Urban Air Pollution and Mortality in a Cohort of Norwegian Men
Nafstad, Per, Haheim, Lise Lund, Wisloff, Torbjorn, Gram, Frederick, Oftedal, Bente, Holme, Ingar, Hjermann, Ingvar, Leren, Paul, Environmental Health Perspectives
We investigated the association between total and cause-specific mortality and individual measures of long-term air pollution exposure in a cohort of Norwegian men followed from 1972-1973 through 1998. Data from a follow-up study on cardiovascular risk factors among 16,209 men 40-49 years of age living in Oslo, Norway, in 1972-1973 were linked with data from the Norwegian Death Register and with estimates of average yearly air pollution levels at the participants' home addresses from 1974 to 1998. Cox proportional-hazards regression was used to estimate associations between exposure and total and cause-specific mortality. During the follow-up time 4,227 men died from a disease corresponding to an ICD-9 (International Classification of Diseases, Revision 9) code < 800. Controlling for a number of potential confounders, the adjusted risk ratio for dying was 1.08 [95% confidence interval (CI), 1.06-1.11] for a 10-[micro]g/[m.sup.3] increase in average exposure to nitrogen oxides (N[O.sub.x]) at the home address from 1974 through 1978. Corresponding adjusted risk ratios for dying from a respiratory disease other than lung cancer were 1.16 (95% CI, 1.06-1.26); from lung cancer, 1.11 (95% CI, 1.03-1.19); from ischemic heart diseases, 1.08 (95% CI, 1.03-1.12); and from cerebrovascular diseases, 1.04 (95% CI, 0.94-1.15). The findings indicate that urban air pollution may increase the risk of dying. The effect seemed to be strongest for deaths from respiratory diseases other than lung cancer. Key words: air pollution, cohort, epidemiology, long-term exposure, mortality, Norway.
The current understanding of the association between long-term exposure to air pollution and mortality is based on results from a few cohort studies, of which two studies have received a lot of attention (Dockery et al. 1993; Pope et al. 1995, 2002). Both of these studies have assessed air pollution exposure on an aggregated (not individual) level. All participants living in the same city were given a city-specific exposure level based on the available measurements. At least two other cohort studies have assessed exposure at an individual level (Beeson et al. 1998; Hoek et al. 2002). Results from these studies strengthen the evidence that urban air pollution is associated with increased risk of dying. Associations have varied and exposure measures have been rather crude. No single epidemiologic study can be the basis for determining a causal relation between air pollution and mortality (Health Effects Institute 2000), and there is still a strong need for exploring the associations between long-term air pollution and mortality further. For a long time Norway has had monthly updated registers for all citizens' home addresses and for deaths. This makes it possible to assess air pollution exposure at home addresses historically by geographical information systems and to link the information with information on deaths and causes of deaths for persons who have lived at these addresses. In this study we have estimated yearly air pollution levels at the home addresses for a cohort of 16,209 male citizens from 1974 to 1998 by geographical information systems (Gram et al. 2003; Haheim et al. 1996; Leren et al. 1975; Nafstad et al. 2003). The cohort was established in 1972-1973. The aim of the present study was to estimate associations between air pollution levels estimated at the participants' home addresses and the participants' risk of dying.
Materials and Methods
Study population. The study population has been described elsewhere (Haheim et al. 1996; Leren et al. 1975; Nafstad et al. 2003). Briefly, in 1972 all 40- to 49-year-old men living in Oslo, Norway (n = 25,915), were invited to participate in a population-based follow-up study of cardiovascular diseases. Those willing to participate (n = 16,209) met for a screening investigation between May 1972 and December 1973.
Health outcome. The National Death Register (Statistics Norway, Oslo, Norway) provided data on all deaths within the cohort including the cause of death according to the International Classification of Diseases, Revision 8 (ICD-8; Statistisk Sentralbyra 1973), Revision 9 (ICD-9; Statistisk Sentralbyra 1990), and Revision 10 (ICD-10; Statens Helsetilsyn 1998). …