The birds had built their nest in the chimney which blocked the flow of air and resulted in a buildup of carbon monoxide when a fire was lit in the fireplace. Carbon monoxide is produced alongside carbon dioxide any time gasoline, wood, oil or natural gas burns. The amount depends on the availability of oxygen; the more limited the supply, the more carbon dioxide forms. It is an insidious poison because it has no smell or taste but can cause headaches, nausea, breathlessness and even death. Basically it kills by depriving cells of oxygen. Carbon monoxide, like oxygen, can bind to hemoglobin, the molecule in red blood cells responsible for oxygen transport. But it binds more than 200 times more strongly than oxygen, meaning that it displaces oxygen from hemoglobin.
The effects of inhaling carbon dioxide were first studied by J.B.S. Haldane, the British physiologist who unraveled many of the mysteries of respiration. Although an excellent scientist, Haldane was pretty foolhardy in his investigation of carbon monoxide. He used himself as a guinea pig and carefully documented symptoms as he inhaled increasing concentrations of carbon monoxide. He also took blood samples and attempted to correlate the symptoms with the amount of carbon dioxide in his blood. When the concentration reached 27%, his vision dimmed, by 40% he was exhausted and on the verge of fainting and when the carbon monoxide concentration hit 56% he could no longer walk. Wisely, he decided it was time to stop. A good decision because it has been learned since that at 60% saturation of hemoglobin there is loss of consciousness, followed by death. Luckily, Haldane suffered no permanent damage because the binding of carbon monoxide to hemoglobin is reversible. Inhaling oxygen will displace the bound carbon dioxide. But not everyone is so lucky. Tennis star Vitas Gerulaitis died of carbon monoxide poisoning when he slept in a room next to one that had a faulty swimming pool heater.
