SmogRank / South Korea / Sejong-si / Sejong
Sejong, month by month
Sejong has a measured daily mean on 909 days since 23 March 2024. Its worst month was March 2026, at 92 US AQI (31.1 µg/m³), 244th of 2,971 cities on that month's world ranking. Its cleanest was July 2024, at 42 US AQI (7.6 µg/m³). 2025 averaged 68 US AQI (18.2 µg/m³), against 62 US AQI (14.8 µg/m³) in 2024.
Every month since 2024
The US EPA's Air Quality Index for PM2.5 (2024 breakpoints). Show values as: µg/m³ · US AQI · India AQI · China AQI · European AQI · UK DAQI · Canada AQHI+. About the scales
Months without a mean, and why
1 month
| Month | Why |
|---|---|
| Mar 2024 | 8 of 31 days measured; a month needs 22. The others: 1 with under 18 hours of readings or readings that failed our checks. |
A day counts once 18 hours of it are measured; a month needs 70% of its days.
Sejong's worst months
| Month | US AQI | World rank | Episode |
|---|---|---|---|
| March 2026 | 9231.1 µg/m³ | 244thof 2,971 | Japan and South Korea, March 2026 South Korea, 15 to 18 March 2026and 1 more |
| March 2025 | 8929.4 µg/m³ | 305thof 2,862 | Japan and South Korea, March 2025 South Korea, 9 to 11 March 2025and 1 more |
| January 2025 | 8828.7 µg/m³ | 111thof 2,474 | Japan and South Korea, 17 to 24 January 2025 South Korea, 13 to 14 January 2025 |
| February 2026 | 8828.7 µg/m³ | 278thof 2,866 | South Korea and Japan, 10 to 15 February 2026 South Korea, February 2026 |
| November 2025 | 8225.7 µg/m³ | 285thof 2,715 | South Korea and Japan, November 2025 South Korea, 13 to 14 November 2025and 1 more |
| December 2025 | 7622.6 µg/m³ | 419thof 3,006 | South Korea, 29 to 30 December 2025 South Korea, 16 to 20 December 2025and 1 more |
Monthly means over the days measured; finished months only. World rank: its place on that month's world ranking of every city with enough days. Each month opens that ranking.
Sejong's worst days
| Day | US AQI | Episode |
|---|---|---|
| 21 Jan 2025 | 16778.1 µg/m³ | Japan and South Korea, 17 to 24 January 2025 |
| 14 Feb 2026 | 16677.0 µg/m³ | South Korea and Japan, 10 to 15 February 2026 |
| 16 Jan 2026 | 16676.8 µg/m³ | South Korea and Japan, January 2026 |
| 5 Feb 2026 | 15865.3 µg/m³ | South Korea, February 2026 |
| 6 Nov 2025 | 15865.1 µg/m³ | South Korea, 5 to 7 November 2025 |
| 22 Jan 2025 | 15864.8 µg/m³ | Japan and South Korea, 17 to 24 January 2025 |
| 13 Feb 2026 | 15662.6 µg/m³ | South Korea and Japan, 10 to 15 February 2026 |
| 15 Jan 2026 | 15358.9 µg/m³ | South Korea and Japan, January 2026 |
| 18 Mar 2026 | 15358.4 µg/m³ | South Korea, 15 to 18 March 2026 |
| 24 Nov 2025 | 15257.1 µg/m³ | South Korea and Japan, November 2025 |
Daily means, the highest since 23 Mar 2024; the WHO's daily guideline is 15 µg/m³. Each date opens that day's world ranking, and each episode the bad-air spell it was part of.
Use this history
SmogRank, PM2.5 monthly and daily means, Sejong, South Korea, 23 Mar 2024 – 12 Oct 2026. Monitor data from Korea Air Ministry of Environment via OpenAQ; sources listed at smogrank.com/sources. smogrank.com/kr/sejong/history (accessed 12 Oct 2026).
You're welcome to reuse a single chart, image or figure with this citation and a link. Terms. Raw monitor readings in bulk are published by OpenAQ, under each provider's licence.
Sejong now: the last 24 hours, its ranks today and the last 12 months day by day.
Updated 2026-10-12 17:07 UTC. Values are PM2.5 (fine particles), the median of each city's monitors, as the US AQI PM2.5 sub-index. Some low-cost readings: Sensor.Community, ODbL (data and licence). Some readings: Uzhydromet (Uzbekistan's Hydrometeorological Service); Department of Environment, Nepal; Vietnam's government monitors; Aurassure; Umhverfisstofnun (Environment Agency of Iceland); RMCAB (Secretaría Distrital de Ambiente, Bogotá); Love My Air (City and County of Denver), no reuse terms published (why we use them).