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

What makes a give gas at a specific inspired partial pressure poisonous?
I know that is somewhat general, so I'll as that I am looking for breathable 
atmospheres. What makes a breathable atmosphere and what does not? not 
referring to pressure here directly, but what makes a gas mixture breathable, 
for example at higher pressures helium has to be mixed in, lower pressure more 
oxygen has to be present, but I don't mean pressure variations, I mean say at 
one atmosphere, what level of fluorine or chlorine (or both) or any other gases 
would be tolerable or toxic, etc. Including high habitable-temperature gases.
Thanks,
Gabriel

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