Now, as we begin to increase an altitude, for example, as we climb a mountain, what happens is the air becomes less dense. So the distance between the gas molecules increases. And what that means is we'll find less gas molecules in the same volume of space at a higher altitude. Now, if the air becomes less dense, then the total pressure in the atmosphere decreases. And if we multiply zero point 21 by a smaller total pressure, that gives us a smaller partial pressure of oxygen. So if we increase the altitude, the partial pressure of oxygen in the atmosphere decreases.