Most of what we learn about carbon monoxide is about emergencies or tragedies. The alarm sounds, everyone gets outside, someone calls 911. That is the right response, but it is only part of the picture. A cracked furnace or a vehicle left running in an attached garage can put CO – which is invisible, tasteless, and odorless – into a home at levels that never set off a standard alarm. Everyone in the house is breathing the same air. Not everyone is affected the same way. The problem is not...