CPAP pressure is measured in units called cmH2O
. One cmH2O is the amount of (additional) air pressure needed to raise a column of water by one centimeter. The cmH2O unit is basically a metric version of "inches of mercury" that we use in the US to measure the barometric pressure for weather forecasting. And just how much additional
pressure does 1 cmH2O represent in terms that are meaningful to an average American? Well, if we go to http://www.unitconverters.net/pressure-converter.html
and scroll way far down the list, we can convert 1cm H2O into (about) 0.029 inches of mercury. Since standard CPAP pressures range from 4 cmH20 to 20 cmH2O, that means they range from 0.116 to 0.58 inches of mercury.
Standard atmospheric pressure varies from day to day, but typical values are in the 28 to 30 inches of mercury range. Right now (Aug 14, 2011 at 8:10am), wunderground
is reporting the air pressure in Buffalo, NY as 29.76 inches. When a typical storm front comes through, the air pressure drops anywhere from 0.25 to 1.5 inches of mercury. Some people are really sensitive to these changes in atmospheric pressure and get "pressure" or "weather" headaches or lots of joint pain. But most people don't notice them very much. At the maximum CPAP pressure of 20cmH2O, the machine is ADDING
about 6/10 of an "inch" of additional pressure on top of the current atmospheric pressure---about as great of a pressure difference between a typical raining day and a typical sunny day.