Kolkata — Seasonal Pollution Patterns

January is the most polluted month in Kolkata on average, with a long-run AQI of 237 — firmly in the Poor band. This is drawn from 7 CPCB monitoring stations across 8 years of daily readings. Through January, residents should expect elevated PM2.5 and PM10, reduced visibility on cooler mornings, and strong recommendations from doctors to limit outdoor exertion, wear well-fitted N95 masks, and run indoor purifiers through evening and overnight hours when pollutant accumulation typically peaks.

July is the cleanest month of the year in Kolkata, averaging AQI 46 in the Good band. The months immediately before and after also tend to sit well below the annual mean of 117, so a visit window centred on July is the safest choice for outdoor sightseeing, marathons, school trips and wedding events. Mornings are usually the crispest time to head out; pollution tends to creep up slightly during the evening commute even in the cleanest months. Always cross-check the day-of live AQI before any high-exertion outdoor plan.

Kolkata shows a clear monsoon-cleansed signature — rain and deeper atmospheric mixing drop AQI to a seasonal trough, and everything else relative to that trough looks elevated. The specific January spike combines pre-monsoon dust, post-rain rebounds and the arrival of cool-season trapping effects.

Across the CPCB record, the week around Diwali in Kolkata averages AQI 190 — 1.46× the normal October baseline of AQI 130, a spike of 59 AQI points. Firework particulates combine with a cooler, more stagnant late-October atmosphere to produce some of the worst air-quality days of the entire year. Sensitive groups should treat Diwali eve and the two days after as peak-alert days: stay indoors, close windows by evening, run purifiers on high, and reserve any outdoor celebrations for daytime hours when mixing is strongest.

Yes — meaningfully. Kolkata's core monsoon window (Jul 15 – Sep 15) averages AQI 67, a 55.9% improvement on the annual mean of 152. Rain removes airborne particulates by wet deposition, and the deeper monsoon boundary layer disperses what remains vertically. Across 725 measured monsoon days we see 89.9% in the Good-or-Satisfactory band.

Between 2017 and 2024, Kolkata's annual average AQI moved from 177 to 101 — a change of -42.9%. In the most recent year-on-year comparison, the winter season specifically improved by 20.1%. The long-run direction is improving — NCAP policy pressure, cleaner fuels and tighter vehicle standards are showing up.

July is the single best month at AQI 46. Based on the 12-month averages, the three cleanest months in Kolkata are July (AQI 46), September (AQI 49), August (AQI 50). These are the safest choices for outdoor itineraries, long walks, open-air concerts and day-trips. Sensitive groups can treat these months as near-normal activity windows but should still check live AQI for the specific date. Avoid planning outdoor-heavy trips in January, when the baseline jumps into Poor territory.

Kolkata is classified as monsoon-cleansed. Based on a 12-month cosine-similarity index computed across all monitored Indian cities, the city whose seasonal signature most closely resembles Kolkata's is Howrah (West Bengal), with its own worst month in December. Cities with similar signatures often respond to similar policy levers — if a neighbouring peer has demonstrated improvements through specific interventions (construction-dust controls, bus electrification, brick-kiln regulation), they are likely candidates for Kolkata too. The peer-city panel on this page lists the closest four additional matches.