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What is AQI? A Complete Guide to the Air Quality Index in India
TL;DR
The Air Quality Index (AQI) converts complex pollution measurements into a single number from 0 to 500. India uses the National Air Quality Index (NAQI) by CPCB, which tracks seven pollutants: PM2.5, PM10, O₃, CO, NO₂, SO₂, and NH₃. AQI is divided into six categories from Good (0–50) to Severe (401–500). The overall AQI equals the highest individual pollutant sub-index. Understanding AQI helps you decide when to exercise outdoors, keep windows open, or use an air purifier.
You check the weather before stepping out. But do you check the air? In a country where 21 of the world's 30 most polluted cities are Indian, understanding AQI isn't optional — it's essential.
🔍 What is AQI?
AQI stands for Air Quality Index. It's a standardised scale that takes complex pollution measurements — parts per billion of ozone, micrograms of particulate matter per cubic metre — and converts them into a single number from 0 to 500 that anyone can understand.
Think of it like a thermometer for air purity:
- 0 = perfectly clean air (rare in most Indian cities)
- 50 = good, safe for everyone
- 100 = acceptable, but sensitive people should take care
- 200+ = unhealthy — everyone starts to feel the effects
- 500 = emergency — stay indoors, seal windows
The beauty of AQI is its simplicity. You don't need to know what "78 μg/m³ of PM2.5" means. You just need to know: is the number green, yellow, orange, or red?
📊 India's NAQI vs US EPA AQI
India doesn't use the same AQI scale as the United States. Here's the key difference:
NAQI (National Air Quality Index) — India's official standard, defined by the Central Pollution Control Board (CPCB):
- Tracks 7 pollutants: PM2.5, PM10, O₃, CO, NO₂, SO₂, and NH₃ (ammonia is unique to India's standard)
- 6 categories: Good, Satisfactory, Moderate, Poor, Very Poor, Severe
- Breakpoints are more lenient than the US EPA for some pollutants
US EPA AQI — the international benchmark:
- Tracks 6 pollutants: PM2.5, PM10, O₃, CO, NO₂, SO₂ (no NH₃)
- 6 categories: Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, Hazardous
- Stricter breakpoints for PM2.5 — what India calls "Satisfactory", the EPA might call "Moderate" or worse
Why does this matter? A city might show NAQI 95 ("Satisfactory") but EPA 120 ("Unhealthy for Sensitive Groups") for the same air. AQI Now shows both standards side-by-side so you can see the full picture.
🧪 The Seven Pollutants Behind Your AQI
Each pollutant gets its own sub-index. The overall AQI equals the highest individual sub-index — meaning one bad pollutant can drive the entire AQI up.
- PM2.5 (Fine Particulate Matter) — Particles smaller than 2.5 micrometres. The deadliest pollutant: enters the bloodstream through lungs. Major sources: vehicle exhaust, cooking fires, industrial emissions, crop burning.
- PM10 (Coarse Particulate Matter) — Particles smaller than 10 micrometres. Sources: construction dust, road dust, pollen. Less dangerous than PM2.5 but still harmful to airways.
- O₃ (Ground-level Ozone) — Not emitted directly. Formed when sunlight reacts with vehicle exhaust and industrial fumes. Peaks in hot afternoons. Irritates lungs and worsens asthma.
- CO (Carbon Monoxide) — Colourless, odourless gas from incomplete combustion. High near traffic junctions and enclosed parking. Reduces oxygen delivery to organs.
- NO₂ (Nitrogen Dioxide) — Reddish-brown gas from vehicle engines and power plants. Triggers airway inflammation. A key ingredient in smog and ground-level ozone formation.
- SO₂ (Sulphur Dioxide) — Sharp-smelling gas from coal power plants and industrial processes. Causes coughing, throat irritation, and worsens asthma. Also causes acid rain.
- NH₃ (Ammonia) — Unique to India's NAQI system. Sources: agricultural fertilisers, livestock waste, industrial coolants. Irritates airways and contributes to secondary PM2.5 formation.
📋 NAQI Categories Explained
| AQI Range | Category | What It Means | What To Do |
|---|---|---|---|
| 0–50 | Good | Air is clean. Minimal health risk. | Enjoy outdoor activities freely |
| 51–100 | Satisfactory | Acceptable for most. Sensitive people may notice mild discomfort. | People with asthma should carry inhalers |
| 101–200 | Moderate | Breathing discomfort for people with lung/heart conditions. | Reduce prolonged outdoor exertion if you have respiratory issues |
| 201–300 | Poor | Discomfort for most people on prolonged exposure. | Limit outdoor time. Consider wearing an N95 mask outside |
| 301–400 | Very Poor | Respiratory illness risk for everyone on prolonged exposure. | Avoid outdoor activities. Use air purifiers indoors |
| 401–500 | Severe | Affects even healthy people. Serious risk for those with existing conditions. | Stay indoors. Seal windows. Run air purifier continuously |
Where Do Indian Cities Fall Today?
Based on AQI Now's analysis of 486 cities tracked across India, here's how the country's air quality typically distributes in a single snapshot:
| Category | Cities | Percentage |
|---|---|---|
| Good (0–50) | 99 | 20% |
| Satisfactory (51–100) | 301 | 62% |
| Moderate (101–200) | 72 | 15% |
| Poor (201–300) | 5 | 1% |
| Very Poor (301–400) | 8 | 2% |
| Severe (401–500) | 1 | <1% |
The majority of Indian cities (82%) fall in the Good or Satisfactory range on any given day — the problem is concentrated in the NCR belt and the Indo-Gangetic plain, especially during winter months.
🏠 How to Use AQI in Daily Life
Morning routine: Check AQI before your commute. If it's above 200, consider working from home or taking a less-traffic-heavy route.
Exercise: Never jog or cycle when AQI is above 150. You breathe 10–20 times more air during exercise, inhaling far more pollutants. Move workouts indoors or to cleaner hours (usually mid-afternoon in winter).
Windows: Open windows when AQI is below 100. Close them when it rises above 150. Cross-ventilation during clean hours is better than keeping windows sealed all day.
Children: Schools should cancel outdoor PE and recess when AQI exceeds 200. Children's lungs are developing and more vulnerable to particulate damage.
Air purifiers: Most effective when AQI is above 150 indoors. Choose HEPA filters rated for your room size. Keep doors and windows sealed when running the purifier.
✅ Quick Reference
- AQI below 50: Green flag — go outside, open windows, exercise freely
- AQI 50–100: Yellow — fine for most, sensitive groups should be aware
- AQI 100–200: Orange — reduce prolonged outdoor exertion
- AQI 200–300: Red — limit outdoor time, wear N95 masks
- AQI above 300: Purple/Maroon — stay indoors, avoid all outdoor activity
🎯 The Bottom Line
AQI is your daily air quality barometer. Checking it takes 5 seconds on AQI Now — and those 5 seconds can change every decision you make about outdoor activities, ventilation, and protecting your family. In a country where clean air is no longer guaranteed, AQI awareness isn't just knowledge — it's a survival skill.
Frequently Asked Questions
What does AQI stand for?
What is the difference between AQI and NAQI?
Which pollutant is most dangerous in Indian cities?
Check Live AQI
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