Why has India's capital's air quality worsened so dramatically?

This month, pollution levels in New Delhi and other parts of northern India have reached deadly levels.

This month, pollution levels in New Delhi and other regions of northern India have reached dangerous levels, a significant decline from October, when residents breathed the cleanest air in four years.

Here are some of the variables contributing to the sudden shifts in air quality, which are exacerbated by crop burning and Diwali holiday firecrackers.