Atmospheric pressure is commonly expressed as which value?

• A. 1034 cm H2O
• B. -5 cm H2O
• C. 760 mmHg
• D. 0 cm H2O

Answer: (C)

Atmospheric pressure at sea level is defined as one atmosphere, a standard that has historically been measured using a column of mercury. The height of that mercury column needed to balance the atmosphere is about 760 millimeters, so atmospheric pressure is commonly expressed as 760 mmHg. In other unit systems it also equals roughly 101.3 kPa or 1034 cmH2O, but the universally recognized reference value is 760 mmHg. The other options represent either different units (centimeters of water) or a gauge-style reference (zero or negative pressure relative to ambient), which isn’t the standard way atmospheric pressure is defined.