When smog was prevalent, it was easy to see. Particulate matter 10 microns in size hover in the air, the famous London Fog was not natural moisture, it was PM10 pollution. In one event, nature combined with smog in London to kill 12,000 people.After that, wealthier nations engaged in pollution control, and then PM10 and its health issues began to dissipate. In the 1990s and with much cleaner air, pollution activists and allied epidemiologists began to 'define pollution down.' PM2.5 was suddenly the new goalpost, they said, and showed air quality maps with red and orange to prove it.