Crackling flames leap up from the fields, spewing out a toxic black smoke as farmers set alight their crop residue in northern India -- an annual practice that exacerbates the pollution crisis in New Delhi. Each winter, tens of thousands of farmers in Delhi's bordering states of Punjab, Haryana and Uttar Pradesh flout laws to clear their fields by burning the rice crop stubble to make way for wheat. The post-harvest burning of stubble covers swathes of the region in poisonous smog, which combines with car and factory emissions to turn Delhi into the world's most polluted capital. IMAGES AND SOUNDBITES