I came up with the following spell
Air that bites (Mu Au 15)
Changes the air into a gas that causes +1 damage per round of exposure (armor clearly doesn't help).
B1, +3 Sight, +1 concentration, +2 size for a big cloud of poison gas
But the problem for me is:
So I've got this huge cloud of poison gas - what happens to those who have a high Parma that the spell can't penetrate. Do they suffocate?
My meander:
It depends on the phenoma of air that the spell is affecting, methinks. It appears you left the target as individual, as opposed to room. As you generally don't treat air as a gas (generally, not always), I'd say you'd have to pick some sort of phenoma- a breeze/wind or fog bank. If the latter, I'd say no, there's non-fog air around, but they may get penalties or have to roll stamina. If the latter, yeah so long as they're in the breeze/wind.
Just my feel on it.
They breath just fine - within their Parma, the air is normal. As a Muto spell, the air is normal underneath the spell anyway.
the pink dot argument strikes again.
Yeah. I should note I don't buy the "suffocation by pink-dotted cloud" tactic - no oxygen is needed for respiration in Mythic Europe, the air will stay just fine.
Thank you all.
I DO need to buy A&A to learn more about medieval physics.
It is a great book. 
One of the best books in 5th edition for me as well 
Since you are not affecting a well defined area, I would say you might need to have Part as a target. <igf you create it around your magus that will not necessarily be true, though. I like this spell, BTW
Cheers,
Xavi