Beijing (CNN) -- An acrid blanket of gray shrouded the Chinese capital for a third day Sunday, leading to a rare smog warning as the index hit record levels.
Data show air quality index soared past the highest level of 500. Anything past that is regarded as "beyond index."
As the number soared, the streets got emptier. The sun, peeking shyly behind the hazy blanket, was barely visible.
Authorities warned residents to cut down on outdoor activities and take shelter indoors.
Reports of respiratory problems -- and sale of masks -- skyrocketed, according to state media.
The U.S. Embassy in Beijing recorded an index of over 700 on Saturday, but the number had lowered by Sunday.
Beijing 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.
CNN's Steven Jiang reported from Beijing, Elizabeth Yuan wrote from Hong Kong and Faith Karimi from Atlanta