Blazing flames light the sky as Indian farmer Ali Sher burns his fields to clear them for new crops, a common but illegal practice that is fueling deadly pollution killing millions.

Burning strips the fertility of fields, has a ruinous impact on India's economy and sends plumes of acrid smoke packed with dangerous cancer-causing particles drifting over a densely-populated belt of northern India, including capital New Delhi's 30 million people.

But it is cheap — for farmers at least — to carry out.