Explain why carbon monoxide is considered a toxic gas and describe its effect on the human body.

Carbon monoxide is toxic because it binds to hemoglobin in red blood cells more effectively than oxygen, reducing the blood's ability to carry oxygen to vital organs and tissues. This can lead to symptoms such as headaches, dizziness, and in severe cases, can be fatal.

A strong answer should directly address the approved learning objective to explain why carbon monoxide is toxic because it reduces the ability of blood to carry oxygen. This question belongs to Properties and effects of atmospheric pollutants within Common atmospheric pollutants and their sources, so the response should use that exact curriculum context rather than a generic statement. The answer is correct when it names the key idea, explains the link to carbon monoxide, and keeps the wording specific to AQA GCSE revision.