What is AQI? A Complete Guide to the Air Quality Index in India

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 Today 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 |

🏠 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 Today — 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.