A time-series analysis of air pollution and preterm birth in Pennsylvania, 1997-2001

Abstract

Preterm delivery can lead to serious infant health outcomes, including death and lifelong disability. Small increases in preterm delivery risk in relation to spatial gradients of air pollution have been reported, but previous studies may have controlled inadequately for individual factors. Using a time-series analysis, which eliminates potential confounding by individual risk factors that do not change over short periods of time, we investigated the effect of ambient outdoor particulate matter with diameter < or = 10 microm (PM10) and sulfur dioxide on risk for preterm delivery. Daily counts of preterm births were obtained from birth records in four Pennsylvania counties from 1997 through 2001. We observed increased risk for preterm delivery with exposure to average PM10 and SO2 in the 6 weeks before birth [respectively, relative risk (RR) = 1.07; 95% confidence interval (CI), 0.98-1.18 per 50 microg/m3 increase; RR = 1.15; 95% CI, 1.00-1. 32 per 15 ppb increase], adjusting for long-term preterm delivery trends, co-pollutants, and offsetting by the number of gestations at risk. We also examined lags up to 7 days before the birth and found an acute effect of exposure to PM10 2 days and 5 days before birth (respectively, RR = 1.10; 95% CI, 1.00-1.21; RR = 1.07; 95% CI, 0.98-1.18) and SO2 3 days before birth (RR = 1.07; 95% CI, 0.99-1.15), adjusting for covariates, including temperature, dew point temperature, and day of the week. The results from this time-series analysis, which provides evidence of an increase in preterm birth risk with exposure to PM10 and SO2, are consistent with prior investigations of spatial contrasts.

Figure 1. RRs and 95% CIs for preterm birth per 50-μg/m³ increase in PM₁₀, lagged 1–7 days before birth, offsetting by the number of gestations at risk and adjusting for long-term trends, temperature, dew point temperature, day of the week, and co-pollutants (NO₂, CO, and SO₂) in four Pennsylvania counties, 1997–2001.

Figure 2. RRs and 95% CIs for preterm birth per 15-ppb increase in SO₂, lagged 1–7 days before birth, offsetting by the number of live births and adjusting for long-term seasonal trend, temperature, dew point temperature, day of the week, and co-pollutants (NO₂, CO, and PM₁₀) in four Pennsylvania counties, 1997–2001.