Carbon monoxide is dangerous when inhaled.

Because carbon monoxide combines with HAEMOGLOBIN about 200-300 times more readily than oxygen does, forming a stable compound called CARBOXYHAEMOGLOBIN. Since this combination is almost irreversible, the haemoglobin can no longer carry oxygen, the tissues are starved, and the person suffers headache, dizziness, unconsciousness and, in excess, DEATH BY ASPHYXIATION.

Give the MECHANISM (the affinity for haemoglobin) and the CONSEQUENCE (oxygen starvation). The figure makes the answer precise: 200-300 times greater affinity, so even a small concentration in the air captures a large share of the haemoglobin.

WHY IT IS SO INSIDIOUS: carbon monoxide is COLOURLESS, ODOURLESS and TASTELESS, so a victim gets no warning and simply becomes drowsy. This is why charcoal stoves and generators must never be used in closed rooms.

Its sources: incomplete combustion — vehicle exhaust, faulty heaters, burning coal, cigarette smoke.