The environment ministry said that air pollution in Athens has surged because of people choosing wood over more expensive fuels to heat their homes in the grips of a continuing economic crisis.
Particulate matter has been measured at 150 milligrammes per cubic metre, or three times the danger level, especially in the northern and western suburbs of the Greek capital, the ministry said.
The visible smog is reminiscent of that which cloaked the mountain-ringed city before it modernised its cars and buses.
"The smog, which appears especially in the evening, is made up of polluting and dangerous particles that can cause respiratory problems," said Evangelos Gerassopoulos, director of the Environment Institute of the Athens Observatory.
"This polluting cloud is the result of the combination of a lack of wind and the burning of wood, which is more prevalent than usual because of the high price of domestic heating fuel," Gerassopoulos told AFP.
The smog contains sulphur dioxide, carbon monoxide, and other carcinogens, he said.
The economic crisis stalking Greece since 2010 initially caused pollution to dip because of a drop in automobile traffic.