While natural gas is odorless, a harmless chemical odor is added so leaks will be easy to detect.
Carbon monoxide — a colorless and odorless gas — is a normal by-product of fuel combustion that can be created by typical heating fuels (e.g., oil, coal, wood, gasoline, kerosene, diesel fuel or natural gas). High levels of carbon monoxide indoors, however, can be dangerous to your health, and can cause serious illness or death if inhaled in large concentrations.
Carbon monoxide can build up to a dangerous level if a fuel-burning appliance isn't operating properly, or is not safely venting fuel combustion by-products. For instance, dangerous levels of carbon monoxide can be produced from improperly vented furnaces, plugged or cracked chimneys, water heaters, space heaters, fireplaces, stoves, and tail pipes.
Running a vehicle inside a garage is the most common carbon monoxide danger. During the heating season, when fresh air circulation is reduced, it's especially important to prevent carbon monoxide buildup indoors.