Carbon dioxide (CO2) and carbon monoxide (CO) are both produced when various fuels are burned. In nature, carbon monoxide is harmful, but carbon dioxide is non-toxic.

Because carbon monoxide forms a complex with haemoglobin (carboxyhemoglobin), which is more stable than the oxygen-haemoglobin complex, it is considered hazardous. Carboxyhemoglobin levels of 3–4% can reduce the oxygen-carrying capacity of our blood. This can lead to blurry vision, headaches, heart problems, and anxiety. Even a small rise in concentration beyond the recommended level can be deadly.

Only very high quantities of carbon dioxide are hazardous.