Carbon Monoxide (chemical abbreviation CO) is an odorless gas that kills. It occurs indoors due to faulty heaters, stoves, grills, generators, house fires of course, and cars with faulty exhaust systems. It can leak in from next-door apartments. Cases have occurred in poorly ventilated areas like at ice rinks, warehouses, and parking garages.
The main symptom is headache. Other early symptoms include feeling yucky, mental dullness, poor concentration, and mild confusion. Textbooks sometimes mention the face turning cherry-red, but this is an uncommon & very late sign (just before death), and should never be relied on.
We absolutely need to think of CO poisoning if someone with a headache gets better after leaving their environment, then symptoms recur once they return. If not, and they go home to sleep, they may never wake up. If two or more people in the place get these same symptoms at the same time, it’s likely CO until proven otherwise. The diagnosis involves finding CO in special blood tests that have to be done in an emergency room. Delay in processing the specimen ruins the test. Treatment is easy once the diagnosis is made.