SmogRank / Japan / Nagano / Okaya
Okaya, month by month
Okaya has a measured daily mean on 932 days since 28 July 2023. Its worst month was April 2024, at 55 US AQI (11.1 µg/m³), 857th of 2,520 cities on that month's world ranking. Its cleanest was September 2026, at 19 US AQI (3.5 µg/m³).
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 |
|---|---|
| Jan 2025 | 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 2024 | 4 of 31 days measured; a month needs 22. The others: 27 with under 18 hours of readings or readings that failed our checks. |
| Nov 2024 | 16 of 30 days measured; a month needs 21. The others: 14 with under 18 hours of readings or readings that failed our checks. |
| Jul 2024 | 14 of 31 days measured; a month needs 22. The others: 17 with under 18 hours of readings or readings that failed our checks. |
| Mar 2024 | 2 of 31 days measured; a month needs 22. The others: 1 missing from OpenAQ's archive, 28 with under 18 hours of readings or readings that failed our checks. |
| Feb 2024 | 1 of 29 days measured; a month needs 21. The others: 1 missing from OpenAQ's archive, 27 with under 18 hours of readings or readings that failed our checks. |
| Jan 2024 | 2 of 31 days measured; a month needs 22. The others: 29 with under 18 hours of readings or readings that failed our checks. |
| Dec 2023 | 11 of 31 days measured; a month needs 22. The others: 20 with under 18 hours of readings or readings that failed our checks. |
| Nov 2023 | 13 of 30 days measured; a month needs 21. The others: 17 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.
Okaya's worst months
| Month | US AQI | World rank |
|---|---|---|
| April 2024 | 5511.1 µg/m³ | 857thof 2,520 |
| August 2023 | 529.8 µg/m³ | 652ndof 1,980 |
| July 2026 | 529.8 µg/m³ | 1,413thof 2,784 |
| June 2024 | 529.6 µg/m³ | 752ndof 2,563 |
| July 2025 | 529.4 µg/m³ | 1,423rdof 2,889 |
| September 2023 | 519.3 µg/m³ | 726thof 2,176 |
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.
Okaya's worst days
| Day | US AQI | Episode |
|---|---|---|
| 26 Mar 2025 | 12143.5 µg/m³ | Japan and South Korea, March 2025 |
| 25 Mar 2025 | 7924.1 µg/m³ | Japan and South Korea, March 2025 |
| 21 May 2025 | 7923.9 µg/m³ | – |
| 19 Apr 2025 | 7723.3 µg/m³ | – |
| 18 Apr 2024 | 7723.0 µg/m³ | – |
| 19 Apr 2024 | 7622.3 µg/m³ | – |
| 21 Apr 2024 | 7220.3 µg/m³ | – |
| 5 Aug 2025 | 7119.9 µg/m³ | – |
| 27 Mar 2025 | 7019.3 µg/m³ | Japan and South Korea, March 2025 |
| 20 May 2025 | 7019.3 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, Okaya, 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/okaya/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.
Okaya now: the last 24 hours, its ranks today and the last 12 months day by day.
Updated 2026-10-12 22:07 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).