Re: What makes a given gas at a specific inspired partial pressure poisonous?

In the most general sense, an atmosphere at 14.7 PSI is breatheable if it 
contains oxygen to about four PSIG partial pressure and the balance is 
more or less chemically inert.

It's possible for chemically reactive gases to be present in the 
atmosphere, but they have to be in very small quantities to prevent 
irrecoverable damage to tissues in the body. The threshold partial 
pressure for gaseous elements can be computed from the concentration in 
parts per million (ppm) or grams per cubic meter given in safety 
literature using the partial pressure 
relation, converting parts to molar weights or mass fraction and 
applying the proper form of the relation.

I hope this helps!