SmogRank / Japan / Niigata / Seirō
Seirō, month by month
Seirō has a measured daily mean on 1,070 days since 28 July 2023. Its worst month was April 2024, at 59 US AQI (13.2 µg/m³), 567th of 2,520 cities on that month's world ranking. Its cleanest was January 2024, at 27 US AQI (4.8 µg/m³). 2025 averaged 46 US AQI (8.2 µg/m³), against 43 US AQI (7.8 µg/m³) in 2024.
Every month since 2023
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
2 months
| Month | Why |
|---|---|
| Mar 2025 | No day measured. The others: 31 with under 18 hours of readings or readings that failed our checks. |
| Jul 2023 | 4 of 31 days measured; a month needs 22. |
A day counts once 18 hours of it are measured; a month needs 70% of its days.
Seirō's worst months
| Month | US AQI | World rank | Episode |
|---|---|---|---|
| April 2024 | 5913.2 µg/m³ | 567thof 2,520 | Japan and South Korea, April 2024 |
| July 2025 | 5612.0 µg/m³ | 839thof 2,889 | Japan and South Korea, June and July 2025 |
| March 2026 | 5612.0 µg/m³ | 1,214thof 2,971 | Japan and South Korea, March 2026 |
| May 2025 | 5611.7 µg/m³ | 614thof 2,826 | – |
| June 2024 | 5410.6 µg/m³ | 528thof 2,563 | – |
| May 2026 | 5410.5 µg/m³ | 698thof 2,874 | – |
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.
Seirō's worst days
| Day | US AQI | Episode |
|---|---|---|
| 24 Mar 2026 | 10838.2 µg/m³ | Japan and South Korea, March 2026 |
| 18 Apr 2024 | 9834.3 µg/m³ | Japan and South Korea, April 2024 |
| 3 Jul 2025 | 9230.9 µg/m³ | Japan and South Korea, June and July 2025 |
| 14 Jul 2024 | 8929.7 µg/m³ | Japan, 12 to 14 July 2024 |
| 4 Jul 2025 | 8828.7 µg/m³ | Japan and South Korea, June and July 2025 |
| 30 Mar 2026 | 8024.6 µg/m³ | Japan and South Korea, March 2026 |
| 28 Apr 2024 | 7723.3 µg/m³ | – |
| 16 May 2025 | 7723.0 µg/m³ | – |
| 19 Apr 2024 | 7622.7 µg/m³ | Japan and South Korea, April 2024 |
| 15 Mar 2024 | 7622.5 µg/m³ | – |
Daily means, the highest since 28 Jul 2023; 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, Seirō, Japan, 28 Jul 2023 – 12 Oct 2026. Monitor data from Ministry of the Environment Air Pollutant Wide Area Monitoring System via OpenAQ; sources listed at smogrank.com/sources. smogrank.com/jp/seiro/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.
Seirō now: the last 24 hours, its ranks today and the last 12 months day by day.
Updated 2026-10-12 18:07 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).