A few brewdays ago, I decided to brew in my detached carport, using my propane burner. The carport is about 400 square feet, with two open sides that I have screened in, to keep the bugs out. I had the burner and kettle set up near the front opening (where I added the red X), with a fan circulating air out of the opening itself.
After mashing and returning the hot wort to the kettle, I stuck it back on the burner and cranked it to high. Fifteen minutes or so had passed, and I realized that I had forgotten to grab a pre-boil gravity, so I headed in to the house to snag my refractometer.
Out in the carport, I had music going, the burner hissing, and was doing some reading while I waited. In the quiet house, I suddenly became aware that I was moderately dizzy, semi-nauseated, kind of fuzzy-brained, and had a weird ringing in my ears. It was kind of like being half-way to drunk, but up to this point, I had consumed no booze. After grabbing the refractometer, I headed back out, killed the heat, and carefully moved my setup outside the screens. After a while, the ringing went away, and I felt normal again.
I had figured that the fan, two "screened-in-but-fully-open" walls, a decent breeze, and the burner's proximity to the outside would have kept the emissions in check, but I guess not. I Amazon'd a CO meter to have on-hand for next time, so we'll see if it was indeed the problem.