We all know that the deeper you go the density of the air you breathe increases, but as humans have evolved to only breathe air at atmospheric pressure, ~1bar, what effects does an increase in gas density have on us?
The recreational depth limit for all SCUBA divers is 40m, at this depth the air that you are breathing is five times as dense as you are used to breathing which has a few effects on the body and your diving. The first effect people tend to notice the most; which is the increase in your air consumption, it takes five times as much air to fill your lungs as it would on the surface. This means for every single breath you take at 40m, you are inhaling the equivalent of five lungful's on the surface. Divers use Surface Air Consumption (SAC) and Respiratory Minute Volume (RMV) to work out precisely how fast they breathe air at depths to better plan their dives.
The first effect on your body changes the concentration gradient between the air in your lungs and the dissolved gases in your blood stream. Your lungs work by balancing the amount of gases you breathe in with the amount dissolved in your blood stream; gases will naturally pass from areas of high concentration to lower concentrations until they reach an equilibrium. The main gas exchanges your cardiopulmonary system is designed for is moving Oxygen (O2) and Carbon Dioxide (CO2) in and out of your body, every time we breathe in O2 rich air enters the lungs and is dissolved into the blood stream, after the O2 is metabolised it becomes CO2 in the blood which then dissolves back into the lungs and is exhaled.
The breathing cycle works well on the surface as air is composed of about 20.9% O2 and 78% Nitrogen (N2) with the remainder made up of CO2 and other trace gases. Down at 40m the air is five times as dense and therefore five times more concentrated to what we are used to. This is where Partial Pressures (PP) come in; the Partial Pressure of O2 (PPO2) on the surface is about 0.21 (the percentage displayed as a decimal of 1bar) and the PPN2 is about 0.78. When the density of air goes from 1bar to 5bar down at 40m the partial pressures change to 1.05 PPO2 and 3.9 PPN2 (the percentages displayed as decimals of 5bar), which is the same as breathing 105% O2 on the surface and vastly increases the concentration gradient meaning we absorb more O2 and N2 than usual.
We all know the dangers of excess Nitrogen in our system and the danger of our PPO2 reaching 1.6 but another danger is the increase Partial Pressure of Carbon Dioxide (PPCO2) which at 40m reaches around 0.2; if there is a high concentration of CO2 in the lungs then CO2 will build up in our blood stream. The brain indirectly analyses the amount of CO2 in the blood and uses that to control our rate of breathing, high amounts of CO2 increase our breathing rate as the brain thinks we are exerting ourselves and need to compensate by breathing more. This mechanism works fine on the surface but the brain doesn't know about the change in pressure, so just by breathing air at that density the brain thinks you're running a marathon.
The other effect on the body is the weight and viscosity of the air you are breathing at that depth; air has weight and at 5 bar of pressure the weight of the air in your lungs is similar to that of a small bottle of water. The intercostal muscles and diaphragm that are the main muscles involved with breathing are not designed to move thick, heavy air in and out of the lungs and struggle beyond this depth.
The density and weight of air also has a profound effect on the lift of your BCD; below a certain depth the lift of your BCD will not be enough to stop you from sinking even when fully inflated. This is the effect of the Archimedes' Principle; the upward force created by the air in your BCD will reach an equilibrium with the weight of the water it is displacing. If you are not careful, weighted properly or have a BCD with sufficient lift you will have to risk ditching some of your lead just to stay neutrally buoyant.
When you see something interesting it is quite easy to drop down a little further than usual and I've seen it many times where one or two divers are much deeper than the rest of the group but they rarely think about the dangers of going too deep. That extra few meters can be quite dangerous, for you and the diver that tries to find you, which is why there are strict depth limits for each qualification. Your body is simply not designed to be breathing air at that depth and doesn't react well when you reach a certain depth without proper training and planning.