Michelle Bell and colleagues have found that the mother's age – whether she is younger than 24 or older than 40 – or the sex of the baby do not make a difference to the relationship between air pollution and low birth weight. The original results also remain the same if the pre-term infants (those born at less than 32 weeks) are excluded from the study. The researchers estimate lower birth weight with higher exposure to pollutants, even those that meet federal limits.

The pollutants considered were nitrogen dioxide (NO2), carbon monoxide (CO), and particulate matter with aerodynamic diameters of less than 10 μm and 2.5 μm (PM10 and PM2.5) during pregnancy. The analysis was based on 358,504 births in the US states of Massachusetts and Connecticut from 1999 to 2002. Birth data was obtained from birth certificate registries from the Division of Vital Statistics, Reproductive Statistics Branch of the National Center for Health Statistics.

For each baby, Bell and colleagues took the following factors into account: place of birth and residence; the mother’s age and racial origin; mother's marital status; parental education level; whether the mother smoked or drank alcohol during her pregnancy; gestational age in weeks; child's sex; birth weight; type of birth (caesarean or natural); quality of prenatal care; and birth order. The dataset was limited to single births with a gestational length of 32 to 44 weeks and weights of 1 kg to 5.5 kg.

The team estimated air pollution in a given area using data from air monitors and weather reports. Air pollution data was obtained from the US Environmental Protection Agency, and weather data from the National Climatic Data Center.

"Findings indicate that while some populations are at higher risk of low birth weight (for example, infants of younger or older mothers, female infants, and infants born in more polluted regions), the relationship between air pollution and low birth weight did not differ by mother's age or infant's sex," say Bell and co-workers. "Results were further robust to exclusion of pre-term births."

The study also showed that lowered birth weight was linked to exposure to particulate pollution in the second and third trimesters of pregnancy, and exposure to carbon monoxide in the first and third trimesters.

The team does stress however, that many questions remain unanswered, such as why exactly air pollution should affect the birth weight of infants. The effect of individual pollutants also needs to be better analysed.

The researchers published their work in Environmental Research Letters.