Which one of these gases is not poisonous in nature but is regarded as a pollutant when its level in the air exceeds a certain extent?

Options

A. Carbon monoxide
B. Sulphur dioxide
C. Carbon dioxide
D. Nitrogen oxide

Explanation

To determine which gas is not poisonous in nature but becomes a pollutant when its concentration exceeds a certain level, we will analyze each of the given options: Carbon monoxide, Sulphur dioxide, Carbon dioxide, and Nitrogen oxide.

### Step-by-Step Solution:

1. **Examine Carbon Monoxide:**
- Carbon monoxide is a colorless, odorless gas that is highly poisonous. It binds with hemoglobin in the blood, forming carboxyhemoglobin, which reduces the blood's oxygen-carrying capacity. This effect occurs even at low concentrations, making it a harmful pollutant.
- **Conclusion:** Carbon monoxide is eliminated as it is poisonous even in small amounts.

2. **Examine Sulphur Dioxide:**
- Sulphur dioxide is released from burning fossil fuels and can cause acid rain when it combines with water vapor. It can damage lungs and plant tissues even in small amounts.
- **Conclusion:** Sulphur dioxide is also eliminated as it is dangerous even in low concentrations.

3. **Examine Carbon Dioxide:**
- Carbon dioxide is a greenhouse gas that helps maintain Earth's temperature. However, when its concentration exceeds a certain level, it can contribute to global warming and is considered a pollutant.
- **Conclusion:** Carbon dioxide is not poisonous in normal levels but becomes a pollutant when present in excess.

4. **Examine Nitrogen Oxide:**
- Nitrogen oxide can also contribute to acid rain and can corrode buildings, particularly marble structures. It is harmful even in small amounts.
- **Conclusion:** Nitrogen oxide is eliminated as it is harmful in low concentrations.

### Final Answer:
The gas that is not poisonous in nature but is regarded as a pollutant when its level in the air exceeds a certain extent is **Carbon Dioxide**.

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