SmogRank / Japan / Chiba / Yotsukaidō
Yotsukaidō, month by month
Yotsukaidō has a measured daily mean on 1,100 days since 28 July 2023. Its worst month was December 2025, at 58 US AQI (13.0 µg/m³), 861st of 3,006 cities on that month's world ranking. Its cleanest was September 2026, at 26 US AQI (4.6 µg/m³). 2025 averaged 52 US AQI (9.5 µg/m³), against 46 US AQI (8.3 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 |
|---|---|
| May 2025 | 19 of 31 days measured; a month needs 22. The others: 12 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.
Yotsukaidō's worst months
| Month | US AQI | World rank | Episode |
|---|---|---|---|
| December 2025 | 5813.0 µg/m³ | 861stof 3,006 | Japan, December 2025 |
| December 2023 | 5712.5 µg/m³ | 400thof 2,247 | – |
| August 2025 | 5611.8 µg/m³ | 823rdof 2,901 | – |
| July 2024 | 5611.7 µg/m³ | 454thof 2,566 | – |
| November 2025 | 5511.2 µg/m³ | 848thof 2,715 | – |
| February 2026 | 5410.8 µg/m³ | 1,230thof 2,866 | – |
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.
Yotsukaidō's worst days
| Day | US AQI | Episode |
|---|---|---|
| 7 Jul 2025 | 10537.1 µg/m³ | Japan and South Korea, June and July 2025 |
| 6 Jul 2025 | 9934.6 µg/m³ | Japan and South Korea, June and July 2025 |
| 8 Jul 2025 | 9633.0 µg/m³ | Japan and South Korea, June and July 2025 |
| 9 Jul 2025 | 9130.5 µg/m³ | Japan and South Korea, June and July 2025 |
| 20 Dec 2025 | 8929.7 µg/m³ | Japan, December 2025 |
| 26 Mar 2025 | 8828.7 µg/m³ | Japan and South Korea, March 2025 |
| 25 Nov 2025 | 8728.2 µg/m³ | – |
| 5 Dec 2023 | 8527.4 µg/m³ | – |
| 31 Dec 2023 | 8527.4 µg/m³ | – |
| 18 Jan 2024 | 8326.1 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, Yotsukaidō, 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/yotsukaido/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.
Yotsukaidō now: the last 24 hours, its ranks today and the last 12 months day by day.
Updated 2026-10-12 20:07 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).