Tehran has been ranked the world's most polluted city with an Air Quality Index of 233, according to Swiss-based air quality monitor IQAir, as critical pollution levels affect all monitoring stations across the Iranian capital.

Pollution has been lingering for days over the Iranian capital as autumnal rains have failed to arrive, only worsening the issue of airborne particles hovering close to the ground due to the so-called "inversion effect", which, due to cool temperatures and a lack of wind, traps pollution close to the ground. 

The Tehran Air Quality Control Company reported that 25 stations registered critical "red" conditions on November 25, with the capital's average Air Quality Index reaching 171 and no areas recording acceptable air quality levels, ISNA reported on November 25.

IQAir's real-time rankings place Tehran ahead of Baghdad (AQI 222), Delhi (209), and Kolkata (206) in the "very unhealthy" category, indicating severe health risks for all residents. The primary pollutant affecting most stations was particulate matter smaller than 2.5 microns (PM2.5).

Fatah Square station in District 9 recorded the highest pollution level with an AQI of 195, marking it as the most polluted monitoring point in the capital. One station registered "very unhealthy" purple-level conditions on November 24, when the citywide average stood at 169 with 21 stations in red status.

Stations recording unhealthy conditions for all groups included Aghdasiyeh, Sharif, Poonak, Tarbiat Modares, Tehran University, Crisis Headquarters, Golbarg and Fatah Square. Critical red-level readings were reported at stations across Districts 11, 13, 15, 17, 18, 19, 20, 21 and 22.

The pollution stems from multiple sources, including vehicular emissions, heavy fuel oil and mazut burning in power plants, and temperature inversion that traps pollutants close to the ground. Vehicle emissions account for approximately 80% of Tehran's air pollution.

Tehran's chronic air pollution stems from a combination of geographic and industrial factors, with the city's location in a valley surrounded by mountains creating an atmospheric inversion effect that traps pollutants close to ground level due to cold weather. 

The Air Quality Index is divided into six categories: zero to 50 indicates clean air, 51 to 100 is acceptable, 101 to 150 is unhealthy for sensitive groups, 151 to 200 is unhealthy for all groups, 201 to 300 is very unhealthy, and 301 to 500 indicates hazardous conditions.

Online social media across Iran has become increasingly critical of government pollution readings, fearing that monitoring stations have been either switched off or producing metered-down readings in recent days, according top previous reports.

Further west, schools and government offices in Tabriz will close on November 26 and 27 after the city's Air Quality Index reached 182, entering the "unhealthy for all groups" red category.

The Emergency Working Group announced the closures due to continuing pollution levels in Iran's fifth-largest city. The AQI reading of 182 places Tabriz firmly in the unhealthy range, which extends from 151 to 200 on the Air Quality Index scale.

The closures affect all educational institutions and government offices across the city for the two-day period, as authorities respond to the sustained deterioration in air quality.

  Other cities across Asia are also suffering under a blanket of smog, including more northernly Tashkent in Uzbekistan, which also reported high readings of airbourne particles. The Uzbek capital, similar in layout to Tehran near mountains is currently suffering with a PM 2.5 level of 198, accordign to the latest readings.