Greek authorities on Monday offered free electricity to low-income families on certain days, after smog rose sharply as hard-hit residents increasingly shun fuel for cheaper firewood to heat their homes.

The health ministry said it had finalised a plan originally announced in November that offers free power to poorer households when smog exceeds safety levels.

The move came after another weekend of extensive wood-burning that choked skies in Athens and other main cities.

In the capital's northern suburbs, particulate matter was double the normal level for the area and approached what authorities have set as the "alarm" threshold of 150 milligrammes per cubic metre.

Authorities said a pollution warning issued when cold temperatures were forecast for this past weekend seemed to have helped control the pollution.

Greeks have turned to burning wood as energy prices have soared over the last two years thanks to tax hikes required by Greece's EU-IMF loan bailout.

A new property tax was added to electricity bills in 2011 and the price of heating fuel also increased in 2012.

Under the new plan, customers who have registered with the state power company's low-income rates are entitled to two days of free electricity for every day that smog hits the "alarm" level.

The smog contains sulphur dioxide, carbon monoxide, and other carcinogens, all judged dangerous for public health.

The economic crisis stalking Greece since 2010 initially caused pollution to dip because of a drop in automobile traffic.