Cooking up problems for babies: wood smoke and low birth weight. (Science Selections).

Low birth weight, defined as less than 2,500 grams (5.5 pounds), places newborns at increased risk of illness and death during infancy. Low birth weight is particularly critical in the developing world, where approximately 18% of newborns are affected. This month, Erick Boy, a senior program specialist at the Micronutrient Initiative in Ottawa, Canada, and colleagues investigate whether maternal exposure to smoke from cook fires, a common indoor environmental pollutant in the developing world, might reduce infants' birth weight [EHP 110:109-114]. Of environmental factors already associated with low birth weight, environmental tobacco smoke (ETS) has been well scrutinized, and the researchers suggest that this model could explain some of the birth weight reduction observed in their study.

The study was conducted in Quetzaltenango, a western Guatemalan province, and included 1,717 women and their infants. Infants were weighed within 72 hours of birth, and the mothers were surveyed on physical, demographic, and socioeconomic characteristics. A key interview question focused on the fuel used for cooking--wood, coal, electricity, or gas--and, if wood or coal, whether it was burned in an open fire or a stove with a chimney. There were so few coal users that they were grouped in with wood users in the paper's subsequent analyses.

Among the 1,717 mothers, 871 cooked with wood over an open fire during pregnancy, 489 used a wood stove with a chimney, and 357 employed electricity or gas. The mothers using an open fire had a slightly higher percentage of low birth weight infants, 19.9% versus 16.8% (stove with a chimney) and 16.0% (electricity or gas). Although small, the relationship between maternal use of open cook fires during pregnancy and birth weight reduction was determined to be statistically significant.

The researchers suggest that the well-studied link between ETS and low birth weight may provide a plausible model to explain this result. As with tobacco, combustion of biofuels such as wood produces carbon monoxide. When inhaled, it binds to hemoglobin and forms carboxy-hemoglobin, which cannot transport needed oxygen to the body's tissues. A developing fetus, deprived of adequate oxygen, suffers intrauterine growth retardation and subsequent low birth weight.

Although the current study does not directly measure carbon monoxide exposure, other studies in the same geographic area indicate that ETS and cook fire smoke are associated with comparable ambient concentrations of carbon monoxide. Also, persons exposed to either may have similar concentrations of carboxyhemoglobin. In ETS studies, these concentrations have been associated with intrauterine growth retardation and low birth weight. The current study is the first to examine whether exposure to smoke from indoor cook fires might have the same effect.

In interpreting the study results, Boy and his colleagues controlled for numerous confounding factors. For one, fuel type is strongly linked to socioeconomic status--rural families use wood more frequently and also tend to be poorer. Further, there are numerous influences on birth weight, including premature birth and maternal nutrition, health, and socioeconomic status. As in other studies, these factors were significant in the Quetzaltenango study.

The researchers note that, although wood smoke may not carry as great a weight as other environmental pollutants such as ETS, the number of people affected by low birth weight around the world gives any risk factor a measure of importance. The researchers acknowledge the difficulties of determining the weight of each factor and suggest that future research might build on their results by quantifying carbon monoxide exposures and detailing them more exactly.