Investigating the Health Risks of Household Air Pollution

How the HAPIN trial is tackling global indoor air pollution, one household at a time

By Shelby Crosier

Across the globe, about one third of all people rely on biomass cooking fuels like wood, charcoal, and dung that create harmful household air pollution. Although biomass is used for cooking around the world (due to cost and accessibility) the pollution it creates can lead to harmful health effects like respiratory disease, heart disease, and cancers. Women and young children are the most affected.

“Household air pollution is associated with a heavy disease burden globally, but it's focused mainly in the Global South and countries that rely heavily on biomass for cooking fuels,” says Thomas F. Clasen, PhD, professor of environmental health. Clasen is one of three principal investigators of the Household Air Pollution Intervention Network (HAPIN) Trial,  a study to investigate the health effects of using liquified petroleum gas (LPG) for cooking, instead of biomass that began in 2018.

The Gold Standard

In the world of research, there are two types of studies: observational and experimental. Although both are important to advancing science, experimental studies offer stronger potential to determine a cause, or the underlying reason why something is happening. The “gold standard” of experimental studies is the randomized control trial, a type of study where researchers compare one group that receives an intervention (like a drug or medical device) and one that does not to see if there is a difference in their health outcomes.

“In a well-designed and well-executed intervention trial, if you see a difference between the two groups, you can reasonably infer that it is causally linked to the presence of that intervention,” says Clasen. “While many environmental interventions cannot be randomly assigned, when we have the opportunity to test an intervention in a randomized control trial, that's our strong preference.”

The HAPIN trial is a randomized control trial. The research team enrolled 3200 pregnant women into the study, then randomly assigned equal numbers either to a treatment group that received an LPG cookstove and fuel, or to a control group that continued to use biomass. For 18 months, the team followed the women and their families to measure their exposure to indoor air pollution and assess their health.

Clasen notes that the benefit of the HAPIN trial’s study design goes beyond just establishing causality. “The randomized control trial gives us an opportunity to test the delivery, adoption, and effects of an intervention on adverse exposures, and to assess its cost and cost-effectiveness. These are all important data for policymakers, donors, and implementers when they determine whether to support scaling up the intervention.”

Reducing Exposure Around the World

Because use of biomass fuel is common around the world, it was important to the HAPIN team to run the trial in multiple countries. They recruited women in Peru, Guatemala, India, and Rwanda—four countries on different continents with very different cultures, resources, and traditions.

“HAPIN’s study design allowed us to not only compare what was going on in different countries, but also step back and say, this is probably what you would expect if you implemented it in other settings,” says Clasen. “The multicenter design increases generalizability of the results, meaning we can be fairly confident that we would see similar results in other settings.”

One thing that the team found across all four settings was that the families in the group who received LPG stoves used them almost exclusively and continued to use them throughout the 18 months of the trial. “Study participants had concerns about safety of LPG and about how it might make cooked food taste different, but our research teams addressed those up front,” notes Clasen. “Ultimately, they embraced the intervention because it was a more convenient and less time-consuming way to cook.”

High usage of the LPG stoves resulted in one very important outcome: less exposure to household air pollution.

“We saw these big reductions in exposure, with most households reaching levels that were below World Health Organization recommended interim levels,” says Clasen. “And we said, all right, we showed that an LPG fuel intervention is effective in reducing household air pollution exposure.  Now we are in a good position to answer the main research question, which ultimately was, does this reduction in exposure improve health?”

Surprising Results

Even though the LPG stoves reduced exposure to household air pollution, results so far show that this did not produce the expected health benefits. In a series of papers in the New England Journal of Medicine, researchers reported that the intervention had no effect on birthweight, stunting, or severe pneumonia in children under 12 months. These results were surprising given prior research linking air pollution with adverse health outcomes. 

“It could be that you need to intervene at an earlier point in gestation in order to achieve the effects that you are looking at,” says Clasen. “In a lot of people's judgment, that's one reason why we might not have seen an effect. One thing you see in these data is that the kids whose moms received the intervention earlier in their pregnancy had stronger effects than the kids who got it later.”

Potential for Long-Term Benefits

It is also possible that some health benefits won’t appear until the children are older. For that reason, the HAPIN team are continuing to follow the participants until their children are five years old.

“We call it the five-year effects of a 500-day intervention, because we still think that there could be effects that don't show up until later,” says Clasen. “That original randomized control trial design allowed us to continue to measure longer-term health outcomes.”

This follow-up will also allow the study team to see if use of LPG stoves is feasible and sustainable for families in the long term, since they were only provided LPG for free for the first 18 months.

“While the intervention period ended, we couldn't tell what would happen with the population,” says Clasen. “Would the intervention arm go back to using biomass or would they somehow find a way to access and continue to afford LPG? That was one thing that we were interested in. If you give people gas for this key period, would they be sufficiently motivated to keep using it?”

The team will find out the answers to these questions after data collection ends in the summer of 2025.