Air quality sits at the intersection of two categories that don't always talk to each other: environment (measuring the air) and mobility (the traffic that mostly causes the problem). The most effective projects in this dataset tend to work on both sides at once.
London's Ultra Low Emission Zone is the largest scheme of its kind by area, charging the most polluting vehicles a daily fee across all 32 boroughs, enforced continuously by automatic number-plate cameras rather than periodic checks. Milan's Area C runs the same ANPR-camera enforcement over a smaller footprint — technically a congestion charge rather than an emissions zone, since it tolls entry rather than banning vehicles by engine class — but ties the revenue directly to financing inner-city cycle lanes, closing the loop between the fee and the alternative it's meant to encourage.
Krakow's Airly network flips the usual order: it started as a student project after the city's 2016 "Smogathon," growing from 6 official monitoring stations to nearly 100 crowd-funded sensors through the public #KrakowOddycha campaign — data that pushed the city toward action rather than the other way round. It's a useful counter-example to the assumption that air-quality monitoring has to start with an official rollout.
Whether the intervention is a camera charging a fee, a signal timed to reduce idling, or a citizen-funded sensor grid, the effective projects here all convert an invisible problem (particulates, NOx) into a number someone is accountable for. See the Environment category for the broader set of measurement and climate projects these belong to.
The same ladder that organises the flood systems in this atlas, applied to air. All four cities measure well. What separates them is whether a number is allowed to oblige anyone to do anything.
| Project | Since | Status | Outcome | Evidence | Run by | Cost | What follows from the reading |
|---|---|---|---|---|---|---|---|
| GRAP: Air Quality Readings as a Legal TriggerNew Delhi, 🇮🇳 | 2017 | Live | Ongoing | Independent | National government, City government, Research institution | not public | The measurement obliges: an index crossing a fixed step brings legal duties into force by itself, from construction bans to school closures. |
| Critical Episode Management: A Next-Day Forecast With Legal ForceSantiago, 🇨🇱 | 1997 | Live | Ongoing | Official | National government, Regional government | not public | The threshold binds but the day is judged — an appointed regional authority signs the declaration, and it acts on a forecast of tomorrow rather than a reading of today. |
| Bangkok PM2.5 Monitoring & Graduated ResponseBangkok, 🇹🇭 | 2023 | Live | Ongoing | Official | City government | not public | The city can close its own schools on a reading, must borrow the disaster act to keep trucks out, and can only ask employers. |
| Court-Ordered Air Quality MonitoringJakarta, 🇮🇩 | 2021 | Live | Ongoing | Independent | City government | not public | A court ordered the network and the network was built — but no index value triggers anything at all. |