Walk past a roadside eatery in Varanasi at six in the morning and you will see a familiar sight: the kettle on, the chulha glowing, and the first batch of puri-jalebi being prepared. What you may not notice is the smoke above it. Multiply that smoke by the lakhs of small commercial eateries across India’s cities, and you begin to see the scale of a problem my colleagues and I have spent the past year trying to measure.