Media playback is unsupported on your device Media caption Dr Kevin Mortimer explains his hopes for the Cooking and Pneumonia Study

An estimated three billion people breathe in poisonous smoke from open-fire cooking, killing more than four million every year. In Malawi, a research team is conducting the largest-ever study on children into the effects of indoor smoke and hopes its results will save millions of young lives, as our science reporter, Victoria Gill, discovered.

Ennere's eyes are watering. Smoke from the fire she is cooking on is filling her tiny, one-room house in a small village in the district of Chikwawa in the south of Malawi.

It is a traditional open fire on the floor of her hut, surrounded by three bricks to keep in the hot embers and on which to rest her cooking pot.

She and her two-year old daughter, who is gripping Ennere's shoulder as she crouches over a warming pan of water, are breathing in the smoke.

Most of the small houses here have no chimney. Some of the smoke escapes through the doorway, but there is no other ventilation. Smoke is filling the little girl's lungs.

We're interested in counting cases of pneumonia. We don't want to count dead children Dr Kevin Mortimer, Liverpool School of Tropical Medicine

This happens every time Ennere cooks.

Like 95% of people in Malawi, her family belong to what the World Health Organization calls the "forgotten three billion", people who have to breathe polluted air in their homes and are being poisoned by their own cooking fires.

"Smoke basically clogs up children's lungs making them more susceptible to infections like pneumonia, explains Dr Kevin Mortimer, one of the lead researchers from the Liverpool School of Tropical Medicine.

In Malawi, pneumonia is the number one killer of children under five.

According to the WHO, throughout sub-Saharan Africa, almost one million children every year die from the disease. Worldwide that number could be more than four million.

Image copyright Victoria Gill Image caption The stove being tested in this cooking-and-pneumonia trial has a battery-operated fan beneath the furnace

Redesigning the stove on which millions cook, the researchers believe, could be the solution to cutting this death toll.

There are several different types of clean cook stove. The ones the team are testing use the same fuel as open fires, wood (or biomass) but they incorporate a battery-powered fan that blows air through a contained furnace.

This produces a more efficient, less smoky fire.

The Liverpool School of Tropical Medicine team intends to distribute 16,000 stoves in the north of Malawi, and here in Chikwawa in the south. It will be the largest trial of its type ever undertaken anywhere in the world.

Ennere and her daughter have been recruited to the control group, meaning they will continue to use conventional cooking methods over a traditional open fire.

That might seem unfair, but it will allow researchers to compare their "stove intervention" with routine, smoky cooking practices.

At the end of the two-year study, during which the researchers will monitor the health of each child, every family involved will receive two of the cleaner, "advanced-burning" stoves.

Image copyright Victoria Gill Image caption One in every hundred children in Malawi dies from pneumonia before the age of five

If this team finds that the stoves they are testing prevent pneumonia, Dr Mortimer says this will be a definitive message for Malawi's government, and policymakers around the world, that this is a priority for their investment.

The 9,000th child

At the nearby Chikwawa District Hospital, the toll that lung disease takes here becomes clear.

Chifundo Ndamala is one of the clinical staff working for the research project, providing care for children involved in the study, as well as recording and tracking every case of pneumonia.

As Dr Mortimer puts it: "We're interested in counting cases of pneumonia, not dead children."

For that reason, the team has employed health workers and invested in diagnostic tools and antibiotics, so every child in the study has access to the treatment they need if they do become ill.

Chifundo is attending to one of the children from the control group - a young boy of about two.

He smiles warmly at the little boy as he examines him, but the toddler is wide-eyed and clings to his mother as they sit together on the bare mattress of the hospital bed.

"He was diagnosed with severe pneumonia three months ago," Chifundo tells me. "But he's doing well, he's responding to treatment now."

Image copyright Victoria Gill Image caption Mothers stay to care for their children at Chikwawa's district hospital

This is a country where healthcare is basic - there are approximately 50,000 people for every doctor in Malawi. And staff here in the children's ward have to work with the few resources they have to care for their young patients, some of whom are critically ill.

But this project does mean that Chifundo is on hand to help.

"This is not a place of comfort," says Dr Mortimer, "but it is a place where people are doing their utmost to provide the best possible care with the resources they have."

Chifundo is even more optimistic.

"We're able to treat children who will be the leaders of this nation one day," he says.

"It's rewarding job; just seeing a child recover and go home, it's good for the soul."

Dr Mortimer also hopes the study and the eventual publication of its findings will "raise the profile" of indoor smoke exposure.

"People should know," he says, "that this is a condition that causes millions of deaths every year".