SmogRank / Japan / Nagano / Kiso
Kiso, month by month
Kiso has a measured daily mean on 1,029 days since 28 July 2023. Its worst month was April 2024, at 54 US AQI (10.8 µg/m³), 907th of 2,520 cities on that month's world ranking. Its cleanest was September 2026, at 22 US AQI (3.9 µg/m³). 2025 averaged 44 US AQI (7.9 µg/m³), against 43 US AQI (7.7 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
7 months
| Month | Why |
|---|---|
| Jun 2025 | 16 of 30 days measured; a month needs 21. The others: 7 with no readings from the monitor, 7 with readings for part of the day (a day needs 18 hours). |
| Mar 2025 | 10 of 31 days measured; a month needs 22. The others: 11 with no readings from the monitor, 10 with readings for part of the day (a day needs 18 hours). |
| Feb 2025 | 19 of 28 days measured; a month needs 20. The others: 5 with no readings from the monitor, 4 with readings for part of the day (a day needs 18 hours). |
| Jan 2025 | 21 of 31 days measured; a month needs 22. The others: 10 with readings for part of the day (a day needs 18 hours). |
| Dec 2024 | 13 of 31 days measured; a month needs 22. The others: 18 with readings for part of the day (a day needs 18 hours). |
| Jan 2024 | 17 of 31 days measured; a month needs 22. The others: 14 with readings for part of the day (a day needs 18 hours). |
| 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.
Kiso's worst months
| Month | US AQI | World rank | Episode |
|---|---|---|---|
| April 2024 | 5410.8 µg/m³ | 907thof 2,520 | – |
| June 2024 | 5410.7 µg/m³ | 508thof 2,563 | – |
| May 2025 | 5410.6 µg/m³ | 759thof 2,826 | Japan, May 2025 |
| April 2025 | 5310.4 µg/m³ | 1,078thof 2,951 | Japan, 19 to 20 April 2025 |
| July 2025 | 5310.0 µg/m³ | 1,271stof 2,889 | – |
| July 2026 | 529.5 µg/m³ | 1,493rdof 2,784 | – |
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.
Kiso's worst days
| Day | US AQI | Episode |
|---|---|---|
| 26 Mar 2025 | 14352.7 µg/m³ | Japan and South Korea, March 2025 |
| 21 May 2025 | 8427.0 µg/m³ | Japan, May 2025 |
| 19 Apr 2025 | 8125.4 µg/m³ | Japan, 19 to 20 April 2025 |
| 2 Feb 2025 | 8125.1 µg/m³ | – |
| 5 Aug 2025 | 7723.2 µg/m³ | – |
| 25 Mar 2025 | 7723.1 µg/m³ | Japan and South Korea, March 2025 |
| 19 Apr 2024 | 7622.5 µg/m³ | – |
| 5 Jul 2025 | 7320.7 µg/m³ | – |
| 20 May 2025 | 7220.2 µg/m³ | Japan, May 2025 |
| 29 Mar 2026 | 7120.1 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, Kiso, 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/kiso/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.
Kiso 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).