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BEIJING – People refused to venture outdoors, and buildings disappeared into Beijing’s murky skyline Sunday as the air quality in China’s notoriously polluted capital went off the index.

The Beijing Municipal Environmental Monitoring Center said on its website that the density of PM2.5 particulates had surpassed 700 micrograms per cubic meter in many parts of the city. The World Health Organization considers a safe daily level to be 25 micrograms per cubic meter.

PM2.5 is tiny particulate matter less than 2.5 micrometers in size, or about one-thirtieth the average width of a human hair. The particles can penetrate deep into the lungs, so measuring them is considered a more accurate reflection of air quality than other methods.

The Beijing center recommended that children and the elderly stay indoors and that others avoid outdoor activities.

The U.S. Embassy also publishes data for PM2.5 on Twitter and interprets the data according to more stringent standards.

In the 24-hour period up to 10 a.m. Sunday, the embassy said 18 of the hourly readings were “beyond index.” The highest number was 755, which corresponded to a PM2.5 density of 886 micrograms per cubic meter.

The U.S. Environmental Protection Agency’s air-quality index goes up to only 500, and the agency advises that anything greater than 300 would trigger a health warning of “emergency conditions,” with the entire population likely to be affected.

While some people vowed to stay indoors with air purifiers turned on, Beijing’s streets were still fairly busy Sunday, and there was the familiar sight of heavy traffic on main thoroughfares.

A young couple strolled along hand in hand in the central business district, with surgical-style masks on their faces.

Two Taiwanese tourists wore masks they said they had brought with them because they heard that Beijing’s pollution was so bad.

“I don’t know why there is such heavy haze these past days. It’s really quite serious, compared with the air quality three days ago,” said a 33-year-old lawyer, who would give only his surname, Liu, as he adjusted his own mask. He said he had ventured out only because he needed to go shopping.

Beijing’s air started to worsen Thursday. The Beijing monitoring center has said the pollution is expected to linger until Tuesday.

Air pollution is a major problem in China due to the country’s rapid pace of industrialization, reliance on coal power, explosive growth in car ownership and disregard to environmental laws. It typically gets worse in the winter.