Pollution in several parts of India escalated to severe levels on Thursday after revellers burst firecrackers long into the night to mark Diwali. The already dismal air worsened, meaning prolonged exposure could lead to severe respiratory illnesses. The concentration of tiny poisonous particulate matter, or PM 2.5, particles that are less than 2.5 microns in diameter in the air can be carried deep into the lungs. A level of 50 or less is considered safe. In parts of Delhi, PM 2.5 levels had soared to 689. Below is a chart that explains how serious a health risk people of India are exposed to due to deteriorating pollution levels.