Environment & Climate17 Oct 2026 · about 6 min

Delhi's AQI jumped from 65 to 216 in a day. Why pollution could worsen

The brief

The Air Quality Index, or AQI, turns air-pollution measurements into a single number and category. It helps people understand whether the air is clean, unhealthy, or poor without examining each pollutant separately. On India's CPCB scale, higher numbers indicate worse air quality. Delhi's reading of 216 falls in the CPCB's "Poor" category, which spans 201 to 300. This is different from AQI.in's label for the same reading: that platform calls 216 "Unhealthy." The labels differ, but both signal deteriorating air. The classification matters because it shows that Delhi's brief improvement did not last. AQI.in recorded the city's air at about 65 earlier in the 24-hour period, then at 216 at 10:31 am on October 17. The CPCB category therefore provides an important warning as winter approaches and dry weather continues.

What is the Air Quality Index (AQI), and what does a reading of 216 mean on India's CPCB scale?

Delhi's reading of 216 falls in the CPCB's "Poor" category, which spans 201 to 300. This is different from AQI.in's label for the same reading: that platform calls 216 "Unhealthy." The labels differ, but both signal deteriorating air.

The classification matters because it shows that Delhi's brief improvement did not last. AQI.in recorded the city's air at about 65 earlier in the 24-hour period, then at 216 at 10:31 am on October 17. The CPCB category therefore provides an important warning as winter approaches and dry weather continues.

How large was Delhi's pollution change—from an AQI of about 65 to 216—and how quickly did it happen?

Delhi's pollution change was both large and rapid. AQI.in's hourly graph shows the index falling to about 65 during the afternoon, then rising to 216 by 10:31 am on October 17. That is an increase of roughly 151 AQI points, more than three times the afternoon reading.

The shift unfolded within the graph's 24-hour window, from 10:31 am on October 16 to 10:31 am on October 17. Pollution climbed sharply in the evening, crossed 200, and reached about 240 overnight. After easing, it rose again on Saturday morning.

This pattern shows how quickly cleaner conditions can disappear. The reading began near 135, dropped to approximately 65, and then returned to the Poor category under the CPCB scale. With no rain expected through October 23, the article warns that high pollution could persist or worsen.

Why can Delhi's air become cleaner during the afternoon but much more polluted in the evening and overnight?

Air pollution is not controlled only by how much pollution is emitted. It also depends on how well the atmosphere mixes and carries pollutants away. During the afternoon, sunlight warms the ground and usually strengthens vertical air movement, spreading pollution through a deeper layer of air. That can lower measured concentrations near the surface.

The article's hourly pattern illustrates this change. Delhi's AQI fell from about 135 to approximately 65 through the afternoon. Pollution then rose sharply during the evening, crossed 200, and reached around 240 overnight. Cooling after sunset can make the lower atmosphere more stable, while weaker mixing allows emissions to build near the ground.

The improvement was therefore temporary, not a lasting reversal. AQI climbed again to 216 by 10:31 am on October 17. The sharp daily swing shows why a clean afternoon does not guarantee clean air later, especially when rain is absent and winter conditions approach.

What happens to pollutants when there is rain, and why could several dry days allow Delhi's pollution to remain high?

Rain can remove some pollution from the atmosphere by bringing particles down with falling water. This process gives the air a chance to clear, although its effect depends on the amount and duration of rain and on ongoing emissions. In the article, rain is described as one potential way to wash pollutants out of Delhi's air.

Delhi is forecast to have mainly clear skies, with no rain expected through October 23. That means pollutants will not receive this weather-driven removal. If emissions continue, the same pollution can remain in the atmosphere or accumulate when winds and mixing are weak.

The dry forecast matters because Delhi's AQI has already risen sharply. It moved from an afternoon low near 65 to 216 at 10:31 am on October 17, after reaching about 240 overnight. IITM projected Poor air quality on October 19 and through its following six-day outlook, making persistent pollution a concern.

Why does approaching winter make it harder for Delhi's pollution to disperse?

Approaching winter matters because atmospheric conditions can become less favorable for dispersing pollution. Cooler air near the ground may mix less readily with air above it, and calmer conditions can reduce the movement of pollutants. When that happens, emissions remain concentrated closer to where people breathe them.

The article does not give a detailed breakdown of winter atmospheric layers, but it warns that changing weather conditions can make pollutants harder to disperse. Its readings show why that warning matters: Delhi's AQI dropped to approximately 65 during the afternoon, then rose above 200 in the evening and overnight.

Winter is therefore arriving as the city faces a fragile pollution situation. The AQI reached 216 at 10:31 am on October 17, and IITM expected Poor air quality from October 19 through its six-day outlook. Without rain, Delhi may have fewer opportunities for relief as the season advances.

Which factors besides rain—such as emissions, wind speed, and wind direction—determine whether Delhi's air improves or worsens?

Delhi's air quality depends on the balance between pollution entering the air and the atmosphere's ability to move it away. The article specifically identifies emissions, wind speed, and wind direction, along with rain and dispersion, as important factors. A high AQI can persist when emissions continue and weather does not carry pollutants away efficiently.

For example, Delhi's AQI fell to about 65 during the afternoon but later climbed sharply. It crossed 200 in the evening, peaked near 240 overnight, and reached 216 at 10:31 am on October 17. This rapid change shows that air quality can shift as atmospheric movement changes, even within one day.

The broader outlook remains concerning. Clear skies and no rain are expected through October 23, while temperatures are forecast to rise by 2–3°C after October 17. IITM projected Poor air quality from October 19, but actual conditions will still depend on emissions and wind behavior.

What health risks can arise when people are exposed to air in the CPCB's 'Poor' AQI range for several days?

The CPCB's Poor range, from 201 to 300, indicates air that can pose health concerns when exposure continues. Fine particles and other pollutants can irritate the eyes, nose, throat, and lungs. Repeated exposure may worsen coughing, wheezing, asthma, or other existing breathing and heart conditions, with greater concern for children, older adults, and people already ill.

The article does not provide a health-effects list, but it records Delhi's AQI at 216 and identifies that level as Poor on the CPCB scale. AQI.in's graph also showed pollution near 240 overnight. Those readings suggest exposure was not limited to a single brief spike.

The outlook adds to the concern. IITM projected that Delhi's air would enter the Poor category on October 19 and remain there through its subsequent six-day outlook. With no rain expected through October 23 and winter approaching, several days of exposure could increase health pressure.

This brief was written by AI from the original reporting and checked by other models. Names, figures and quotes come from the source; read it for full context.