Centimeter Water (4 °C) to Inch Mercury

Respiratory medicine adopted cmH₂O because the original ventilators literally used water columns to regulate pressure — a jar of water with a submerged tube set the pressure at whatever depth the tube was immersed. A CPAP setting of 10 cmH₂O meant the air bubbled out at 10 cm depth. The unit stuck even after electronics replaced water seals, because clinicians, patients, and device manuals all speak the same scale.

Most adults are prescribed between 6 and 14 cmH₂O, with 10 cmH₂O being a common starting point. Severe obstructive sleep apnoea may require 15–20 cmH₂O. Auto-titrating (APAP) machines vary pressure within a set range — typically 4–20 cmH₂O — adjusting breath by breath. Higher pressures are more effective at splinting the airway open but can cause discomfort and air swallowing.

ICU ventilators also use cmH₂O. Positive end-expiratory pressure (PEEP) is usually set at 5–15 cmH₂O to keep alveoli open. Peak inspiratory pressure above 30–35 cmH₂O raises the risk of lung injury. Plateau pressures are monitored to stay below 30 cmH₂O. The entire field of mechanical ventilation runs on this single unit because it directly corresponds to the pressures inside the lung.

Measured via lumbar puncture with the patient lying on their side, normal CSF pressure is 10–18 cmH₂O in adults. Above 25 cmH₂O suggests raised intracranial pressure — potentially from a tumor, meningitis, or hydrocephalus. Below 6 cmH₂O indicates low pressure, often from a CSF leak. Neurologists use cmH₂O rather than mmHg because spinal fluid is essentially water, making the unit a direct physical analogue.

1 cmH₂O ≈ 0.981 mbar ≈ 0.0981 kPa. For bedside estimates, 1 cmH₂O ≈ 1 mbar is close enough (error under 2%). A CPAP setting of 12 cmH₂O is about 11.8 mbar or 1.18 kPa. Since respiratory equipment universally reads cmH₂O, conversion is mainly needed when interfacing with industrial instruments or when charting pressures alongside blood gas data reported in mmHg.

The US National Weather Service inherited the convention from early American meteorology, which used mercury barometers calibrated in inches. A typical sea-level reading of 29.92 inHg is easy to remember and fits weather maps without decimal clutter. Most other countries switched to millibars or hectopascals, but the US stuck with inHg for the same reason it kept Fahrenheit — familiarity and institutional inertia.

US air traffic controllers broadcast the local barometric pressure in inches of mercury — for example, "altimeter two niner niner two" means 29.92 inHg. Pilots dial this into their altimeter so the instrument reads correct altitude above sea level. If the setting is wrong by just 0.1 inHg, the altimeter reads roughly 100 feet off — enough to matter during instrument approaches in fog.

At sea level, 29.92 inHg is standard. Readings above 30.20 inHg are high-pressure (clear skies, calm winds). Below 29.50 inHg is considered low pressure and often signals approaching storms. The lowest sea-level pressure ever recorded was Typhoon Tip in 1979 at 25.69 inHg (870 mbar). A household barometer swinging from 30.50 down to 29.30 is a reliable sign that weather is deteriorating.

Refrigeration techs evacuate AC system lines to remove moisture before charging with refrigerant. They measure the vacuum in inHg below atmospheric pressure — a reading of 29 inHg (out of 29.92 max) means near-total vacuum. Industry best practice requires pulling to at least 29.92 inHg (or equivalently, below 500 microns on a micron gauge) to ensure all moisture has boiled off at room temperature.

1 inHg ≈ 33.86 mbar ≈ 0.491 psi. So standard atmosphere (29.92 inHg) is about 1013 mbar or 14.7 psi. For quick mental math: multiply inHg by 34 to get millibars, or divide by 2 to get a rough psi estimate. These conversions come up constantly when comparing US weather data with international sources or converting aviation altimeter settings for foreign aircraft.