In some places, air pollution has reached a crisis point. Those places are a warning to other cities across the world of what unregulated and unmitigated air pollution looks like. Take New Delhi, which many consider to be the most polluted capital in the world. During peak pollution periods, PM2.5 concentrations in Delhi regularly exceed WHO guideline levels by roughly 24 times. According to the Air Quality Life Index, that exposure is estimated to reduce life expectancy by almost 12 years. Exposure at these levels is associated with increased risk of heart attacks, strokes, chronic respiratory disease and impaired brain development in children. The result is that 15% of all deaths in Delhi can be attributed directly to air pollution. It’s a slow and invisible poison, which is why it attracts far less attention than more immediate health threats.