In October the Mayor of London produced a report titled, “PM2.5 in London. Roadmap to Meeting World Health Organisation Guidelines by 2030.” What is PM2.5? PM2.5, also known as fine particulate matter, refers to particles or liquid droplets in the air that have a diameter less than 2.5 micrometres across (about 3 per cent of … Continue reading PM2.5 in London. Report from Mayor of London.