By James Reynolds
BBC News, Shanxi
The village of Gezhuotou is in the heart of China's coal belt
At a temperature of -10C (14F), in the grey-blue dawn, two schoolchildren have a thankless job to complete.
They are meant to sweep away the soot, dirt and grime from the school gate.
But this village is surrounded by coal mines and power stations, so it is impossible to get anything clean.
Inside, a class of 10-year-olds works its way through its early-morning reading lesson.
The children all have dirty hands and faces. In this village, once you get grubby, you stay grubby. Winter makes things worse.
"When it comes to this time of year, one quarter of students get respiratory diseases," says the head teacher, Zhao Xiangjing.
"We sometimes give them shots to try to prevent them all getting ill. But we always have someone coughing."
This small village, Gezhuotou, is in the middle of China's central Shanxi province. It is the heart of the country's coal belt.
All around, coal-fired power stations provide energy for the much of the rest of the country.
But it comes at a price. China suffers from some of the worst pollution in the world.
Every year, it is estimated that around 400,000 people in China die prematurely from pollution-related illnesses.
On top of a hill, the windows of Wang Demeng's family home rattle whenever a coal train goes by.
From the front door, patrolled by guard dogs, you can make out the nearby mines and factories dumping smoke into the air.
At dinner time, they try to wash away the taste of pollution with some weak soup.
They say they feel abandoned by their leaders.
"The village leaders don't live here," says Zhang Xianjiang, pointing his finger. "They live in the city where it's cleaner. But we don't have any money, so we have to stay where we are."
An oxygen cylinder stands in the corner of the one-room house belonging to 73-year-old Zhang Mingzhi. He suffers from lung disease.
He lies in bed, his face swollen, barely able to move. His wife, Feng Lingmei, has to spoon-feed him. Her eyes are red.
"The air is so bad," she says. "On winter days like this, he can't go out, he gets worse, he just can't breathe."
The village clinic is just down the road. Wang Derong sits in his office, smoking a cigarette.
The pollution gets worse in winter
"I've been working here for 20 years," he says, "With more and more mines, the pollution has got worse. More and more people get respiratory diseases. Some people just can't pay for their medicine - so we let them write IOUs. I haven't told the village leaders about this."
In mid-afternoon, the haze is so bad that cars almost need headlights to see where they are going.
There is no wind, so the pollution from nearby mines and factories just sits in the air. Dozens of coal trucks head along the main road.
China may be trying to develop alternative energy sources. But right now in winter, more than a billion people need to keep warm, and carry on working.
The coal from this province does the job. The residents of Gezhuotou can feel it for themselves with every breath.