The guide to sigil says that people entering sigil lose points of constitution for a while due to the lack of air.
Also, if you enter the lower ward, you must make a constitution check to save against the smog and if you fail, you get -1 to STR and DEX. Plus, you make additional rolls when doing a strenuous activity to prevent a coughing fit for a few rounds. Combined with thin air, it can be an almost deadly combination :twisted:
I like these ideas since they intensify the dark and gloomy feeling of a blade-decorated city and also scare the players a bit, so they don't find too much comfort in the "yes, planes have a dangerous enviroment, but there's always Sigil to relax" thought
Then I started looking for equivalents in the d20 enviroment rules. I found this:
A character who breathes heavy smoke must make a Fortitude save each round (DC 15, +1 per previous check) or spend that round choking and coughing. A character who chokes for 2 consecutive rounds takes 1d6 points of nonlethal damage.
...which is a bit strong but nice :twisted:... At least if you remove this +1 or even turn it into -1 or something :oops:
But then I found the thin atmosphere rule in d20 modern/future SRD:
Many thin atmospheres are the equivalent of being at a high elevation on Earth, such as on top of a mountain or in the upper atmosphere. A creature exposed to a thin atmosphere must succeed on a Fortitude save (DC 20) every hour. On the first failed save, the creature is fatigued. A fatigued creature that fails a subsequent save becomes exhausted for as long as it remains in the thin atmosphere. After 1 hour of complete, uninterrupted rest in a normal atmosphere, an exhausted creature becomes fatigued. After 8 hours of complete, uninterrupted rest, a fatigued creature is no longer fatigued.
...and when you look at what being exhausted or fatigued is all about, this rule would be way too harsh, especially for low/medium level characters. Also, this check should be made every hour as long as you are in thin air which would reduce Sigil from a million-people city to a few-planar-creatures-and-their-mother city. Any thoughts on these?
I would say that natives of that area would have developed a tolerance. If that's another reason why people don't like to go to the Lower Ward, so be it. Seems to me that this rule wouldn't be enforced when the DM feels like overlooking it, which works.
Cagers wouldn't be affected by the thin air at all, since acclimation can adapt anyone to higher elevations.