A thick haze of smog has descended on parts of northern China, pushing pollution to more than 20 times what the World Health Organization considers a safe limit.

The heavy smog reduced visibility in some areas to such an extent Friday that highways were closed and residents were warned to stay indoors. Beijing authorities raised the city’s air pollution alert to orange, its second highest level.

In Hebei, a province bordering Beijing to the north, concentrations of pollutant particles called PM2.5 reached upwards of 500 micrograms per cubic meter, according to Beijing’s air quality index. The WHO’s recommendation for maximum healthy exposure is 25 micrograms.

These particles are considered the most dangerous type of air pollution because they lodge deep inside the lungs. In Beijing, where PM2.5 concentration has remained above 300 since Wednesday, the city’s Meteorological Bureau is advising citizens, especially those with heart or lung conditions, to limit outdoor activities.

According to the U.S. State Department’s air quality guide, levels of PM2.5 between 301 and 500 micrograms are considered hazardous to health and constitute a “serious risk of respiratory effects in the general population,” while anything above 500 is literally off the charts.

Most U.S. cities have a PM2.5 concentration of 20 or below.

Many Beijing residents are wearing gas masks or covering their faces with cloth when outdoors to avoid the toxic gas.

“It’s the smog season of the year again. Now I wonder if I can live long enough to see it solved,” Beijing resident Su Zhiyu told the South China Morning Post.

“If the government can’t solve the problem for the residents, [we] should at least have a day off on smoggy days,” Zhiyu said.

Beijing’s Environmental Bureau blamed the recent onset of smog on unusually calm weather conditions that prevent the pollution from dispersing, as well as on farmers burning the remains of crops in rural parts of Hebei, according to Bloomberg.

Environmental groups such as Greenpeace criticized Beijing officials for only raising the pollution alert to orange, rather than red, the most severe warning. A red alert, which has never been issued in the time since the system was inducted last year, would result in public school closures — and force half the city’s cars off the road.

Activists from Greenpeace East Asia projected the message “Blue sky now!” on Beijing’s historic Drum Tower.

Just last month, the Chinese government pledged to cut down its carbon dioxide emissions and “reach a peak as soon as possible” at the U.N. Climate Summit — a lofty goal, considering the country has been increasing emissions by an average of 10% for most of the last decade.

Rampant economic growth, built primarily on cheap dirty fossil fuels, has made China the largest emitter of greenhouse gases in the world, according to a 2013 European Commission report.

But public pressure has forced the government to commit to more vigilantly regulating heavy industries — particularly coal power plants, which supply 70% of the country’s energy intake. Chinese Vice Premier Zhang Gaoli reported at the conference that the nation’s carbon intensity, measured in carbon emissions per unit of gross domestic product, has dropped by nearly 29% since 2005.

Zhang also said the country has fined companies more than $2 million in the past six months, as part of air quality initiatives.

Beijing was engulfed in similarly dangerous levels of smog last winter until the weather caused the conditions to subside. At the time, public contention with the government’s inability to deal with the problem prompted one citizen to file the country’s first-ever lawsuit against a municipality over toxic smog exposure.