I have another idea:Low air areas:If the envirosuit is going to be put in then areas with low air sound good,after 15 turns you get a warning and 5 turns after the warning you start losing health because your suffocating [duh].
Just we have to decide whether the planet was terraformed enough, so we get enough air outside - or not.
In case the planet was not terraformed, any damage to the towers will cause the loss of air accumulated there, so it would be a little hardcore...
In AvPs planets were terraformed enough not to worry about the oxygen too much.
Also, we would have to estimate how much time does "one action" last (like walking one cell). If walking one cell = 1 sec, then slow death after 20 secs may be a bit extreme.
Additionally, if we include such details, we have to remember about the fact that even the weakest hit to a person wearing Env. Suit would probably damage it. So no Env. Suit after the first alien that happened to teleport next to us and do some damage.