At the same time, the smoke from the burning of wood — and often just about anything else that will catch fire — has caused spikes in air pollution that worry health officials. On some nights, the smog is clearly visible above Thessaloniki, Greece’s second-largest city, and in Athens, where particulate matter has been measured at three times the normal levels.

“Places that in 2008 wouldn’t even think about using their fireplaces for heating, now they are obliged to do so,” said Stefanos Sabatakakis, a health supervisor with the Hellenic Center for Disease Control and Prevention. He said the rise in pollution could cause eye irritation and headaches in the short term and far more serious problems in the long term. The air is particularly bad for asthma sufferers.

The agency has asked that anyone who is lighting living-room fires just for the aesthetics give them up. It has also uploaded information on its Web site about what not to burn — anything that is painted or lacquered, for instance. But in these times, Mr. Sabatakakis acknowledged, people are not that picky.

Government officials say it is too early to judge the new tax. The winter is not yet over. It has not been particularly cold, they say, and many people may have stocked up on fuel oil last season. In the north of Greece, temperatures often dip to freezing at night, while in Athens they are more likely to stay in the low 40s.

“This is a very complex environment,” said Harry Theoharis, the secretary general of the Ministry of Finance, adding that many factors were affecting people’s behavior. “It is not easy to isolate and say: ‘O.K., this tax, this is the effect it had.’ ”

He said there were no clear indications yet that the tax had discouraged illicit sales of heating oil as diesel, though he had detected a slight change in buying patterns that might indicate some change.