(Source: CGTN)

After a brief break in the haze and a few days of blue skies in Beijing, the city's infamous smog is back.

The air quality index in the capital rose above 150 on Saturday morning, a level bad enough to affect people's health.

A report from Peiking University shows there has been an improvement in the city's air quality over the past four years. 

Thanks to environmental measures imposed by the government, the density of some major pollutants has declined. These include PM-10 particles and sulphur dioxide, among others.

However, there's been no noticeable decline in the density of PM 2-point-5 particles in 2016 over 2015.

The PKU study also shows an increase in some pollutants in the atmosphere. The report says cutting the city's smog will be a challenge. A study in the US journal Science Advances says climate change may also be contributing to the pollution.