The one big difference between regular and radiation fog is the way they form, said Sun, but there is no difference in the severity in terms of potential pollution.

"Radiation fog generally occurs in a focused area, while regular fog could be caused by the collision of different air masses," he said.

"Since the air is from different regions, regular fog could even be more polluted and harmful than radiation fog," he noted.

Radiation fog is just the professional name for experts to differentiate it from other types, said Sun.

"But ordinary people don't need to know this," he said.

Zhang Yuanxun, a professor of resources and environment at the Chinese Academy of Sciences, said that fog, particularly in a large urban area like Beijing, can contain pollution.

"It depends on its contents. Fog can form when there is too much pollution or where there is moisture in the atmosphere," said Zhang.

Yet he noted that since Beijing is a polluted city, all kinds of fog are likely to be harmful.

"Fog doesn't allow the diffusion of contaminants and can cause disease," he said.