Miasma.

Two uses for it. First, a highly unpleasant odor or vapor. Second, a unpleasant or oppressive atmosphere.

Quick story regarding it's use by me. I walked into the restroom at work when they were having plumbing issues, and I mentioned to my coworker there that there was quite the miasma in the air. He replied, "I don't know what you're talking about, but it smells like burnt piss in here!"