Air pollution and mortality in Christchurch, New Zealand

Although numerous time series epidemiological studies have reported an association between daily mortality and air pollution, there is uncertainty about the extent of life shortening due to air pollution. It is unclear whether the increase in daily deaths as demonstrated by time series studies is just an indicator of the short-term displacement of the time of death of frail individuals by a few days, a phenomenon known as “harvesting”. This is an important question for the public health task of estimating the impact of air pollution on mortality. The impact will be much lower if the mortality displacement is very short-term i.e. just by a couple of days, rather than a substantial reduction of life expectancy by months or years. While several time series studies have provided evidence of the acute effects of air pollution on mortality, there is limited research analysing the extended effects of air pollution over a few weeks and the effects of long-term exposure to air pollution on mortality. Christchurch has amongst the worst air quality in New Zealand. The frequent occurrence of calm periods and temperature inversions on cold winter nights, in combinations with emissions from the burning of coal and wood as domestic heating, exacerbate winter air pollution levels in Christchurch. For several decades, air pollution from the burning of coal and wood in the winter has been a major concern, primarily as an environmental nuisance. In addition to the emissions from domestic heating, air pollution from other sources, mostly vehicle emissions, is also a growing concern. Because of mounting evidence of the association between air pollution and mortality, air pollution has been increasingly recognised as a public health threat in Christchurch. This study examines whether or not particulate matter is associated with an increase in mortality in Christchurch. The four main objectives are: 1) to estimate the acute effects of daily air pollution on daily mortality, up to a few days after exposure; 2) to test whether an association between short-term exposure to particulate matter and mortality can be attributed to “harvesting”; 3) to estimate the extended effects of particulate matter on daily mortality up to a few weeks after exposure; and 4) to quantify the association between long-term exposure to particulate matter and annual mortality. Time series studies were carried out for the first three objectives and an ecological cross-sectional study was conducted for the fourth objective. Mortality data for the years 1988 1999 were obtained from the New Zealand Health Information Service. Environment Canterbury provided routinely monitored air pollution and weather data. For the first three objectives, daily mortality, air pollution and weather data for the period from June 1988 to December 1999 were analysed. For the fourth objective of estimating the effect of long-term exposure to PMio on mortality at annual level, census area unit based mortality data from 1996 to 1999 were associated with spatial annual average PMio exposure estimates provided by the University of Canterbury. Modelling techniques included Poisson regression, polynomial distributed lag, time series analysis over the mid-scale variations of daily data and logistic regression across spatial units. Poisson regression models were used to associate daily air pollutant concentrations with daily mortality controlling for a long-term trend, seasonal variation in daily mortality and the confounding effects of weather variables. Polynomial distributed lag modelling was used to analyse the extended effects of PMio on mortality. The mid-scale variations of daily data were analysed to examine the presence of harvesting. Logistic regression across spatial units was used to associate annual average PMio concentrations with annual mortality. A significant positive association was observed between daily PMio and daily non-external mortality. An increase of 10 pg/nr in daily PMio was associated with a 1.5% increase (95% Cl: 0.6, 2.5%) in the same day non-external deaths in the population aged 65+ years. When deaths up to several weeks were considered using polynomial distributed lag models, the total estimated effects of PMio on non-external cause mortality, circulatory mortality and respiratory mortality were found to be much larger. There was a total estimated increase of 14.9% (95% Cl: 6.9, 23.4%) in non­ external cause mortality in the population aged 65+ years during 41 days after exposure (including the day of exposure) for each 10 pg/nr increase in daily PMio. The analysis of long-term exposure to PMio estimated an increase of 17% (95% Cl: 9, 26%) in annual non-external deaths in the population aged 65+ years for each 10 pg/nr increase in annual average PMi0. The estimated PM]0 associated risk of mortality was higher for respiratory mortality than for other causes. This study provides evidence of both short-term and long-term effects of PMio on mortality in Christchurch and suggests that studies focussing only on the effects of PMio on mortality on the same day or a couple of days after exposure underestimate the total effect of PMio. It also shows that the short-term association between PMio and mortality can not be entirely attributed to harvesting. These findings contribute to the scientific evidence of mortality effects of PMio both in the short-term and long-term, and suggest that air pollution may be a greater threat to public health than what has been thought until now.