“PM2.5” seems an odd and wonky term for the blogosphere to take up, but that is precisely what has happened in China in recent weeks. It refers to the smallest solid particles in the atmosphere—those less than 2.5 microns across. Such dust can get deep into people’s lungs; far deeper than that rated as PM10. Yet until recently China’s authorities have revealed measurements only for PM10. When people realised this, an online revolt broke out.