PlanetWatch decentralizes and incentivizes environmental monitoring. We are partnering with a state-of-the-art blockchain and with one of the leading research centers in the world to build the world's first immutable air quality ledger and to reward any contribution to our ecosystem. Air pollution is the fourth largest health risk in the world, causing one in ten deaths in 2013 (Source: World Bank). Air quality monitoring is therefore a global challenge for governments, regulators, city administrators and citizens. Today, air quality is monitored via large, expensive scientific instruments installed at a relatively small number of locations. Data is usually not available in real time. This approach is inadequate: without a fine mesh and real-time data, it is not possible to obtain early indications of pollution peaks and issue timely warnings for the population. Our innovative approach to air quality monitoring leverages a new generation of fixed and mobile sensors and the direct participation of the population to rapidly deploy, on demand and at low cost, a dense surveillance network whose data can be released in real time.