Technicalities:
While spelling is agreeably a very minor detail when it comes only to card design, 'Airborne' is what the card should state.
For the cost, it should go before the ability name and followed by a colon. This only affects the unupgraded card.
Mechanics:
I take it ATT cannot become negative due to this card.
I think -2/-1 | -3/-1 is too much. Maybe -2/0 | -2/-1 ?
Other:
As for name, Toxic Smog | Toxic Pollution seems to me to be redundant and not as fluid. Would Acid Rain [unable to run search on used names right now] be a better name?
Why is 
unaffected?
Addressing Technicalities:
I can make those issues be gone. It takes 20 minutes. No biggie.
Addressing Mechanics:
In any situation of
GAINING a negative amount of attack/hp, I assumed that going into the negative was an effect that was not only possible, but likely for many weak creatures.
The reason why they lose so much in stats, in my reasoning, is this:
Any creature who loses -2 or -3 in Atk without going into the negative is likely to have enough health to survive. But creatures who are 2|1 or 1|1 are meant to die - It's basically a card that demands strong monsters only be on the field.
Addressing 'Other':
The name came from where I got the inspiration - A smokey steel mill. Smog/Pollution came to mind, and I'm quite fond of the name. Acid rain (Also yes, it has been suggested before) more or less sounds like a spell that deals an instant effect to all creatures, like Rain of Fire or Pandemonium. My card is supposed to be more like a field modifier, like Flood or Cloak.
And
are unaffected by natural law - Death thrives in a polluted/smoggy wasteland, and light shines through it. Think of a lighthouse shining through a thick mist off the horizon. Even though
is the opposite of
, and not
, Captain Scibra described it best: Toxic Smog is supposed to lay a dark curse-like effect on the entire field, but more like a cloud of poisonous air rather than a curse.
