Aljazeera:  Out of the world’s 100 most polluted cities, 94 are in India, China, and Pakistan…How is air quality measured? Air quality is determined by the levels of air pollutants PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide.Particulate matter (PM) comprises tiny particles that negatively impact health. PMs vary in size, most damaging are PM2.5 and PM10 – with a diameter of less than 2.5 μm and 10μm respectively. A human hair’s diameter is 50-70 μm. PM2.5 levels lower than 12 are considered good, 55-150 unhealthy and 250 or above is hazardous…”