Is contaminated compressed air more dangerous to the diver at the surface or at a depth of 30 meters?

"Bad air" is more dangerous at depth, for several reasons.

1. Breathing contaminated gas at the surface, and becoming symptomatic (unconscious for example) will be less likely to result in the diver drowning.

2. Boyles law (simplified) tells us that pressure and volume are inversely proportional. So, as depth increases, the volume of the gas decreases. With the molecules in our breathing gas pressed ever so close together, we will take on more of the contaminant per breath, and transport it throughout our body.

Now in some cases, (carbon monoxide, for example) the higher partial pressure of Oxygen (we get more of that per breath too!) at depth will mask the symptoms of the contaminant. When the divers ascend, the partial pressure of O2 drops... Suddenly the diver is acutely aware of his dilemma.

A (much) more detailed description of the above is available in the US Navy Diving Manual, Revision 6. It is available for download free of charge on the USN SUPSALV website. Be aware: It's a big book!