospheric pressure in what we now call mmHg. Torricelli's instrument was refined over the following centuries but the basic principle — and the unit of measurement — remained unchanged.

Medical Adoption

In 1896, Italian physician Scipione Riva-Rocci developed the mercury sphygmomanometer, which measured blood pressure by balancing arterial pressure against a column of mercury. Riva-Rocci's device measured only systolic pressure. In 1905, Russian physician Nikolai Korotkoff discovered the auscultatory method of measuring both systolic and diastolic pressure by listening for characteristic sounds (Korotkoff sounds) with a stethoscope while the cuff deflates. This technique, combined with the mercury sphygmomanometer, established mmHg as the universal unit for blood pressure.

Meteorological Use

Mercury barometers remained the standard for weather observation stations from the 17th century through the 20th century. Aneroid barometers (mechanical, mercury-free) were calibrated against mercury instruments and displayed readings in mmHg or inHg. The transition to hectopascals (hPa) in meteorology began in the 1980s, but some weather services — notably the US National Weather Service — still report pressure in inches of mercury (inHg) for public forecasts.

The Mercury Phase-Out

Mercury's toxicity has led to the gradual phasing out of mercury instruments. The European Union's Restriction of Hazardous Substances (RoHS) directive and the 2017 Minamata Convention on Mercury have restricted the manufacture and sale of mercury-containing devices. Most hospitals now use aneroid or digital sphygmomanometers, though they continue to display readings in mmHg. The unit survives even as the physical mercury column disappears from medical practice.

Blood Pressure Measurement

The most visible use of mmHg is in blood pressure measurement. Normal blood pressure is defined as below 120/80 mmHg (systolic/diastolic). Hypertension Stage 1 is 130-139/80-89 mmHg. Hypertension Stage 2 is 140+/90+ mmHg. Hypertensive crisis is above 180/120 mmHg. These thresholds, defined by medical organizations worldwide, are universally stated in mmHg.

Ophthalmology

Intraocular pressure (IOP) is measured in mmHg using tonometry. Normal IOP ranges from 10 to 21 mmHg. Elevated IOP is a major risk factor for glaucoma, a leading cause of blindness. IOP measurements guide the diagnosis and treatment of glaucoma, with target pressures typically below 15-18 mmHg depending on disease severity.

Respiratory Medicine

Partial pressures of blood gases are reported in mmHg. Normal arterial oxygen pressure (PaO₂) is 80-100 mmHg. Normal arterial CO₂ pressure (PaCO₂) is 35-45 mmHg. These values are critical for diagnosing respiratory failure, managing ventilator settings, and monitoring patients in intensive care units.

Vacuum and Laboratory

Laboratory vacuum systems sometimes use mmHg, particularly in older equipment and protocols. Rotary evaporators, vacuum ovens, and filtration systems may have gauges calibrated in mmHg. A "water aspirator" vacuum reaches approximately 10-25 mmHg. A typical laboratory rotary vane pump reaches 0.01-0.1 mmHg.

At the Doctor's Office

Every visit to a doctor begins with a blood pressure measurement in mmHg. The nurse or medical assistant wraps a cuff around your upper arm, inflates it to temporarily stop blood flow, then slowly releases pressure while listening for pulse sounds. The result — for example, 120/80 mmHg — tells the doctor your systolic pressure (when the heart contracts) and diastolic pressure (when the heart relaxes). Understanding these numbers is one of the most practical health literacy skills.

Eye Exams

During a routine eye exam, your ophthalmologist may measure intraocular pressure using a tonometer. The "puff of air" test (non-contact tonometry) or the gold-standard Goldmann applanation tonometry provides a reading in mmHg. Results above 21 mmHg may prompt further investigation for glaucoma.

Weather Reports

In the United States, barometric pressure in weather forecasts is given in inches of mercury (inHg) — a close relative of mmHg. Standard atmospheric pressure is 29.92 inHg, which equals 760 mmHg. In many other countries, weather has transitioned to hectopascals, but the mercury-column heritage remains visible in the US.

Understanding Altitude Effects

Oxygen partial pressure, measured in mmHg, explains altitude sickness. At sea level, oxygen partial pressure is about 159 mmHg (21% of 760 mmHg). At 3,000 meters altitude, it drops to about 110 mmHg. At 5,500 meters (Everest Base Camp), it is about 80 mmHg. At the summit of Everest (8,849 m), it falls to approximately 53 mmHg — less than one-third of the sea-level value.

Physiology and Medicine

In physiology, mmHg is used for virtually all pressure measurements within the body. Central venous pressure: 2-6 mmHg. Pulmonary artery pressure: 15-30 mmHg (systolic). Intracranial pressure: 7-15 mmHg. Cerebrospinal fluid pressure: 5-15 mmHg. Portal vein pressure: 5-10 mmHg. These values are memorized by medical students and used daily by physicians.

Blood Gas Analysis

Arterial blood gas (ABG) analysis reports partial pressures in mmHg. PaO₂ (oxygen): 80-100 mmHg normal. PaCO₂ (carbon dioxide): 35-45 mmHg normal. Alveolar oxygen tension is calculated using the alveolar gas equation: PAO₂ = FiO₂ × (PB - PH₂O) - PaCO₂/RQ, where PB is barometric pressure in mmHg and PH₂O is water vapor pressure (47 mmHg at body temperature).

Vapor Pressure

Vapor pressures of liquids are often tabulated in mmHg. Water vapor pressure at 20°C is 17.5 mmHg. At 37°C (body temperature), it is 47 mmHg. Mercury vapor pressure at 20°C is 0.0012 mmHg. Ethanol vapor pressure at 20°C is 44 mmHg. These values are essential for chemical engineering, meteorology, and occupational health.

Osmotic Pressure

Osmotic pressure in biological systems is sometimes expressed in mmHg. Normal blood plasma osmotic pressure is approximately 5,400 mmHg (about 720 kPa). The oncotic pressure (colloid osmotic pressure) of plasma proteins is approximately 25-28 mmHg — a critical value for understanding fluid exchange across capillary walls (Starling forces).