Air Hub: Engineering healthy indoor environments  aims to build research capacity to understand what is in particulate matter (PM2.5) inside, what its health impacts are, and what we should do about it. Although we know that PM2. 5 is a significant contributor to ill health, we know little about the constituents (and therefore toxicology) of particulate matter in our buildings. The network will run for two years with a series of online and in-person workshops, sandpits and pilot studies.