Research report 1Department of Public Health, University of Otago, Wellington, New Zealand 2University of Otago, Wellington, New Zealand 3University of Auckland, Auckland, New Zealand Correspondence to Dr Simon Hales, Department of Public Health, University of Otago, Wellington, 23a Mein Street, Wellington 6242, New Zealand; simon.hales@otago.ac.nz Statistics New Zealand Security Statement The New Zealand Census Mortality Study is a study of the relation between social factors and mortality in New Zealand, based on the integration of anonymised population census data from Statistics New Zealand and mortality data from the New Zealand Health Information Service. This project was approved by Statistics New Zealand as a data laboratory project under the Microdata Access Protocols in 1997. The datasets created by the integration process are covered by the Statistics Act and can be used for statistical purposes only. Only approved researchers who have signed Statistics New Zealand’s declaration of secrecy can access the integrated data in the data laboratory. Access to the data used in this study was provided by Statistics New Zealand under conditions designed to give effect to the security and conﬁdentiality provisions of the Statistics Act, 1975. The results presented in this study are the work of the author, not Statistics New Zealand. The authors take full responsibility for the paper and Statistics New Zealand will not be held accountable for any error or inaccurate ﬁndings. Accepted 8 July 2010 Published Online First 21 October 2010 This paper is freely available online under the BMJ Journals unlocked scheme, see http:// jech.bmj.com/site/about/ unlocked.xhtml 468 Air pollution and mortality in New Zealand: cohort study Simon Hales,1 Tony Blakely,2 Alistair Woodward3 ABSTRACT Background Few cohort studies of the health effects of urban air pollution have been published. There is evidence, most consistently in studies with individual measurement of social factors, that more deprived populations are particularly sensitive to air pollution effects. Methods Records from the 1996 New Zealand census were anonymously and probabilistically linked to mortality data, creating a cohort study of the New Zealand population followed up for 3 years. There were 1.06 million adults living in urban areas for which data were available on all covariates. Estimates of exposure to air pollution (measured as particulate matter with an aerodynamic diameter less than 10 mm, PM10) were available for census area units from a previous land use regression study. Logistic regression analyses were conducted to investigate associations between cause- speciﬁc mortality rates and average exposure to PM10 in urban areas, with control for confounding by age, sex, ethnicity, social deprivation, income, education, smoking history and ambient temperature. Results The odds of all-cause mortality in adults (aged 30e74 years at census) increased by 7% per 10 mg/m3 increase in average PM10 exposure (95% CI 3% to 10%) and 20% per 10 mg/m3 among Maori, but with wide CI (7% to 33%). Associations were stronger for respiratory and lung cancer deaths. Conclusions An association of PM10 with mortality is reported in a country with relatively low levels of air pollution. The major limitation of the study is the probable misclassiﬁcation of PM10 exposure. On balance, this means the strength of association was probably underestimated. The apparently greater association among Maori might be due to different levels of co-morbidity. Anthropogenic air pollution is thought to cause a substantial global burden of disease, even if conservative assumptions are used for the analysis.1 Most epidemiological studies of air pollution effects use a time series design, in which day-to-day changes in morbidity or mortality (usually in a single city) are related to day-to-day changes in air pollution exposure. These studies have the advan- tage of controlling for time invariant (or slowly varying) confounding factors by design. Time series studies provide extensive and detailed evidence of short-term air pollution effects, but are not the best basis for public health policy on air pollution.2 This is because cohort studies suggest that long-term exposures have much greater public health impact than short-term exposures. Because it is difﬁcult to estimate air pollution exposure for large populations over long periods, relatively few cohort studies have been reported and few such studies have been conducted outside north America and Europe.3e18 We report here the ﬁndings from a longitudinal study in New Zealand, based on the national census. In New Zealand, major sources of urban air pollution are home heating and motor vehicle emissions and levels of pollution are low by inter- national standards. Typical annual average levels of particulate air pollution (particulate matter with an aerodynamic diameter less than 10 mm, PM10) at monitoring stations in the main urban centres are in the range 15e25 mg/m3. In this study, we analyse the spatial association between average PM10 exposure and mortality over the years 1996e9. We also investigate potential modiﬁcation by social factors (age, sex, ethnicity, income, education and area level deprivation). We hypothesise a priori that effects of PM10 on mortality will be stronger for cardiorespiratory causes of death than for all diseases, because these causes are most sensitive to air pollution effects. METHODS The New Zealand Census-Mortality Study is a population-wide cohort study, in which the cohort consists of the entire 1996 resident popula- tion (N¼3 732 000) and the outcome of interest is mortality. For this analysis, records from the 1996 census were anonymously linked to 3 years of subsequent mortality data, creating a cohort study of the New Zealand population followed up for 3 years.19 20 In New Zealand, urban areas are deﬁned as cities having a population of at least 30 000 people; about half of the approximately 2000 census areas are classiﬁed as urban. Because the deﬁnition of ‘urban areas’ in New Zealand includes small towns and suburbs, many urban census areas have low levels of air pollution. Record linkage The method of record linkage has been described in detail elsewhere.19 Brieﬂy, probabilistic record linkage is a process used to link two ﬁles of records, in which records in one ﬁle have a corresponding record in the other ﬁle. Records from the ﬁrst ﬁle are compared with records from the second ﬁle in order to ﬁnd ‘matching’ record pairs (ie, two records belonging to the same individual). Census and mortality records were linked using date of birth (day, month and year as separate matching variables), country of birth, sex, ethnicity and address of usual residence. (Geocoded addresses were the most discriminatory matching variable.) Linkage was restricted to individuals aged 74 years or below at the time of the census. The proportion of mortality records linked overall was 78%,20 and varied by sex, age, ethnicity and the deprivation index.19 Weights were therefore applied to adjust for linkage bias, by strata of sex, age, J Epidemiol Community Health 2012;66:468e473. doi:10.1136/jech.2010.112490