The farm fires helping to fuel India's deadly air

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 fuelling 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.