"At least" four guests were "sickened" by carbon monoxide gas at the Fourth Street Marriott on Thursday night. The deadly fumes sent stricken guests to San Francisco General Hospital and St. Francis Memorial Hospital for treatment. They are expected to survive. But still, yikes.

No word yet on what exactly caused the noxious fumes to takeover.

"Initially we believed it to be the heating system in the rooms," fire Lt. Mindy Talmadge told BCN (via SF Appeal). "But after further investigation we determined it couldn't be the heating system because they use a steam system which doesn't use carbon monoxide."

An air intake mechanism on Stevenson Street, one that draws air in from the outside, could be to blame.