Japan's Air Quality Standards Explained
If you look for a single 0-to-500 air-quality number in Japan, you won't find an official one. Japan takes a different approach: it sets legally defined Environmental Quality Standards (EQS) for each pollutant and issues targeted alerts — most visibly the summer photochemical-oxidant ("smog") warning. Third-party apps often map Japanese data onto a US-style AQI, but that's their conversion, not Japan's system. This guide explains the real thing.
Environmental Quality Standards, not a composite index
Japan's Ministry of the Environment publishes a target threshold for each major pollutant. Air is assessed against these per-pollutant standards rather than collapsed into one index number:
| Pollutant | Standard |
|---|---|
| PM2.5 | ≤ 15 µg/m³ (annual) and ≤ 35 µg/m³ (24-hour) |
| Suspended particulate matter (SPM) | ≤ 0.10 mg/m³ (24-hour) and ≤ 0.20 mg/m³ (1-hour) |
| Photochemical oxidants (Oₓ) | ≤ 0.06 ppm (1-hour) |
| Nitrogen dioxide (NO₂) | within or below 0.04–0.06 ppm (daily average of 1-hour values) |
| Sulphur dioxide (SO₂) | ≤ 0.04 ppm (24-hour) and ≤ 0.1 ppm (1-hour) |
| Carbon monoxide (CO) | ≤ 10 ppm (24-hour) and ≤ 20 ppm (8-hour) |
The alerts that actually drive public action
- Photochemical-oxidant warning. Japan's signature summer hazard is photochemical smog. When the 1-hour oxidant concentration reaches 0.12 ppm and conditions are expected to persist, prefectures issue a warning (注意報) advising people — especially children — to avoid strenuous outdoor activity. This is twice the EQS threshold of 0.06 ppm.
- PM2.5 provisional advisory. When the daily mean PM2.5 is forecast to exceed 70 µg/m³ (double the 24-hour standard), authorities advise the public to limit prolonged outdoor exertion. Spring spikes are often driven by transboundary transport and Asian dust from the continent — see Asian Dust (Hwangsa).
Where the data lives
Real-time readings for PM2.5, SPM, oxidants, NO₂, SO₂, CO, and more are published nationwide through the Ministry of the Environment's Soramame ("AtmoSphere Monitoring Atmospheric Environmental Regional Observation System") network.
So what does a "Japan AQI" in an app mean?
Because Japan has no official composite index, any single AQI number you see for a Japanese city in a third-party app is that app converting raw concentrations onto another country's scale — usually the US EPA AQI. It's a reasonable convenience, but it isn't Japan's official metric. For the authoritative picture, read the per-pollutant values against the standards above, and watch for oxidant warnings in summer. For how other countries' indices compare, see AQI Around the World.
Air quality on your iPhone — free
Smog Report shows real-time air quality with widgets, Live Activities, and Apple Watch. Free, no account, no tracking.
Download for iOSPrimary sources: Japan Ministry of the Environment — Environmental Quality Standards (Air) · Soramame — national air-quality monitoring system