Why is carbon monoxide bad when it's so similar to carbon dioxide?

Carbon dioxide (CO2) in large quantities is also poisonous; just not as much so as the much-deadlier carbon monoxide (CO).

CO is easily absorbed by the body, which is just as reluctant to release it. It is also completely undetectable: it is invisible, and has no odor or taste. It binds with the hemoglobin in the blood in preference to oxygen; what's worse, it makes the hemoglobin reluctant to release the oxygen it already has. The combination rapidly leads to oxygen deprivation and potential cell death, and may trigger a possible heart attack.

To make matters even more dangerous, CO can also affect central nervous system function, in part by causing certain fatty compounds in the brain to break down into harmful chemical byproducts.

The body is designed to expel CO2, so it rarely builds up to a level high enough to cause damage. And the amount of CO2 necessary to cause injury is measured in parts per thousand, whereas the amount of harmful levels of CO can be measured in the much-smaller unit of parts per million (ppm).