The smog smothering millions in the Indian capital of New Delhi is so thick, it's plainly visible from space, showing up as a milky blanked covering northern India.

Visible satellite imagery posted by NASA's Worldview tool shows persistent smog across northern India since late October. This coincides with the advent of winter weather patterns that often bring more stagnant air masses to the region.

Pollution levels are so high, Indian authorities have shut schools and construction sites this week. The smog poses serious public health threats, including causing or aggravating heart disease, heightening cancer risks and elevating mortality among more vulnerable populations.

Delhi has some of the most polluted skies in the world, rivaling if not beating parts of China for this distinction. Among some of the varied causes for this? Garbage fires, dung-powered cookstoves, diesel-powered vehicles and coal-fired power plants.

In addition, this event may be partly attributed to agricultural fires in areas close to the capital.

Satellite view of smog in northern India, taken Nov. 4, 2016. Image: nasa worldview

In addition, weather patterns have helped to trap pollutants in the lower atmosphere.

"Higher air pollution levels may take days to settle," said Dr. Surya Kant Tripathi, who heads the pulmonary medicine department at King George's Medical University in Lucknow, the capital of Uttar Pradesh.

Lucknow city director J.P. Gupta said the smog covering New Delhi was wafting over western parts of Uttar Pradesh and would soon cover the entire state, India's most populous, with around 210 million people.

In Delhi, the level of the most dangerous particulate matter in the air — tiny particles that are just 2.5 microns in diameter, about thirty times smaller than that of a human hair — were at least above 400 micrograms per cubic meter on Monday.

That's over 40 times what's considered safe by the World Health Organization, and over six times the limit set by Indian law. Some monitoring points were registering even higher levels than that.

Delhi has also ordered the temporary closure of a nearby coal-burning power plant, as well as construction and demolition sites blamed for sending huge plumes of fine dust particles into the air. It also ordered that roads be doused with water to settle dust, and banned diesel-powered electricity generators for 10 days except at hospitals and cellphone towers.

The Center for Science and Environment, a Delhi-based research and lobbying organization, said government data shows that the smog that has covered the city for the last week is the worst in 17 years.

The Indian government is seeking to drastically scale up the use of cleaner, renewable energy sources such as solar and wind power as a way to combat air pollution as well as global warming.

The Associated Press contributed reporting.