Survival Analysis of Long-Term Exposure to Different Sizes of Airborne Particulate Matter and Risk of Infant Mortality Using a Birth Cohort in Seoul, Korea

Article excerpt

BACKGROUND: Several studies suggest that airborne particulate matter (PM) is associated with infant mortality; however, most focused on short-term exposure to larger particles.

OBJECTIVES: We evaluated associations between long-term exposure to different sizes of particles [total suspended particles (TSP), PM [less than or equal to]] 10 [micro]m in aerodynamic diameter ([PM.sub.10]), [less than or equal to] 10-2.5 [micro]m ([PM.sub.10-2.5]), and [less than or equal to] 2.5 [micro]m ([PM.sub.2.5])] and infant mortality in a cohort in Seoul, Korea, 2004-2007.

METHODS: the study includes 359,459 births with 225 deaths. We applied extended Cox proportional hazards modeling with time-dependent covariates to three mortality categories: all causes, respiratory, and sudden infant death syndrome (SIDS). We calculated exposures from birth to death (or end of eligibility for outcome at 1 year of age) and pregnancy (gestation and each trimester) and treated exposures as time-dependent variables for subjects' exposure for each pollutant. We adjusted by sex, gestational length, season of birth, maternal age and educational level, and heat index. Each cause of death and exposure time frame was analyzed separately.

RESULTS: We found a relationship between gestational exposures to PM and infant mortality from all causes or respiratory causes for normal-birth-weight infants. For total mortality (all causes), risks were 1.44 (95% confidence interval, 1.06-1.97), 1.65 (1.18-2.31), 1.53 (1.22-1.90), and 1.19 (0.83-1.70) per interquartile range increase in TSP, [PM.sub.10], [PM.sub.2.5], and [PM.sub.10-2.5], respectively; for respiratory mortality, risks were 3.78 (1.18-12.13), 6.20 (1.50-25.66), 3.15 (1.26-7.85), and 2.86 (0.76-10.85). For SIDS, risks were 0.92 (0.33-2.58), 1.15 (0.38-3.48), 1.42 (0.71-2.87), and 0.57 (0.16-1.96), respectively.

CONCLUSIONS: Our findings provide supportive evidence of an association of long-term exposure to PM air pollution with infant mortality.

KEY WORDS: air pollution, Cox proportional hazards model, infant mortality, long-term effect, particulate matter, [PM.sub.2.5], [PM.sub.10], [PM.sub.10-2.5], survival analysis, time dependent, TSP. Environ Health Perspect 119:725-730 (2011). doi:10.1289/ehp.1002364 [Online 17 December 2010]

Numerous epidemiologic studies have demonstrated associations between ambient air pollution and health outcomes, including mortality (Chen et al. 2008; Dales et al. 2004; Qian et al. 2007), hospitalizations (Dominici et al. 2006; Lin et al. 2004; Villeneuve et al. 2007; Wellenius et al. 2005), and lung function (Delfino et al. 2008; Jalaludin et al. 2000; Tang et al. 2007). Infant mortality is still a major contributor to childhood mortality (Glinianaia et al. 2004a). Infants and children are potentially susceptible because of their young immune systems, developing respiratory and other systems, and common viral infections (Bateson and Schwartz 2008; Glinianaia et al. 2004b; Koranteng et al. 2007).

Findings for air pollution and infant health are relatively consistent for particulate matter (PM) compared with other pollutants (Sram et al. 2005). Several studies suggest that PM exposure is associated with infant mortality (Bobak and Leon 1999; Ha et al. 2003; Hajat et al. 2007; Lipfert et al. 2000; Romieu et al. 2004; Tsai et al. 2006; Woodruff et al. 1997). However, most studies focused on short-term exposure to larger particles such as total suspended particulate (TSP) or [PM.sub.10] (PM [less than or equal to] 10 [micro]m in aerodynamic diameter) and to exposures that occurred postneonatally. Research on effects of [PM.sub.2.5] ([less than or equal to] 2.5 [micro]m) or coarse particles [PM.sub.10-2.5] (2.5-10 [micro]m) in infants is limited.

Only two studies evaluated associations between long-term [PM.sub.2.5] exposure and infant mortality. Woodruff et al. (2006) observed an association between postneonatal respiratory mortality and [PM. …