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Reducing women's exposure to air pollution from cooking with wood or other biomass fuels didn't improve the birth weight of their babies, a randomized trial showed.

Daily personal exposure to fine particulate matter dropped more than 66% for pregnant women who switched to gas cookstoves compared with controls who continued using biomass fuel (23.9 vs 70.7 μg/m³), reported Thomas F. Clasen, JD, PhD, of Emory University in Atlanta, and colleagues.

However, mean birth weight averaged 2,921 g for those switched to liquefied petroleum gas (LPG) and 2,898 g for controls, a nonsignificant adjusted mean difference of 19.5 g (95% CI -10.1 to 49.2), they noted in the .

The lack of benefit from the clean-cooking intervention was unexpected given previous observational studies linking household during pregnancy to lower birth weight for the some 3 billion people who cook on open fires or traditional stoves using wood, dung, charcoal, or agricultural waste -- "a number that has not changed substantially in the past three decades," the researchers wrote.

A few prior failed trials of clean-cooking interventions had been attributed to poor performing stoves that made too little difference in household air pollution, partial or inconsistent use of the stove intervention, or other sources of indoor air pollution.

None of those were the case in this trial, which had near complete uptake of the free LPG stove and fuel provided.

The trial had three other primary endpoints, though, that have yet to read out: stunting in infants, severe pneumonia in infants, and high systolic blood pressure in women.

The Household Air Pollution Intervention Network (HAPIN) trial included 3,200 pregnant women ages 18 to 34 in Guatemala, India, Peru, and Rwanda who were at 9 to <20 weeks' gestation (confirmed on ultrasonography).

They were randomized to the intervention -- a free LPG cookstove with at least two burners and a continuous supply of free fuel delivered to the home during pregnancy and until the infant was 1 year of age, along with education to encourage its exclusive use -- or to continue use of their customary practices, with monetary compensation similar to the value received by the other group to minimize loss to follow-up.

While not all the infants born to the participating women had birth weight collected within 24 hours by a trained nurse or study staff directly (in part due to the pandemic), nearly all had valid birth weight measurements collected by some means, typically the healthcare facility where they were born.

Secondary outcomes showed no substantial impact of the intervention on gestational age at birth, preterm birth, early preterm birth, preterm delivery, or stillbirth. No serious adverse events occurred with the intervention.

One of the potential explanations for the neutral results, as suggested by the researchers, was that the reduction in air pollution exposure wasn't sufficient.

More than twice as many women dropped below the threshold for median 24-hour exposure to with the intervention versus controls (69% vs 23%), but "that standard is seven times as high as the recently revised annual WHO guideline value of 5 μg per cubic meter," Clasen's group pointed out.

Another possibility was that earlier intervention was needed than the trial's typically second trimester start.

There was a signal for greater impact on birth weight when the intervention was implemented at less than 18 weeks' gestation compared with later in gestation (between-group difference 33.8 vs 5.3 g). Also, infants born to women for whom the intervention started in the first trimester had a mean z score for birth weight that was estimated to be 0.15 standard deviations higher than those in the control group.

"Moreover, because we did not have information on the locations of the biomass cookstoves, it is possible that in some cases indoor LPG cookstoves replaced outdoor biomass cooking, thereby limiting the benefits of the intervention with respect to indoor air pollution," the researchers noted.

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