There appears to be no end in sight for China‘s air pollution problems as Beijing issued its first pollution warning of the new year. The capital, along with Tianjin and a number of cities in Hebei province are again enveloped in a shroud of hazardous smog.

Some of Beijing’s most well-known buildings were barely visible on Thursday as the municipal government issued a yellow smog alert.

Pollution levels reached more than 20 times the levels considered unhealthy by the World Health Organisation and the the city’s residents, especially children and the elderly were advised not to go outdoors. Early on Thursday morning levels of fine particulate pollution known as PM2.5 reached around 500 micrograms per cubic metre, according to the US Embassy which measures air pollution and publishes the levels on its Twitter feed. It is thought to be the highest reading since January 2013.

Visibility in some parts of the city dropped to less than 500 metres and four highways out of the city were closed temporarily. The current high pollution levels began on Wednesday afternoon and it was reported by Chinese state media that it is expected to last until Friday when winds will increase.

High levels of air pollution have become commonplace in Beijing and other Chinese cities, especially during winter months when temperatures drop and more coal is burned when heating systems are switched on. This combined with stagnant air conditions leave many cities covered in a dense layer of smog for days at a time.

There has been much debate in China about what the main cause of the air pollution is. Cars, coal-fired power stations, factories and even barbeques have been blamed.

On Thursday, Beijing’s mayor Wang Anshun announced an “all-out effort” to tackle air pollution. He said that coal use would be cut by 2.6 million tonnes and that measures will be taken to stop coal burning in the city and the surrounding areas.

China’s central government is also taking the problem seriously and in September the State Council, China’s cabinet, announced a plan to bring improvements in air quality by 2017. Under the plan many polluting factories would be closed and China’s massive reliance on fossil fuels would be reduced.