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I'm trying to come up with a hazard and I was wondering how you determine a challenge rating for a hazard. I guess, I'd have to compare it to traps and creatures to get a good feel for a challenge rating.
The hazard is a greenish cloud of gas in a sewer. A mite alchemist has opened up shop in an abandoned warehouse directly above the sewer, and he's developed the habit of letting his alchemical waste drain into the sewer. (The mite is producing shiver for a local crimelord and has several dream spiders stashed away in his lair. This doesn't have much to do with my questions, but I wanted to give a little context.)
Inhaling the vapors can trigger a random first level spell effect. This would usually be a transmutation effect like increase person/reduce person, but it can also be something like inflict light wounds. A DC11 fortitude save is enough to resist the effect. It's not that difficult to notice the green mist, but you'd need a DC16 spellcraft check and detect magic to identify the properties of the cloud. A DC21 spellcraft check will tell you that you fire will neutralize the transmutating gases, though it's somewhat dangerous to do so. Creatures adjacent to the cloud will take 1d6 fire damage, DC11 reflex save for half.
Now based on magical traps, this should be roughly equivalent to a trap based on a first level spell and thus have a CR of 2. Does that sound right?
Another question, what are good spells to use as effects for this hazard?