The air quality in Beijing was designated hazardous over the weekend, according to readings by Chinese and US government, with particulate matter in the air at up to 36 times what is considered healthy, Financial Times reported. Environmentalists have called the levels of pollution the worst on record, and people were advised not to venture out. The Chinese government recorded 900 micrograms per square meter of the most harmful matter particles, those smaller than 2.5 micrometers. Levels under 50 are considered healthy. Residents who went outside reported headaches, chest pain, sore throats and stinging eyes. The primary source of the pollution was the industrial and domestic burning of coal across northern China, as temperatures across the region plummeted, said Zhou Rong, climate spokeswoman for Greenpeace. State media said the pollution is likely to continue for a few days due to weather conditions.