This video contains a nice animation showing how carbon monoxide poisons people:

Inside each red blood cell, there are binding sites that oxygen and carbon dioxide use for transport to and from the lungs. When carbon monoxide gets into your lungs, it attaches to those same binding sites. But carbon monoxide won’t let go. If enough of the binding sites get gummed up with carbon monoxide, there are not enough binding sites left for oxygen, and the cells in your body suffocate.

If you have a gas furnace, stove, water heater, dryer or fireplace, then there is some risk of carbon monoxide poisoning in your home. Any kind of engine exhaust carries the same risk.

Here’s another perspective from the CDC:

Carbon monoxide detectors are now very inexpensive. This one has a display that will tell you the concentration in parts per million:

Kidde KN-COPP-B Front Load Battery-Operated Carbon Monoxide Alarm with Digital Display

If you google for “safe” levels of CO, you want to have levels below 10 PPM. Here’s more information on alarms:

See also: Carbon Monoxide Questions and Answers