Cluster analysis of social and environment inequalities of infant mortality. A spatial study in small areas revealed by local disease mapping in France

Abstract

Mapping spatial distributions of disease occurrence can serve as a useful tool for identifying exposures of public health concern. Infant mortality is an important indicator of the health status of a population. Recent literature suggests that neighborhood deprivation status can modify the effect of air pollution on preterm delivery, a known risk factor for infant mortality. We investigated the effect of neighborhood social deprivation on the association between exposure to ambient air NO2 and infant mortality in the Lille and Lyon metropolitan areas, north and center of France, respectively, between 2002 and 2009. We conducted an ecological study using a neighborhood deprivation index estimated at the French census block from the 2006 census data. Infant mortality data were collected from local councils and geocoded using the address of residence. We generated maps using generalized additive models, smoothing on longitude and latitude while adjusting for covariates. We used permutation tests to examine the overall importance of location in the model and identify areas of increased and decreased risk. The average death rate was 4.2‰ and 4.6‰ live births for the Lille and Lyon metropolitan areas during the period. We found evidence of statistically significant precise clusters of elevated infant mortality for Lille and an east-west gradient of infant mortality risk for Lyon. Exposure to NO2 did not explain the spatial relationship. The Lille MA, socioeconomic deprivation index explained the spatial variation observed. These techniques provide evidence of clusters of significantly elevated infant mortality risk in relation with the neighborhood socioeconomic status. This method could be used for public policy management to determine priority areas for interventions. Moreover, taking into account the relationship between social and environmental exposure may help identify areas with cumulative inequalities.

Figure 1

A. Spatial distribution of infant mortality by census block in the Lyon and Lille MAs during 2002-2009. B. Spatial distribution of the deprivation index by census block in Lyon and Lille MA during 2002-2009. C. Spatial distribution of the NO₂ air pollution by census block in Lyon and Lille MA during 2002-2009.

Figure 2

Yearly averages of the NO₂ ambient air concentrations during the period 2002-2009 for the Lille and Lyon MAs. Figure 2 show the temporal trends of the NO₂ concentration during the period 2002 to 2009. In Lille MA, a meteorology penalizing in terms of dispersion, combined with majority of traffic emissions for the years 2003, 2005, led to annual levels of nitrogen dioxide higher. The pattern is quite stable in both graphs, but the mean NO₂ in Lyon MA is higher than 40µg/m³ which represents the referent limit by the WHO.

Figure 3

Comparison of the NO₂ average concentrations according to the classes of deprivation in the Lille and Lyon MAs, period 2002-2009.

Figure 4

(A) Prevalence of infant mortality estimated by the GAMs in the Lille Metropolitan Area for crude model (A), according to (B) NO₂ air pollution exposure, (C) deprivation index and (D). with the interaction term. Light grey to dark grey shading indicates lower to higher prevalence. Solid lines identify areas with significantly increased rates (hotspots) and dashed lines identify areas with significantly decreased rates (coldspots).

Figure 5

(A) Prevalence of infant mortality estimated by the GAMs in the Lyon Metropolitan Area for crude model (A), according to (B) NO₂ air pollution exposure, (C) deprivation index and (D). with the interaction term. Light grey to dark grey shading indicates lower to higher prevalence. Solid lines identify areas with significantly increased rates (hotspots) and dashed lines identify areas with significantly decreased rates (coldspots).