The effect of ambient carbon monoxide on low birth weight among children born in southern California between 1989 and 1993

We evaluated the effect of carbon monoxide (CO) exposures during the last t rimester of pregnancy on the frequency of low birth weight among neonates b orn 1989-1993 to women living in the Los Angeles, California, area. Using b irth certificate data for that period, we assembled a retrospective cohort of infants whose mothers resided within 2 miles of 1 of 18 CO monitoring st ations. Based on the gestational age and birth date of each child, we estim ated last-trimester exposure by averaging the corresponding 3 months of dai ly CO concentrations registered at the monitoring station closest to the mo ther's residence (determined from the birth certificate). Where data were a vailable (at 6 stations), we also averaged measurements taken daily for nit rogen dioxide and ozone and those taken at 6-day intervals for particulate matter less than or equal to 10 mu m (PM10) to approximate last-trimester e xposures to other pollutants. Overall, the study cohort consisted of 125,57 3 singleton children, excluding infants born before 37 or after 44 weeks of gestation, those weighing below 1,000 or above 5,500 g at birth, those for whom fewer than 10 days of CO measurements were available during the last trimester, and those whose mothers suffered from hypertension, diabetes, or uterine bleeding during pregnancy. Within the cohort, 2,813 (2.2%) were lo w in birth weight (between 1,000 and 2,499 g). Exposure to higher levels of ambient CO (>5.5 ppm 3-month average) during the last trimester was associ ated with a significantly increased risk for low birth weight [odds ratio ( OR) = 1.22; 95% confidence interval (CI), 1.03-1.44] after adjustment for p otential confounders, including commuting habits in the monitoring area, se x of the child, level of prenatal care, and age, ethnicity, and education o f the mother.