Carbon monoxide is a common by-product of the combustion (burning) of fossil fuels. Most fuel-burning equipment (natural gas, propane and oil), if properly installed and maintained, produces little CO. The by-products of combustion are usually safely vented to the outside. However, if anything disrupts the venting process (such as a bird's nest in the chimney) or results in a shortage of oxygen to the burner, CO production can quickly rise to dangerous levels.
The burning of wood, kerosene, coal and charcoal produce CO. Gasoline engines produce CO. CO production is at a maximum during the startup of a cold engine. Starting, then idling, your car or gas mower in the garage can be dangerous.The fumes that contain CO can enter a home through connecting walls or doorways and can quickly rise to dangerous levels.
So this much we'd agree.... as far as ventilation goes....don't sit in a 2x6 closet with your space heater. That might not be healthy.
Then why do we have gas ranges to cook on in our kitchens that are run by propane? If it was a significant danger they(gubmint) would not allow it.