Long term exposure to air pollution, mortality and morbidity in New Zealand: Cohort study

To investigate associations between long-term exposure to PM_2.5, NO₂, mortality and morbidity in New Zealand, a country with low levels of exposure.

Retrospective cohort study.

The New Zealand resident population.

The main analyses included all adults aged 30 years and over with complete data on covariates: N = 2,223,507. People who died, or were admitted to hospital, (2013-2016) were linked anonymously to the 2013 census, and to estimates of ambient PM_2.5, and NO₂ concentration. We fitted Poisson regression models of mortality and morbidity in adults (≥30) for all natural causes of death, and by sub- group of major cause. Person-time of exposure, censored at the time of death, was included as an offset. We adjusted for confounding by age, sex, ethnicity, income, education, smoking status and ambient temperature. Further analyses stratified by ethnic group, and investigated respiratory hospital admissions in children.

There were statistically significant positive associations between pollutants and natural causes of death: RR (per 10 μg/m³) for PM_2.5 1.11 (1.07 to 1.15) and for NO₂ 1.10 (1.07 to 1.12). For morbidity, the strongest associations were for PM_2.5 and ischaemic heart disease in adults, RR: 1.29 (1.23 to 1.35) and for NO₂ and asthma in children, RR: 1.18 (1.09 to 1.28). In models restricted to specific ethnic groups, we found no consistent differences in any of the associations.

The results for NO₂ are higher than those published previously. Other studies have reported that the dose-response for PM_2.5 may be higher at low concentrations, but less is known about NO₂. It is possible NO₂ is acting as a proxy for other traffic-related pollutants that are causally related to health impacts. This study underlines the importance of controlling pollution caused by motor vehicles.