BEIJING (AP) — One of Beijing's worst rounds of air pollution kept schoolchildren indoors and sent coughing residents to hospitals, but this time something was different about the murky haze: the government's transparency in talking about it.

While welcomed by residents and environmentalists, Beijing's new openness about smog also put more pressure on the government to address underlying causes, including a lag in efforts to expand Western-style emissions limits to all of the vehicles in Beijing's notoriously thick traffic.

"Really awful. Extremely awful," Beijing office worker Cindy Lu said of Monday's haze as she walked along a downtown sidewalk. But she added: "Now that we have better information, we know how bad things really are and can protect ourselves and decide whether we want to go out."

"Before, you just saw the air was bad but didn't know how bad it really was," she said.

Even state-run media gave the smog remarkably critical and prominent play. "More suffocating than the haze is the weakness in response," read the headline of a front-page commentary by the Communist Party-run China Youth Daily.

Government officials — who have played down past periods of heavy smog — held news conferences and posted messages on microblogs discussing the pollution.

The wave of pollution peaked Saturday with off-the-charts levels that shrouded Beijing's skyscrapers in thick gray haze. Expected to last through Tuesday, it was the severest smog since the government began releasing figures on PM2.5 particles — among the worst pollutants — early last year in response to a public outcry.

A growing Chinese middle class has become increasingly vocal about the quality of the environment, and the public demands for more air quality information were prompted in part by a Twitter feed from the U.S. Embassy that gave hourly PM2.5 readings from the building's roof.

The Chinese government now issues hourly air quality updates online for more than 70 cities.

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"I think there's been a very big change," prominent Beijing environmental campaigner Ma Jun said, adding that the government knows it no longer has a monopoly on information about the environment. "Given the public's ability to spread this information, especially on social media, the government itself has to make adjustments."

Air pollution is a major problem in China due to the country's rapid pace of industrialization, reliance on coal power, explosive growth in vehicle ownership and disregard for environmental laws, with development often taking priority over health. The pollution typically gets worse in the winter because of an increase in coal burning.

"The pollution has affected large areas, lasted for a long time and is of great density. This is rare for Beijing in recent years," Zhang Dawei, director of Beijing's environment monitoring center, told a news conference Monday.

According to the government monitoring, levels of PM2.5 particles were above 700 micrograms per cubic meter on Saturday, and declined by Monday to levels around 350 micrograms — but still way above the World Health Organization's safety levels of 25.

In separate monitoring by the U.S. Embassy, levels peaked Saturday at 886 micrograms — and the air quality was labeled as "beyond index."

City authorities ordered many factories to scale back emissions and were spraying water at building sites to try to tamp down dust and dirt that worsen the noxious haze.

Schools in several districts were ordered to cancel outdoor flag-raisings and sports classes, and in an unusual public announcement, Beijing authorities advised all residents to "take measures to protect their health."

The Beijing Shijitan Hospital received 20 percent more patients than usual at its respiratory health department, most of them coughing and seeking treatment for bronchitis, asthma and other respiratory ailments, Dr. Huang Aiben said.

PM2.5 are tiny particulate matter less than 2.5 micrometers in size, or about 1/30th the average width of a human hair. They can penetrate deep into the lungs, and measuring them is considered a more accurate reflection of air quality than other methods.

"Because these dust particles are relatively fine, they can be directly absorbed by the lung's tiny air sacs," Huang said. "The airway's ability to block the fine dust is relatively weak, and so bacteria and viruses carried by the dust can directly enter the airway."

Prolonged exposure could result in tumors, he added.

Demand spiked for face masks, with a half dozen drugstores in Beijing reached by phone reporting they had sold out. A woman surnamed Pang working at a Golden Elephant pharmacy said buyers were mainly the elderly and students, and that the store had sold 60 masks daily over the past few days.

The bulk of the smog choking Chinese cities is belched out by commercial trucks, but authorities have put off enforcing tougher emissions standards to spare small businesses the burden of paying for cleaner engines.

"It is not a problem of technology. It's more about consumer affordability. Increasing the emissions standard greatly increases the cost," said John Zeng, Asia-Pacific director for LMC Automotive Ltd., a research firm. "Most buyers are small business owners, and they are very price-sensitive."

Upgrading to cleaner engines would cost about 20,000 yuan ($3,200), adding about 8 percent to a typical sticker price of a vehicle, according to Zeng.

The haze even inspired a song parody, widely circulated online. "Thick haze permeates every street in Beijing, the pollutant index is worse than the charts can read. I'm surrounded by buildings in a fairyland and I see people wearing masks all over the city," go the lyrics. "Who is traveling in fog and who is crying in fog? Who is struggling in fog and who is suffocating in fog?"

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Associated Press writers Louise Watt and Joe McDonald and researchers Flora Ji and Yu Bing in Beijing contributed to this report.