Carbon monoxide is a common industrial hazard. It's the byproduct of internal combustion engines, like the ones we find in regular vehicles, diesel engines, or forklifts. Carbon monoxide, or CO, is also the result of the incomplete burning of natural gas and any other material containing carbon. This means that some pretty standard appliances, like water heaters, space heaters, propane cooking ranges, or blast furnaces might have a carbon monoxide risk -- especially if they aren't properly maintained.