Beijing’s skyline vanished as a thick, foul-smelling layer of smog fell over the city as air quality in the Chinese capital worsened to hazardous levels on Saturday night.

The density of PM 2.5 particulates had surpassed 700 micrograms per cubic meter in many parts of the city, or 36 times World Health Organization’s recommended safe daily level of 25 micrograms per cubic meter, the Beijing Municipal Environment Monitoring Center said.

Beijing’s air started to worsen on Thursday and the toxic mixture of fog, coal fumes, car exhaust, construction dust and industrial emissions could linger until Tuesday because of the lack of winds, according to the center.

Children and the elderly were recommended to stay indoors while most people refused to venture outside and turned on air purifiers in their homes in the capital – home to about 20 million people.

Residents who braved the hazy surroundings complained of sore throats, stinging eyes, chest pains and headaches, reports the Financial Times.

China has the worst air pollution due to its rapid growth, driven by industrialization, reliance on coal power particularly during cold weather, and ballooning car ownership, which triggers the release of harmful particulates in the air.

PM2.5 particulates are considered the most hazardous because they are small enough – not more than 2.5 micrometers in size – to enter the bloodstream and damage lung tissue.