In the cities of our country, which among the following atmospheric gases are normally considered in calculating the value of Air Quality Index?

1. Carbon dioxide

2. Carbon monoxide

3. Nitrogen dioxide

4. Sulfur dioxide

5. Methane

Select the correct answer using the code given below:

The correct answer is Option 2: 2, 3 and 4 only — that is, Carbon monoxide (CO), Nitrogen dioxide (NO₂), and Sulfur dioxide (SO₂) are among the gases considered when calculating the Air Quality Index (AQI) in Indian cities.

To understand why this is the correct answer, let us first understand what the Air Quality Index is and how it works.

What is the Air Quality Index (AQI)?

The Air Quality Index is a standardized numerical scale used by government agencies to communicate to the public how polluted the air currently is, or how polluted it is forecast to become. In India, the Central Pollution Control Board (CPCB) calculates the AQI based on a defined set of pollutants that are known to have measurable, harmful effects on human health and the environment at the concentrations found in urban air.

The Eight Pollutants Used in India's AQI Calculation:

India's AQI framework considers the following eight key pollutants:

1. Particulate Matter (PM_2.5)
2. Particulate Matter (PM₁₀)
3. Carbon Monoxide (CO)
4. Nitrogen Dioxide (NO₂)
5. Sulfur Dioxide (SO₂)
6. Ozone (O₃)
7. Ammonia (NH₃)
8. Lead (Pb)

As you can see, from the five gases listed in the question, Carbon monoxide (Statement 2), Nitrogen dioxide (Statement 3), and Sulfur dioxide (Statement 4) are officially part of the AQI calculation. This makes Option "2, 3 and 4 only" the correct choice.

Now let us go through each statement and explain exactly why it is included or excluded:

Statement 1 — Carbon Dioxide (CO₂): NOT included ✗

This is a very common point of confusion. Although CO₂ is a major greenhouse gas responsible for global warming and climate change, it is not a criteria pollutant for AQI calculation. This is because CO₂, at the concentrations naturally and currently present in urban ambient air (around 400–420 ppm), does not pose a direct, immediate health hazard to humans through breathing. The AQI focuses on pollutants that cause direct short-term and long-term health effects at ambient concentrations. CO₂ is a climate concern, not a direct urban air toxicity concern at current levels. Hence, Statement 1 is incorrect for AQI purposes.

Statement 2 — Carbon Monoxide (CO): Included ✓

Carbon monoxide is a colourless, odourless, and highly toxic gas produced primarily by the incomplete combustion of fossil fuels in vehicle engines, industrial boilers, and generators. Even at relatively low concentrations, CO binds to haemoglobin in the blood (forming carboxyhaemoglobin) with an affinity about 200 times greater than oxygen, thereby reducing the blood's oxygen-carrying capacity. This can lead to serious health effects including headaches, dizziness, and even death at high levels. Because of this direct and severe health impact at ambient levels found in cities, CO is officially included in India's AQI. Statement 2 is correct.

Statement 3 — Nitrogen Dioxide (NO₂): Included ✓

Nitrogen dioxide is a reddish-brown gas with a sharp, biting smell, formed during high-temperature combustion processes — mainly from motor vehicle exhaust and thermal power plants. NO₂ is a major respiratory irritant; it inflames the airways, aggravates asthma, and can increase susceptibility to respiratory infections. It also contributes to the formation of ground-level ozone and particulate matter (secondary pollutants). Because of its widespread presence in urban areas and its well-documented health impacts, NO₂ is a standard pollutant included in India's AQI calculation. Statement 3 is correct.

Statement 4 — Sulfur Dioxide (SO₂): Included ✓

Sulfur dioxide is a colourless gas with a pungent, suffocating smell, released primarily from the burning of sulfur-containing fossil fuels (especially coal) in thermal power plants and industrial processes. SO₂ causes respiratory problems, aggravates asthma and bronchitis, and reacts in the atmosphere to form sulfuric acid — a key component of acid rain and fine particulate matter (PM_2.5). Due to its significant health and environmental impact at concentrations observed near industrial and urban areas, SO₂ is included in India's AQI. Statement 4 is correct.

Statement 5 — Methane (CH₄): NOT included ✗

Methane is a potent greenhouse gas, primarily emitted from livestock, paddy fields, landfills, and natural gas leaks. Similar to CO₂, methane at ambient atmospheric concentrations does not directly harm human health through inhalation. It is not a respiratory irritant or toxic pollutant at the concentrations found in urban air. While it contributes to climate change and can indirectly affect air quality (by contributing to ground-level ozone formation in certain conditions), it is not classified as a criteria pollutant under India's AQI framework. Hence, Statement 5 is incorrect for AQI purposes.

Summary Table:

Key Takeaway: The AQI is designed to measure pollutants that have direct, measurable, short-term and long-term health effects at concentrations found in city air. Greenhouse gases like CO₂ and CH₄ are climate concerns, not direct urban air health hazards, and are therefore excluded from AQI calculations. CO, NO₂, and SO₂, on the other hand, are well-established health hazards at urban ambient levels and are firmly included. This is why the answer is definitively "2, 3 and 4 only".