NEW DELHI (REUTERS) - India's capital New Delhi was shrouded in a toxic haze for the second straight day on Thursday (Dec 12), and visibility dropped due to cooler temperatures and lower wind speeds that let deadly pollutants hang in the air.

The air quality index crossed 400 on a scale of 500, indicative of "severe" conditions that pose a risk for healthy people and can seriously impact those with existing diseases.

The index measures the concentration of deadly pollutant PM2.5 - tiny particles that can enter the bloodstream.

Chronic exposure to such pollutants can contribute to the risk of developing diseases such as lung cancer, according to the World Health Organisation.

Federal pollution control officials were tracking the air quality status, Mr Prashant Gargava, member secretary at the Central Pollution Control Board, told Reuters.

The board falls under the federal environment ministry.

Under an emergency action plan, the authorities shut down brick kilns and halted all construction activity during the day.

During the last two months, the capital's 20 million residents have breathed "moderate" to "satisfactory" air only for four days, according to a record of official data compiled by Reuters.

The air quality index was "very poor" on most days this month.

Air quality levels have crossed 400 for a second time this month despite farm fires from Delhi's neighbouring states - blamed by the authorities as the primary cause for poor air quality in recent weeks - coming to an end with the onset of winter.

"Now fire counts are almost stopped except in a few routine incidences and hence no contribution to Delhi's air quality is expected now onwards for the season," government-run monitor SAFAR said.

The relentless focus on stamping out farm fires every year tends to deflect scrutiny from the authorities that are falling behind on cleaning up industry or improving public transport, critics say.

Vehicular exhausts, along with emissions from industry, contribute more than 50 per cent of Delhi's air pollution on most days through the year, according to official estimates.

SAFAR forecast rain later on Thursday, but added that Delhi's air quality was likely to deteriorate next week due to foggy conditions.