Ordinary air pressure is the weight of a column of air that extends to the top of the Earth's atmosphere divided by the cross-sectional area of the column. Air pressure can be measured using a mercury barometer by the following method.
At 00C the height of a Mercury column supported by air pressure is 760 mm under normal conditions.
In the case of a Mercury Barometer the pressure exerted by 760 mm column of mercury is the same as that of a column of air extending to the top of the atmosphere. Thus my measuring the height of column of mercury one can measure the pressure of the air. This method is not in much use these days because Mercury is considered a biohazard. One could use water instead of mercury except it would not be very practical since the a column of water that will equal that of air pressure is 10.3 m high.
QT4 demonstration of air pressure holding up a column of water
The Force of Air Pressure on a Person:
Force on your body due to air pressure is enormous. Assuming a surface area of typical body @ 1.3 m2, the total pressure on your body would be approximately (105 Pa)(1.3 m2) = 130,000 N @ 30,000 lb. This means that any cavities in our body tissues contain gasses pushing outwards with same pressure. The maximum free-dive is to a depth of 344 ft. At this depth the total pressure on the body is approximately 10 atm.