Centimeter Water (4 °C) to Pascal

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.

One pascal is the pressure of a single newton spread over an entire square meter — roughly the weight of a small apple pushing on a dining table. Atmospheric pressure is 101,325 Pa, so bare pascals produce unwieldy five- and six-digit numbers. That is why real-world use gravitates to kilopascals (tire pressure), hectopascals (weather), and megapascals (structural steel). The pascal earned its place as the SI base because it ties cleanly to other SI units, not because it matches human-scale pressures.

Sound pressure level is measured in pascals, then converted to decibels relative to 20 micropascals — the faintest sound a healthy young ear can detect. Normal conversation is about 0.02 Pa (60 dB), a rock concert hits roughly 2 Pa (100 dB), and the threshold of pain is around 20 Pa (120 dB). Even loud sounds are astonishingly small pressures compared with atmospheric pressure.

They are all the same unit at different scales: 1 hPa = 100 Pa, 1 kPa = 1,000 Pa. Meteorologists favor hectopascals because 1 hPa equals 1 millibar, making the switch from the old millibar scale painless. Engineers and tire manufacturers prefer kilopascals because car tire pressure (about 220–250 kPa) lands in a tidy two- to three-digit range. Megapascals (MPa) handle material strengths.

Pascal was a 17th-century French mathematician who demonstrated that pressure applied to a confined fluid transmits equally in every direction — now called Pascal's law. His famous "barrel experiment" used a long narrow tube of water to burst a sealed barrel, proving that pressure depends on height, not volume. That principle powers every hydraulic brake, lift, and press in existence today.

When the World Meteorological Organization switched from millibars to SI units in 1986, they chose hectopascals because 1 hPa = 1 mbar exactly. Decades of weather records, pilot training, and forecast charts did not need recalibrating — only the unit label changed. Using kilopascals would have meant rewriting every pilot's altimeter reference (1013.25 mbar became 1013.25 hPa, not 101.325 kPa).