Short-term effects of gaseous pollutants on cause-specific mortality in Wuhan, China.

ABSTRACT

In Asia, limited studies have been published on the association between daily mortality and gaseous pollutants of nitrogen dioxide (N[O.sub.2]), ozone ([O.sub.3]), and sulfur dioxide (S[O.sub.2]). Our previous studies in Wuhan, China, demonstrated long-term air pollution effects. However, no study has been conducted to determine mortality effects of air pollution in this region. This study was to determine the acute mortality effects of the gaseous pollutants in Wuhan, a city with 7.5 million permanent residents during the period from 2000 to 2004. There are approximately 4.5 million residents in Wuhan who live in the city's core area of 201 [km.sup.2], where air pollution levels are highest, and pollution ranges are wider than the majority of the cities in the published literature. We used the generalized additive model to analyze pollution, mortality, and covariate data. We found consistent N[O.sub.2] effects on mortality with the strongest effects on the same day. Every 10-[micro]g/[m.sup.3] increase in N[O.sub.2] daily concentration on the same day was associated with an increase in nonaccidental (1.43%; 95% confidence interval [CI]: 0.87-1.99%), cardiovascular (1.65%; 95% CI: 0.87-2.45%), stroke (1.49%; 95% CI: 0.56-2.43%), cardiac (1.77%; 95% CI: 0.44-3.12%), respiratory (2.23%; 95% CI: 0.52-3.96%), and cardiopulmonary mortality (1.60%; 95% CI: 0.85-2.35%). These effects were stronger among the elderly than among the young. Formal examination of exposure-response curves suggests no-threshold linear relationships between daily mortality and N[O.sub.2], where the N[O.sub.2] concentrations ranged from 19.2 to 127.4 [micro]g/[m.sup.3]. S[O.sub.2] and [O.sub.3] were not associated with daily mortality. The exposure-response relationships demonstrated heterogeneity, with some curves showing nonlinear relationships for S[O.sub.2] and [O.sub.3]. We conclude that there is consistent evidence of acute effects of N[O.sub.2] on mortality and suggest that a no-threshold linear relationship exists between N[O.sub.2] and mortality.

INTRODUCTION

Ambient air pollution has been associated with a wide range of effects on human health, including measurable decreases in lung function (1) and increases in respiratory symptoms and diseases, (2) hospital and emergency department admissions, (3) and mortality. (4) It is well documented that there is a positive association between ambient particle air pollution and daily mortality in the United States, (5) Canada, (6) Eastern Germany, (7) China, (8) Korea, (9) Greece, (10) and Brazil. (11) However, limited studies have reported an association between daily mortality and gaseous pollutants, including nitrogen dioxide (N[O.sub.2]), ozone ([O.sub.3]), or sulfur dioxide (S[O.sub.2]). Among the limited published studies, reported associations are not in agreement because of several uncertainties: (1) the mortality rates attributed to gaseous pollutants are not specific and may also be attributed to exposure to particulate matter (PM), change of climate or temperature, or personal and socio-demographic factors affecting gaseous pollutant exposure, such as use of air conditioners, level of education, and influence of other comorbidities; (2) insufficient evidence about the general shape of exposure-response relationships between gaseous pollutants and observed daily cause-specific mortality; (3) heterogeneity of reported gaseous pollutant mortality effects; (4) model misspecification; and (5) lack of biologically demonstrable mechanisms for the observed associations. In addition, similar studies from Asia have been limited compared with the studies in developed countries. (12) For example, our previous studies in Wuhan, China, have demonstrated long-term respiratory health effects of ambient air pollution (13) and acute coarse PM ([PM.sub.10]) mortality effects. In recent years, combustion of gas is the most common method for domestic cooking, and motor vehicle emissions are a more important pollution source than before. Both the combustion of gas and motor vehicles emissions could produce more N[O.sub.2]. However, no study has been carried out in Wuhan to determine acute mortality effects of ambient gaseous pollutants.

In this study, we examined the associations of daily cause-specific mortality (nonaccidental, cardiovascular, stroke, cardiac, respiratory, and cardiopulmonary) with daily mean concentrations of N[O.sub.2], [O.sub.3], and S[O.sub.2] while controlling for [PM.sub.10]. The central hypothesis of this study is that daily gaseous pollutant concentrations are associated with daily cause-specific mortality. We tested the central hypothesis addressing the following three study questions: (1) is daily cause-specific mortality associated with N[O.sub.2], [O.sub.3], and S[O.sub.2]?; (2) can any shape of the exposure-response relationships (threshold) other than linear be identified?; and (3) are there any specific subgroups, such as the elderly (age [greater than or equal to]65 yr), more susceptible to the ambient gaseous air pollution?

EXPERIMENTAL WORK

Study Area

Wuhan is the capital of Hubei Province in China. It located in the middle of the Yangzi River delta at 29 [degrees]58'-31 [degrees]22' north latitude and 113 [degrees]41'-115 [degrees]05' east longitude. Wuhan is one of the biggest hubs for land, water, and air transportation in China. It has a subtropical humid monsoon climate, and the average daily temperature in July is 37.2 [degrees]C. Because the maximum daily temperature often exceeds 40 [degrees]C in summers, Wuhan has been called an "oven city" in China. Wuhan has approximately 7.5 million permanent residents, and approximately 4.3 million of them reside in nine urban core districts within an area of 201 km. (2) The major industries in Wuhan include ferrous smelters, chemical plants, power plants, and machinery plants. The major sources of air pollution in the city are motor vehicles and the use of coal for domestic cooking, heating, and industrial processes. In recent years, combustion of gas has been the most common method for domestic cooking.

Mortality Data

This study focuses on the 4.3 million permanent residents in nine urban core districts within an area of 201 [km.sup.2] in Wuhan. We obtained all of the mortality data from the Wuhan Centres of Disease Control and Prevention (WCDC) during the study period of July 1, 2000, to June 30, 2004. In the event of a death in Wuhan, the government requires that the decedent's family obtain a death certificate from a hospital or a local community clinic to remove the deceased person from the government-controlled household registration. In addition, the decedent's family must submit the death certificate to both the district center of disease control and prevention (CDC) and the local police station to obtain a cremation certificate. Thus, the decedent's family obtains two certifications, one from the police station and the other from the district CDC. The latter develops electronic records of all death certificates and reports them to the WCDC.

It is the regulatory policy in Wuhan that the WCDC electronically archives all death certificates. In 1992, the WCDC became the first center in China standardizing its system for mortality data collection. This system was approved and recommended by the Chinese Department of Health. This system requires the following: (1) mortality data must be validated four times a year; (2) death events collected from the WCDC must conform with those collected from the Wuhan Police Department; (3) no missing data exist on any death certificates; (4) unclear cause of death and death from other reasons are below 2% in urban districts; and (5) a correct coding rate above 98% must be achieved for cause-specific deaths.

In this study, total mortality was divided into the following major causes of death: nonaccidental mortality (International Classification of Diseases, Ninth Revision [ICD9] 1-799 or International Classification of Diseases, Tenth Revision [ICD10] A00-R99), cardiovascular diseases (CVDs; ICD9 390-459 or ICD10 I00-I99), stroke (ICD9 430-438 or ICD10 I60-I69), cardiac diseases (ICD9 390-398, 410-429 or ICD10 I00-I09, I20-I52), respiratory diseases (RD; ICD9 460-519 or ICD10 J00-J98), and cardiopulmonary diseases (RD + CVD).

Air Pollution and Climate Data

Daily concentrations of N[O.sub.2], [O.sub.3] (8-hr mean concentrations of [O.sub.3] from 10:00 a.m. to 6:00 p.m.), S[O.sub.2], and [PM.sub.10] were obtained from the Wuhan Environmental Monitoring Center (WEMC). These measurements have been collected automatically and continuously for 24 hr per day and 365 days per year without interruption. The monitoring system has been operated by the WEMC and certified by U.S. Environmental Protection Agency (EPA). The operation of this system has strictly followed the quality assurance/quality control procedure set by the State Environmental Protection Administration of China. (14) Meteorological data were collected for daily average temperature and daily average relative humidity (RH) from the Wuhan Meteorological Administration.

Statistical Methods and Data Analyses