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The air of Delhi-NCR has remained in ‘poor’ category for three consecutive days. Here’s a guide to the air quality index, how the readings are taken, and what pollutants like PM2.5 mean
Vehicles continued to move on the Gurugram-Delhi Expressway amid low visibility. (PTI photo)
For the third consecutive day, Delhi morning dawned under a slightly hazy sky. air quality index, which measures how clean or polluted According to the Central Pollution Control Board (CPCB), the air temperature reached 237 on Thursday morning, causing the temperature in the capital to rise. ‘Poor’ categoryAQI was 210 on Wednesday and 211 on Tuesday.
The minimum temperature dropped to 18.1 degrees Celsius, about one and a half degrees below normal, while the humidity stood at 87 per cent at 8.30 am, the India Meteorological Department (IMD) said. Fog was observed in the morning, after which the sky cleared during the day.
But that mist carried more than moisture. At five places- Anand Vihar (345)DU North Campus (307), CRRI Mathura Road (307), Dwarka Sector 8 (314) and Wazirpur (325) – the air had already deteriorated to ‘very poor’ category.
There are 40 air quality monitoring stations in Delhi, of which data were available for 38.
On Wednesday, twenty stations reported AQI in the ‘poor’ category, while 13 stations fell in the ‘moderate’ category.
There’s a bigger story behind these figures than how the city’s air is measured, what those readings mean, and why the same cycle of smog and health warnings returns every winter.
What is Air Quality Index?
The air quality index is a single number that summarizes how polluted the air is, or how polluted it is expected to be. It combines data on key pollutants such as PM10, PM2.5, nitrogen dioxide, sulfur dioxide, carbon monoxide and ozone to help people understand. How air quality affects their health,
The AQI scale runs from zero to 500. The higher this number, the worse the air. In India, the categories are: good (0–50), satisfactory (51–100), moderately polluted (101–200), poor (201–300), very poor (301–400), and severe (401–500).
India launched its colour-coded AQI system in 2014 under the Ministry of Environment, Forest and Climate Change with technical inputs from IIT Kanpur. An expert group of doctors, scientists and advocacy groups designed it to simplify the interpretation of pollution data.
How is AQI calculated?
Every day, monitoring stations across India record concentrations of major pollutants. These raw values ​​are then converted into sub-indices that reflect the impact of each pollutant on health. The pollutant with the highest sub-index defines the AQI for that location.
Both the CPCB and the US Environmental Protection Agency follow a 500-point scale, but India’s system applies health breakpoints adapted to local conditions.
This process turns complex data into a single, comparable number that tells the public when the air becomes unsafe to breathe. But to understand why these numbers rise so sharply during the colder months, it helps to know which pollutants are the main culprits.
Danger of PM2.5 and PM10
Among the pollutants tracked, PM2.5 is the most harmful. These are microscopic particles smaller than 2.5 micrometres, which is about three percent the width of a human hair. Because they are so small, they can pass through the nose and throat and go deep into the lungs, even entering the bloodstream.
PM10 particles, which are up to 10 micrometres wide, mainly come from road dust and construction. But PM2.5 comes from vehicles, power plants, industrial emissions and combustion sources like stubble burning. Long-term exposure to high PM2.5 levels has been shown to cause asthma, bronchitis, heart attacks and reduced lung function. This is also the main reason for the persistent winter smog in Delhi.
As these pollutants increase, governments initiate short-term emergency responses to limit further damage.
GRAP-I implemented in Delhi-NCR
As pollution levels rose this week, authorities activated emergency measures. Commission for Air Quality Management (CAQM) on Tuesday implemented Stage-I Graded Response Action Plan (GRAP) After Delhi’s average AQI reached 211, the entire National Capital Region.
Stage-I applies when the AQI falls between 201 and 300. It requires agencies to implement 27 measures to prevent further declines. These include anti-smog guns, water sprinkling, curbing dust on roads and construction sites and strict checks on open burning and vehicle emissions. Construction sites must follow strict dust-control guidelines, and local bodies are directed to monitor compliance.
In its order, the CAQM said: “The AQI of Delhi has been recorded at 211 (‘Poor’ category) on 14.10.2025. Further, the IMD/IITM forecast predicts the AQI to remain in the ‘Poor’ category in the coming days. The Sub-Committee, accordingly, decides to implement all actions under Phase-I of GRAP in the entire NCR with immediate effect.”
Why is AQI important for health?
AQI is not just a number; It is a snapshot of how safe or unsafe the air is to breathe. When air goes ‘bad’, sensitive groups such as children, the elderly and people with asthma or heart conditions are at greater risk of irritation and respiratory distress. Even healthy people may experience cough or fatigue if pollution persists.
Authorities use the AQI to decide when to impose restrictions such as GRAP, while the public can use it to plan outdoor activities or exercise schedules. Regular monitoring also helps to identify which areas or pollutants require policy attention.
Where can you check AQI level?
Keeping an eye on air quality has become easier. CPCB’s SAMEER app and website provides live AQI readings for hundreds of stations across India. The SAFAR app (System of Air Quality and Weather Forecasting and Research) provides city-wise forecasts and hourly updates. Global platform AQICN.org tracks real-time data for Indian cities, and most smartphones now show AQI details through Google or weather widgets.
But an app only shows numbers; The smog over Delhi-NCR looks like those numbers in the sky. Each winter, as pollution deepens and visibility decreases, AQI figures take on a visible form – the gray blanket of smog that defines the season.
What is smog?
Smog is a combination of smoke and fog that turns the air brown on days with high pollution. There are two main types: photochemical smog and sulfurous smog. Photochemical smog, or Los Angeles smog, is formed when sunlight reacts with nitrogen oxides and volatile organic compounds (VOCs) from fuels, paints, and industrial sources.
Sulphurous smog, also known as London smog, forms in colder months when particulate matter, sulfur oxides and soot mix with fog. At present it is affecting Delhi-NCR. This mixture comes from vehicles, coal burning, industries and crop fires, which are trapped close to the ground due to low temperatures and weak winds.
According to the Center for Science and Environment, Delhi’s air pollution causes between ten thousand and thirty thousand premature deaths every year. From damaging lungs to damaging crops, the effects of this smog extend far beyond what the eye can see.
Haze’s impact on people and planet
Prolonged exposure to smog causes coughing, wheezing, difficulty breathing and irritation in the eyes and throat. This worsens asthma and heart disease and may increase the risk of cancer. The damage extends beyond humans: When sunlight is blocked, crop yields fall, and pollutants accumulate on leaves and soil, damaging ecosystems.
In Delhi, smog usually peaks around the festival season, when traffic increases, temperatures drop and stubble fires rage, creating the familiar toxic mix that reduces visibility and fills the air with fine dust.
Can smog be controlled?
Controlling smog requires both collective and individual action. Authorities have promoted a gradual transition to clean fuels, emissions standards and renewable energy. Under GRAP, use of coal and firewood, diesel generators and open burning in eateries are banned, while parking charges may be increased to discourage private vehicles.
Experts recommend long-term reforms: replacing stubble burning with sustainable alternatives, reducing the number of vehicles through carpooling and public transportation, and expanding green spaces. At the individual level, residents can use air purifiers, wear N95 masks and avoid outdoor activity during peak hours.
Yet, even with these measures, the same smog reappears every winter.
Why is the pollution cycle repeated every year?
Despite repeated warnings and annual action plans, Delhi’s pollution cycle has become routine. Each winter, a temperature inversion traps pollutants near the surface while wind speeds decrease, preventing dispersion.
The city’s landlocked geography, combined with smoke from crop burning in Punjab and Haryana, compounds the impact.
Phase I of GRAP is now active and the forecast predicts ‘poor’ air to continue, with officials trying to prevent another drop into the ‘severe’ category, the level that has forced school closures and emergency restrictions in the past.
As Delhi moves towards winter, the fight against smog begins again.
Karishma Jain, Deputy Editor in Chief, News18.com, writes and edits opinion pieces on a wide range of topics including Indian politics and policy, culture and arts, technology and social change. Follow him @kar…read more
Karishma Jain, Deputy Editor in Chief, News18.com, writes and edits opinion pieces on a wide range of topics including Indian politics and policy, culture and arts, technology and social change. Follow him @kar… read more
October 16, 2025, 14:04 IST
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