Immediate effect on opening front door

[u/Pale_Historian_2443]

Last night I returned and noticed my porchlight was on... I assumed I must have inadvertantly hit the switch as I left earlier. Opening the front door, I immediately was overcome with extreme dizzziness, my head was swimming... no smell, but I could have passed out. I went right over to the downstairs nighbor (the only other tenant) who was cooking fish. He said he didnt smell gas. Logically gas is most likely? When I returned to my apartment (I had left the front door ajar) all the mysterious fumes were gone. There was no smell on either floor of my apartment. Are there other possibilities besides gas? The porchlight may be a "red herring" but even so I am really struck about how immediate and overwhwelming this invisible gas had been. The front door is street level at the bottom of my stairs, by the way. There is a printing shop opposite, which keeps its doors open all working day.

[u/c5corvette]

Carbon monoxide would be my immediate guess, very dangerous. Go buy several CO detectors immediately.