Carbon monoxide (CO) is harmful to man because

Options

A. it forms carbolic acid
B. generates excess `CO_2`
C. it is carcinogenic
D. it compete with `O_2` for haemoglobin.

Explanation

### Step-by-Step Solution:

1. **Understanding Carbon Monoxide (CO):**
- Carbon monoxide is a colorless, odorless gas that can be produced from incomplete combustion of carbon-containing fuels.

2. **Identifying the Harmful Effects of CO:**
- The question asks why carbon monoxide is harmful to humans. We need to analyze its interaction with hemoglobin in the blood.

3. **Role of Hemoglobin:**
- Hemoglobin (Hb) is a protein in red blood cells responsible for transporting oxygen (O2) from the lungs to the tissues and returning carbon dioxide (CO2) from the tissues back to the lungs.

4. **Interaction of CO with Hemoglobin:**
- Carbon monoxide competes with oxygen for binding sites on hemoglobin. The affinity of CO for hemoglobin is about 200 times greater than that of oxygen.

5. **Formation of Carboxyhemoglobin:**
- When CO binds to hemoglobin, it forms a stable compound known as carboxyhemoglobin (COHb), which prevents hemoglobin from carrying oxygen.

6. **Consequences of CO Binding:**
- The binding of CO to hemoglobin reduces the amount of oxygen that can be transported in the blood, leading to tissue hypoxia (lack of oxygen) and potentially resulting in suffocation or poisoning.

7. **Conclusion:**
- Therefore, the correct reason why carbon monoxide is harmful to humans is that it competes with oxygen for binding to hemoglobin, impairing oxygen transport in the body.

### Final Answer:
Carbon monoxide (CO) is harmful to man because it competes with O2 for hemoglobin.

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