The capitals of the world's two most populous nations, China and India, were blanketed in hazardous, choking smog on Monday as climate change talks began in Paris, where leaders of both countries are among the participants.

China's capital Beijing maintained an “orange” pollution alert, the second-highest level, on Monday, closing highways, halting or suspending construction and prompting a warning to residents to stay indoors. The Chinese smog was 25 times higher than World Health Organization maximums for particulate levels of major air pollutants including ozone and carbon monoxide; last month it reached levels 50 times over those limits.

Emissions in northern China soar over winter as urban heating systems are switched on and low wind speeds mean that polluted air has not been dispersed. The choking pollution was caused by the “unfavourable” weather, the Ministry of Environmental Protection said Sunday.

Meanwhile in New Delhi, a thick smog blanketed the city and visibility was down to about 200 yards. Air pollution levels reached well into “hazardous” territory according to the U.S. embassy's monitoring station — which recorded an air quality index (AQI) of 372. The AQI is calculated for major pollutants including ozone, particle pollution, carbon monoxide and sulfur dioxide. The AQI can be thought of as a yardstick that runs from 0 to 500, with greater levels being of greater health concern; an AQI of over 300 represents air quality so hazardous that everyone — not just those with respiratory or other health issues — may experience serious effects.

Like China, air quality in the city of 16 million is usually bad in winter — when coal fires are lit by the poor to ward off the cold. Traffic fumes, too, are trapped over the city by a temperature inversion and the lack of wind.

However, the government has not raised any alarm over the current air quality and no advisories have been issued to the public. Thirty thousand runners took part in a half marathon over the weekend, when pollution levels were just as high.

In Beijing, a city of 22.5 million, the air quality index in some parts of the city soared to 500, its highest possible level. At levels higher than 300, residents are encouraged to remain indoors, according to government guidelines.

As the smog creates hazardous conditions for residents in India and China, it may also contribute to extreme weather conditions in the United States and Canada, according to a study released last year.

Those toxins ride the Pacific storm track to the west coasts of Canada and the U.S., across North America, and could lead to increased frequency and intensity of storms or even severe droughts. The smog may also exacerbate climate change.

Soot particles in the form of black carbon can collect on ice packs in the poles — which attract more heat from the sun and accelerates ice-cap melting. Melting ice caps results in the release of methane — hastening climate change and accelerating sea level rise.

The hazardous air underscores the challenge facing the government as it battles pollution caused by the coal-burning power industry and will raise questions at the ongoing Paris talks about its ability to clean up its economy.

Chinese President Xi Jinping and Indian Prime Minister Narendra Modi are both in Paris and both were scheduled to meet U.S. President Barack Obama on Monday to give momentum to the two-week negotiations.

Modi sought to highlight India's green credentials in an article for the Financial Times on Monday, writing: “The instinct of our culture is to take a sustainable path to development. When a child is born, we plant a tree.”

But at Connaught Place, a city center landmark in New Delhi, people chided the government for failing to minimize the risks to their health from air pollution.

“The pollution level is so high it's just unbelievable,” said Aisha, a 19-year-old student.

For Beijing's residents, the poor air makes breathing hard.

“This sort of weather, you can see that all of Beijing has been completely enveloped in smog … and for every breath, getting up every morning, your throat will feel particularly uncomfortable,” said Zhang Heng, a 26-year-old architect.

The Beijing Environmental Protection Bureau said on Sunday that it had requested factories to limit or suspend output and had also stopped construction work throughout the city.

State-run Xinhua news agency said more than 200 expressway toll gates in east China's Shandong province were closed on Monday due to smog. The province issued a yellow alert.

China launched a “war on pollution” last year following a spate of smog outbreaks in Beijing and surrounding regions. China has vowed to slash coal consumption and close down polluting industrial capacity, but environmental officials admit that the country is unlikely to meet state air quality standards until at least 2030.

Reducing coal use and promoting cleaner forms of energy are set to play a crucial role in China's pledges to bring its climate warming greenhouse gas emissions to a peak by around 2030.

Al Jazeera and Reuters