Novel green space measures and birthweight in Queensland, Australia

Abstract

IntroductionEnvironmental exposures can contribute both benefits and risks to human health. Green space represents an environmental exposure that may confer benefits to human health. There are several proposed benefits to green space exposure, such as improving mental wellbeing, increasing physical activity, improving the social cohesion of local neighbourhoods, reducing the negative effect of environmental risk factors, and improving exposure to beneficial microbiota. One of the proposed health benefits is an improvement in the health outcomes of neonates. Maternal exposure to green space has been associated with improvements in birthweight, among other birth outcomes. The developmental origins of health and disease hypothesis suggests that investing in healthy foetal development in Utero confers benefits across the life span. This makes an assessment of the potential impacts of environmental exposures on foetal development an important undertaking in population health. The current body of evidence on birthweight and maternal green space exposure is limited to a handful of exposure measures and coarse allocation of exposure. Newer measures of green space have been developed, which allows for an exploration of the effect of different ground covers (green, dry and bare earth), as well as measures of biodiversity, on health. This thesis explores the association of these novel green space measures with birthweight in a large birth cohort in Queensland, Australia.ObjectivesThis study has several objectives: 1) to examine the maternal and socio-demographic characteristics associated with birthweight; 2) to test for an association between green space, dry cover and bare earth and birthweight outcomes, controlling for the maternal and social factors; 3) to test whether foliage projective cover, a measure of crown canopy shading, is associated with birthweight outcomes; 4) to explore whether effects of green space differ between rural and urban communities; 5) to explore the interaction of green space with other environmental variables, namely air pollution and ambient temperature; and 6) to explore whether biodiversity is associated with birthweight outcomes.MethodsSeveral data sets were acquired and merged to create the data set used in this study. The outcome was birthweight (in grams and age-standardised), and this was acquired along with maternal covariates from the routine health records available for all births in Queensland, Australia. Social and neighbourhood variables were acquired from the Australian Bureau of Statistics. The exposures of interest to this study were green space (fractional cover, biodiversity and foliage projective cover), ambient temperature and air pollution. Directed acyclic graphs were developed to guide variable selection in all analyses. Mixed-effects linear regression and generalised linear mixed-effects models were developed, with random intercepts for maternal residential locality and year of birth. Results are presented as standardised beta coefficients or odds ratios, with 95% confidence intervals.ResultsMaternal characteristics such as smoking, maternal pre-pregnancy BMI, migration status, hypertension and diabetes remain important predictors of low birthweight. Analysis ofenvironmental factors showed that in urban areas, each percentage increase of green cover (0.59g, 95% CI 0.28 – 0.91) and foliage projective cover (0.52g, 95% CI 0.22 – 0.83) are associated with increases in estimated birthweight. Each increasing percentage of dry cover (-0.69g, 95% CI -1.21 – -0.17) and bare earth (-0.35g, 95% CI -0.66 – -0.05) are associated with an estimated decrease in birthweight. Mothers living in rural areas had similar results, with each percentage increase in green cover (0.36g, 95% CI 0.06 – 0.65), but not foliage projective cover, associated with an estimated increase in birthweight, while bare earth (-0.55g, 95% CI -0.91 – -0.19) was associated with a decrease in birthweight. The biodiversity measure used in this study was not associated with any birthweight outcomes.ConclusionThis study finds that the types of ground cover within the maternal residential locality are associated with small, but significant, changes in estimated birthweight. Higher levels of greencover were associated with small increases in birthweight for women residing in both urban and rural locations. The current study finds that the beneficial effects of green space, and the potentially negative effects of other ground cover types, is not limited to urban areas. The difference in effect across the rural-urban divide suggests that rural areas should be considered in future research of environmental factors. While this study finds highly significant changes in birthweight with increasing green space, the change is very small, and the current evidence base is lacking on whether these small changes are important to long-term health of the individual. In the context of what is currently known, this study concludes that the changes in birthweight found in this population associated with green space are unlikely to be of biological importance to the individual, but may be of importance in population health.