Carbon monoxide, the silent killer. Episode twenty-nine of the Gas Service Animated series.

It starts like the flu. A whole household — headaches, fatigue, nausea, feeling worse indoors and better once they step outside. But no fever. That missing fever is the tell. Carbon monoxide is the great imitator, and at low levels over days or weeks it gets misread as a virus, again and again. The house itself is making them sick.

So what is it? Carbon monoxide forms during incomplete combustion. Burn a hydrocarbon fuel with plenty of air, and you get carbon dioxide and water — a clean blue flame. Starve that flame of oxygen — a blocked vent, a dirty burner, not enough combustion air — and the carbon can't fully oxidize. Instead of carbon dioxide, you get carbon monoxide.

Here is why it's so dangerous. Carbon monoxide is colorless, so you can't see it. Odorless, so you can't smell it. Tasteless. And non-irritating — it won't sting your eyes or your throat to warn you. Every alarm your body has stays silent. That is why it earned the name: the silent killer.

Now the mechanism. Your red blood cells carry oxygen on a protein called hemoglobin. Carbon monoxide grabs the very same site — but about two hundred times more tightly than oxygen. Some sources put it as high as two hundred fifty times. So when CO is present, it wins. It locks onto hemoglobin, forming carboxyhemoglobin, and every seat it takes is a seat oxygen can't use. Your blood's ability to carry oxygen quietly drops.

But it's worse than just taking seats. Carbon monoxide changes the hemoglobin that remains. It shifts the oxygen dissociation curve to the left, so the little oxygen clings tighter and won't let go into the tissues that need it. And CO doesn't stop in the blood. It poisons the cell's own engines — binding an enzyme called cytochrome c oxidase and jamming the mitochondria that turn oxygen into energy. Less oxygen delivered, and the cells can't use what little arrives.

Which organs feel it first? The ones that burn the most oxygen — the brain and heart. Early poisoning looks like headache, confusion, and chest tightness. Losing consciousness is an ominous sign; a faint from CO strongly predicts severe toxicity. A warning: blood CO levels line up poorly with how sick someone is. A low number never rules poisoning out.

Some people are hit harder and faster. Infants, the elderly, and anyone with heart disease or anemia get sick at lower levels. And the unborn are the most vulnerable of all. Carbon monoxide crosses the placenta, fetal hemoglobin holds it even tighter, and it clears more slowly. In severe poisoning of the mother, fetal death can exceed fifty percent. The same air is never the same dose for everyone.

Time matters, because the body can clear carbon monoxide — but slowly. On ordinary room air, half of it leaves in about four to five hours. Put the patient on one hundred percent oxygen through a mask, and that falls to forty to eighty minutes. In a hyperbaric chamber, under pressurized oxygen, about twenty-three minutes. Oxygen is the antidote — and more oxygen means a faster clock.

Carbon monoxide: invisible to every sense, two hundred times hungrier for blood than oxygen, a double hit, delivery and cell. Oxygen is the clock that saves you. A low reading never rules it out. Next: CO by the numbers.