A community sensor logged four hundred and sixty-nine days of Bali's breath before it fell silent. A single government monitor reports intermittently for 4.4 million residents. Since the first of April, seven tons of waste a day have been pulled from Denpasar's rivers. The smoke in the alleys rises without an instrument to record it. The science underneath, the practical mask guide, and — closing the collection — what would actually help, in the order it should be deployed.