Poisonous and sleeping substance

Question

common in surroundings

Answer 1

Carbon monoxide?  It's formed by incomplete combustion (such as in car engines, some propane/butane heaters) and fairly common and harmless in the low concentrations in which it's normally found.  However, without proper ventilation, one of the above sources can cause it to build up in the blood when breathed (It bonds with hemoglobin in the blood in place of the oxygen we all need and is very slowly removed).  This leads to drowsiness due to lack of oxygen and can eventually lead to death from the same cause.  Just ask Dr. Kevorkian!