Story highlights Beijing recording "hazardous" levels of air pollution Monday

Children in worst-hit areas ordered to stop outdoor sports activities

Reports of respiratory problems -- and sale of masks -- skyrocket

U.S. Embassy in Beijing records an index of more than 700 micrograms Sunday

Schoolchildren were ordered to halt outdoor sports activities until Tuesday this week, as a dirty cloud of smog continued to shroud China's capital.

This was among a series of emergency response measures adopted in Beijing Sunday when the city's Air Quality Index exceeded 500 micrograms, the highest level. Anything above this is regarded as "beyond index."

Reports of respiratory problems -- as well as the sale of masks -- have skyrocketed, according to state media, and over the weekend, streets appeared emptier, as a sun was barely visible amid a hazy blanket.

On Monday morning the U.S. Embassy in Beijing recorded "hazardous" levels of pollution, with a "Beyond Index" reading of 515 at 3:00 a.m. (2 p.m. Sunday) local time, last seen on Saturday when the air quality index, based on U.S. national air quality standards, hit as high as 755.

Beijing Hyundai Motor Company suspended production on Sunday, and nearly 30 construction sites halted construction, the China Daily cited the city's environmental protection bureau as saying.

JUST WATCHED Beijing's bad air quality Replay More Videos ... MUST WATCH Beijing's bad air quality 05:06

JUST WATCHED 'I haven't seen the sun' in four days Replay More Videos ... MUST WATCH 'I haven't seen the sun' in four days 03:00

JUST WATCHED Greenpeace on Beijing's hazardous smog Replay More Videos ... MUST WATCH Greenpeace on Beijing's hazardous smog 03:25

The city's residents say pollution is worsening despite authorities' assertions that air quality has improved since the city hosted the 2008 Olympic Games.

Last year, heavy haze and smog forced the cancellation of almost 700 flights at Beijing airports.

The city is comparable to Los Angeles, another gray city, according to experts.

"With their difficult meteorological conditions and a large number of pollution sources, addressing pollution is a long-term and difficult task," Deborah Seligsohn, adviser of the World Resources Institute, said last year. The agency runs a climate energy and pollution program in China.

Los Angeles has battled air pollution since the 1950s, well before U.S. national regulation, according to Seligsohn.

"Stilll in the 1970s, 20 years later, it was famous for its smog," she said. "In the 40-plus years since the Clean Air Act was passed, L.A. has never been fully in compliance with EPA standards, even though it has continued to improve."

European Commission figures show that China produced 9.7 million kilotons of carbon dioxide while the United States had 5.42 million kilotons in 2011, the latest available numbers.

The weekend's pollution levels have prompted an "orange fog" warning in Beijing because of diminished visibility, according to state-run Xinhua news agency.

"Pollutants have gradually accumulated over the course of recent windless days, making the air quality even worse," said Zhu Tong, an environmental sciences professor at Peking University told news agency.

Beijing will remain covered in gray until Wednesday, when the wind will sweep in to the rescue and blow the smog away, according to state media.