Carbon monoxide and natural gas are not the same thing. Carbon monoxide detectors will not detect natural gas leaks. Natural gas contains and odourant that makes it smell like rotten eggs, even in very small concentrations. If you smell rotten eggs, there could be a leak in your natural gas piping. Open all doors and windows, do no use electrical equipment, extinguish all open flames/heat sources, and leave the building. Call Enbridge Gas immediately at 1-866-763-5427.