Why London is on high pollution alert despite having AQI under 100?

London has issued a high air pollution alert despite its overall AQI remaining below 100. The warning is instead linked to the concentration of specific pollutants, particularly PM2.5, which consists of tiny airborne particles that can enter the respiratory system. While AQI remained under check, London's PM2.5 levels were reported to be around five times the WHO's recommended guideline level.