Why is carbon monoxide (CO) considered a dangerous air pollutant from the point of view of human health?

Introduction / Context:
Carbon monoxide is a colourless, odourless gas produced by incomplete combustion of carbon containing fuels. It is a major indoor and outdoor air pollutant and is responsible for many cases of accidental poisoning. Understanding the mechanism by which carbon monoxide harms the body is an important part of environmental and health related chemistry. This question asks why carbon monoxide is considered such a dangerous pollutant.

Given Data / Assumptions:
- Carbon monoxide is inhaled along with air in polluted environments.
- Haemoglobin is the oxygen carrying protein in red blood cells.
- Options suggest various possible biochemical effects, but only one matches standard medical explanations.
- We assume typical exposure scenarios from vehicle exhausts, faulty heaters and industrial emissions.

Concept / Approach:
Haemoglobin in red blood cells normally binds oxygen and transports it from the lungs to body tissues. Carbon monoxide binds to the same iron sites in haemoglobin but with a much greater affinity than oxygen. This forms carboxyhaemoglobin, a stable complex that reduces the capacity of blood to carry oxygen. As a result, tissues become starved of oxygen, leading to symptoms such as headache, dizziness, unconsciousness and in severe cases, death. This binding with haemoglobin is the primary reason carbon monoxide is such a dangerous pollutant.

Step-by-Step Solution:
Step 1: Recall that haemoglobin transports oxygen by reversible binding in the lungs and releasing it in tissues. Step 2: Recognise that carbon monoxide can also bind to haemoglobin at the same binding sites as oxygen. Step 3: Understand that the affinity of haemoglobin for carbon monoxide is many times higher than for oxygen, so even small concentrations of CO can occupy a large fraction of binding sites. Step 4: Realise that when haemoglobin forms carboxyhaemoglobin with carbon monoxide, it cannot carry oxygen efficiently, leading to oxygen deficiency in tissues. Step 5: Conclude that the dangerous nature of carbon monoxide as a pollutant arises from its strong and harmful reaction with haemoglobin.

Verification / Alternative check:
Medical literature and environmental health sources consistently describe carbon monoxide poisoning in terms of reduced oxygen delivery to tissues due to carboxyhaemoglobin formation. Treatment often involves administering pure oxygen or hyperbaric oxygen to displace carbon monoxide from haemoglobin and restore oxygen transport. Symptoms of carbon monoxide exposure, such as weakness, confusion and loss of consciousness, are consistent with systemic oxygen deprivation rather than direct inhibition of metabolism pathways like glycolysis. This confirms that binding to haemoglobin is the central mechanism of harm.

Why Other Options Are Wrong:
Carbon monoxide does not simply make the entire nervous system inactive in a direct way; its effects on the nervous system arise from lack of oxygen rather than immediate direct action on nerve cells. It does not react quickly with atmospheric oxygen to become harmless; rather, it can persist and remain dangerous for some time. There is no standard evidence that carbon monoxide directly inhibits glycolysis in all body cells; the main issue is oxygen transport, not a direct block of metabolic enzymes. Carbon monoxide also does not play a major role in producing protective ozone in the upper atmosphere. Thus, options B, C, D and E do not accurately describe why CO is a dangerous pollutant.

Common Pitfalls:
Learners may confuse carbon monoxide (CO) with carbon dioxide (CO2) or with other pollutants that have different modes of action. Another pitfall is thinking of toxicity in overly general terms, such as assuming that any harmful gas must simply paralyse the nervous system. Understanding the specific biochemical interaction between carbon monoxide and haemoglobin helps make sense of both the symptoms and the recommended treatments for exposure. This precise understanding is valuable for both exam answers and real world awareness.

Final Answer:
Carbon monoxide is a dangerous air pollutant because It reacts strongly with haemoglobin to form stable carboxyhaemoglobin, reducing the blood ability to carry oxygen.