Correct Answer: C. Severe
The Air Quality Index (AQI) classification in India follows the National Air Quality Index (NAQI) framework established by the Central Pollution Control Board (CPCB). The AQI of 407 on November 23 falls into the "Severe" category, which is defined as AQI ≥ 401. This index is calculated using the worst sub-index among the six pollutants measured: O₃, CO, NO₂, PM₁₀, PM₂.₅, and SO₂. On November 23, the PM₁₀ value of 512 µg/m³ is the dominant pollutant driving the overall AQI to 407. According to CPCB guidelines, AQI values are stratified as: Good (0–50), Satisfactory (51–100), Moderately Polluted (101–200), Poor (201–300), Very Poor (301–400), and Severe (≥401). The Severe category indicates that air quality has reached a critical level where even healthy individuals may experience serious respiratory and cardiovascular effects. In the Indian context, Delhi frequently experiences severe air pollution episodes during winter months (October–January) due to crop burning in neighboring states, vehicular emissions, and meteorological conditions that trap pollutants. The AQI of 407 represents a public health emergency requiring immediate intervention and advisories for vulnerable populations.
Why the other options are wrong
A. Moderately polluted — Moderately Polluted corresponds to AQI 101–200 according to CPCB classification. An AQI of 407 is far beyond this range—it is more than double the upper limit. This option represents a much lower pollution level and would be appropriate for days with AQI in the 100–200 range, not 407. The trap here is confusing the general term 'polluted' with the specific CPCB category boundaries. B. Very poor — Very Poor is the category immediately below Severe, defined as AQI 301–400. Although November 23's AQI of 407 is close to this range, it exceeds the upper limit of 400 by 7 points. The CPCB classification is strict: any AQI ≥401 is classified as Severe, not Very Poor. This is a common NBE trap—students may round down or confuse adjacent categories when the value is near a boundary. D. Poor — Poor is defined as AQI 201–300, which is significantly lower than the observed AQI of 407. This category would apply to days like November 21 (AQI 350) or November 24 (AQI 356) if they fell in the 201–300 range, but they do not. Selecting 'Poor' suggests misunderstanding the CPCB AQI scale and its numerical thresholds for classification.
High-Yield Facts
- CPCB AQI Severe threshold: AQI ≥ 401 is classified as Severe; any value below 401 is Very Poor (301–400).
- AQI calculation: The overall AQI is the worst sub-index among O₃, CO, NO₂, PM₁₀, PM₂.₅, and SO₂—not an average.
- November 23 driver: PM₁₀ at 512 µg/m³ is the dominant pollutant determining the Severe classification on this date.
- CPCB categories in order: Good (0–50) → Satisfactory (51–100) → Moderately Polluted (101–200) → Poor (201–300) → Very Poor (301–400) → Severe (≥401).
- Delhi winter pollution: Severe AQI episodes in October–January are driven by crop burning, vehicular emissions, and temperature inversions that trap pollutants.
Mnemonics
AQI Boundary Memory: 'Good-Sat-Mod-Poor-VeryPoor-Severe' 0–50 (Good) → 51–100 (Sat) → 101–200 (Mod) → 201–300 (Poor) → 301–400 (VeryPoor) → ≥401 (Severe). Each boundary is a multiple of 50 or 100; the Severe cutoff at 401 is the only non-round number—remember it as 'just over 400 = Severe.' Worst Sub-Index Rule AQI = maximum of all six pollutant sub-indices, not the average. On Nov 23, PM₁₀ (512) is worst → drives AQI to 407 → Severe. Use this when multiple pollutants are given.
NBE Trap
NBE often places AQI values just above or below category boundaries (e.g., 400 vs. 401, 300 vs. 301) to test whether students know the exact CPCB thresholds. An AQI of 407 is only 7 points above the Very Poor ceiling (400), tempting students to choose 'Very Poor' instead of 'Severe'—this is the primary cognitive trap.
Clinical Pearl
In Indian clinical practice, a Severe AQI (≥401) is a public health emergency: hospitals in Delhi see a surge in respiratory and cardiovascular admissions during such episodes. Clinicians must counsel patients with asthma, COPD, and cardiac disease to remain indoors and use air purifiers; even healthy individuals may develop acute respiratory symptoms at this pollution level.
_Reference: Park's Textbook of Preventive and Social Medicine (K. Park), Chapter on Environmental Health and Air Pollution; CPCB National Air Quality Index (NAQI) Guidelines_