Pressure is usually measured in the unit of atmospheres. Another common unit of pressure measurement is millimeters of mercury or mm Hg. This unit corresponds to the height to which mercury will rise in a mercury column.

1 atmosphere of pressure will cause a rise of about 760 mm in the mercury. Thus,

1 atm = 760 mm Hg.

We can use this equivalence to convert the pressure from mm mercury to atmospheres.

760 mm Hg = 1 atm

1 mm Hg = 1/760 atm/mm Hg

454 mm Hg = 1/760 x 454 atm = 0.597 atm or 0.6 atm.

Thus, the hiker will only feel an atmospheric pressure of about 0.6 atmospheres at an elevation of 14,000 ft and hence will find it very difficult to exert much at this level. In simple terms, we often say that oxygen availability is decreased at high altitudes - this is what the hiker will feel, hence the difficulty in exertion.

Hope this helps.