SmogRank / Japan / Yamagata / Shonai
Shonai, month by month
Shonai, Yamagata has a measured daily mean on 993 days since 28 July 2023. Its worst month was July 2025, at 56 US AQI (12.0 µg/m³), 839th of 2,889 cities on that month's world ranking. Its cleanest was October 2023, at 29 US AQI (5.3 µg/m³). 2025 averaged 47 US AQI (8.4 µg/m³), against 44 US AQI (7.9 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
10 months
| Month | Why |
|---|---|
| Oct 2026 | 7 of 12 days measured; a month needs 9. The others: 3 with no readings from the monitor, 2 with readings for part of the day (a day needs 18 hours). |
| Jan 2026 | 14 of 31 days measured; a month needs 22. The others: 17 with under 18 hours of readings or readings that failed our checks. |
| Dec 2025 | 20 of 31 days measured; a month needs 22. The others: 11 with under 18 hours of readings or readings that failed our checks. |
| Feb 2025 | 13 of 28 days measured; a month needs 20. The others: 15 with under 18 hours of readings or readings that failed our checks. |
| Jan 2025 | 18 of 31 days measured; a month needs 22. The others: 13 with under 18 hours of readings or readings that failed our checks. |
| Dec 2024 | 9 of 31 days measured; a month needs 22. The others: 22 with under 18 hours of readings or readings that failed our checks. |
| Feb 2024 | 19 of 29 days measured; a month needs 21. The others: 1 missing from OpenAQ's archive, 9 with under 18 hours of readings or readings that failed our checks. |
| Jan 2024 | 11 of 31 days measured; a month needs 22. The others: 20 with under 18 hours of readings or readings that failed our checks. |
| Dec 2023 | 18 of 31 days measured; a month needs 22. The others: 13 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.
Shonai's worst months
| Month | US AQI | World rank | Episode |
|---|---|---|---|
| July 2025 | 5612.0 µg/m³ | 839thof 2,889 | Japan and South Korea, June and July 2025 |
| July 2024 | 5511.4 µg/m³ | 506thof 2,566 | Japan, 12 to 14 July 2024 |
| April 2024 | 5511.2 µg/m³ | 841stof 2,520 | Japan and South Korea, April 2024 |
| June 2024 | 5511.1 µg/m³ | 454thof 2,563 | – |
| March 2026 | 5410.5 µg/m³ | 1,539thof 2,971 | Japan and South Korea, March 2026 |
| June 2025 | 5310.3 µg/m³ | 1,216thof 2,508 | Japan and South Korea, June and July 2025 |
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.
Shonai's worst days
| Day | US AQI | Episode |
|---|---|---|
| 4 Jul 2025 | 9331.9 µg/m³ | Japan and South Korea, June and July 2025 |
| 18 Apr 2024 | 9130.6 µg/m³ | Japan and South Korea, April 2024 |
| 24 Mar 2026 | 9030.1 µg/m³ | Japan and South Korea, March 2026 |
| 14 Jul 2024 | 8929.6 µg/m³ | Japan, 12 to 14 July 2024 |
| 3 Jul 2025 | 8829.0 µg/m³ | Japan and South Korea, June and July 2025 |
| 13 Jul 2024 | 8627.7 µg/m³ | Japan, 12 to 14 July 2024 |
| 14 Feb 2026 | 8527.5 µg/m³ | Japan, 14 to 15 February 2026 |
| 16 Jul 2024 | 7924.0 µg/m³ | – |
| 19 Apr 2025 | 7823.7 µg/m³ | – |
| 5 Jul 2025 | 7723.0 µg/m³ | Japan and South Korea, June and July 2025 |
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, Shonai, Yamagata, 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/shonai/history (accessed 13 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.
Shonai now: the last 24 hours, its ranks today and the last 12 months day by day.
Updated 2026-10-13 00:07 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).