Pollution levels in New Delhi have hit their worst this year in the past two days, earning a "severe" to "emergency" rating and indicating conditions that can spark a public health crisis. Senior government officials said the main reasons for the surge in the amount of toxic smog trapped over New Delhi were unusually cold air, including fog, and a lack of wind. This mean that vehicle fumes, pollution from coal-fired power plants and industries, as well as smoke from fires being burned to keep people warm hangs over the city.

Data from the government's Central Pollution Control Board (CPCB) showed the air quality index, which measures the concentration of poisonous particulate matter, was an average of 449 yesterday, only slightly better than the 450 on Sunday. The index measures the concentration of tiny poisonous particulate matter, or PM 2.5, that are less than 2.5 microns in diameter, which can be carried deep into the lungs. The previous highest recording this year was 447 on June 15, when there was a dust storm. Anything above 100 is considered unhealthy.

In some parts of Delhi, pollution levels hit 654 yesterday, among the worst recorded this year, and visibility in some parts of the city was just 200 meters, the weather department said.

Environmentalists said that inaction by the authorities was inexcusable and a concerted effort was needed to reduce pollution from vehicles and industry. "If this is not an emergency, then what is?" asked Delhi-based environmentalist Vimlendu Jha.

The "severe" to "emergency" ratings mean that the air is not only hazardous for citizens with existing respiratory problems but can also seriously affect healthy people. Conditions are likely to remain severe today.