Numerous studies have linked ambient air pollution to various adverse health outcomes. Most studies were conducted in developed countries and only a small number of studies have been conducted in developing countries of Asia [1]. Large cities in China have been experiencing substantial socio-economic developments in the past three decades and often accompanied by substantial environmental pollution. Beijing, capital city of China, has had serious ambient air pollution over decades. In addition to coal combustion, rapid increase in motor vehicles has made emissions from motor vehicles another major source of air pollution [2], which has brought significant public health issues in recent years. Although several studies have examined the health consequences of air pollution in Beijing [3]–[7], most of them were conducted before the 2008 and little is known about air pollution and health outcomes after the 2008 Beijing Olympics.

In the present study, we conducted a time-series analysis to evaluate the associations between the major air pollutants, including carbon monoxide (CO), nitrogen dioxide (NO₂) and particulate matter <10 µm in aerodynamic diameter (PM₁₀), and daily mortality (nonaccidental death, cardiovascular disease and respiratory disease) in Beijing from January 1, 2009 to December 31, 2010.

A total of 152,714 deaths were recorded in Beijing between January 1, 2009 and December 31, 2010, 0.83% in those aged 0–4 years old, 5.17% in those aged 5–44 years, 19.81% in those aged 45–64 years and 74.19% in those aged 65 years or older, respectively. Overall, there were 200.4 nonaccidental deaths per day in average, including 101 from cardiovascular diseases, which accounted for 60.6% of the total of nonaccidental deaths, and 20.6 from respiratory diseases, respectively (Table 1). During our study period, the mean daily average concentrations of CO, NO₂ and PM₁₀ were 1.54 mg/m³, 55.02 µg/m³ and 121.04 µg/m³, respectively (Table 1). The mean daily temperature, relative humidity and barometric pressure were 13.0°C, 51% and 1012 hPa.

Spearman correlation coefficients between air pollutant concentrations and weather conditions are listed in Table 2, showing close correlation between ambient air CO, NO₂ and PM₁₀ concentrations and weather conditions (all p<0.05), with the highest of ambient air CO and NO₂ concentrations primarily from traffic sources. Ambient air pollutant concentrations significantly inversely correlated with temperature and positively with relative humidity, but weakly correlated with barometric pressure.

Estimated effects of ambient air CO, NO₂ and PM₁₀ on nonaccidental, cardiovascular and respiratory mortalities were all statistically significant in single pollutant model (Table 3). The effects decreased but remained significant after adjusting collinearity by principal component analysis (Table 3).The sensitivity analysis indicated that the increasing of the degrees of freedom led to a decrease in the effect estimates (results not shown).

Air pollution effects on nonaccidental and cardiovascular mortality varied by sex and age (Table 4). For both males and females, effect estimates of all the three air pollutants were statistically significant. And effect estimates for females were a little bit higher than those for males, but between-sex difference was non-significant for all these three pollutants.

We observed that the effects of air pollutants were statistically significant in residents aged 65 years or older for nonaccidental and cardiovascular mortalities (Table 4), but only significant for nonaccidental mortality in residents aged 45 or older. For nonaccidental and cardiovascular mortality, effect estimates in residents aged 65 years or older for all these three air pollutants were higher than those in residents aged 45–64 years, but not reaching the level of statistically significantly difference.

Figure 2 presents the lag pattern of effects of air pollutants on cardiovascular mortality, with statistically significant associations for most lag days of all three pollutants except for lag3 and lag4. Generally, cumulative exposure lag01 had larger effects than single-day exposure (lag0 to lag4) and the effects of cumulative exposures (from lag01 to lag04) showed a decreasing trend.

The results of this time-series study found that ambient air pollutants CO, NO₂ and PM₁₀ remained strongly associated with nonaccidental deaths and mortalities caused by major cardiovascular and pulmonary diseases in Beijing after the 2008 Olympics (2009∼2010). After adjusting collinearity, all the three air pollutants were associated with nonaccidental and cardiopulmonary mortalities. Women and elderly were more vulnerable to air pollution than men and younger people.

Air pollution has been a major environmental and public health problem in Beijing for the past decades. In addition to coal combustion, the rapid increase in motor vehicles has made exhaust emissions from them another major source of air pollution [2]. In Beijing, number of automobiles increased substantially from 1.46 million in 1999 to around 5 million in 2011, which became major source of ambient air CO (86%) and NO_x (56%) emissions there [17], indicating Beijing still faces a substantial challenge of balancing economic growth and environment protection. Our study evaluated relationship between air pollutants and mortality after the 2008 Beijing Olympics and therefore provides important information for policy makers.

A meta-analysis of 109 time-series studies of air pollution and daily mortality, most of which were conducted in North America and Europe, indicated that 1 mg/m³ change of CO and 10 µg/m³ change of NO₂ and PM₁₀ were associated with 1.35 percent (95% CI, 0.95, 1.75), 0.61 percent (95% CI, 0.46, 0.76) and 0.64 percent (95% CI, 0.48, 0.77) increases in all daily natural deaths, respectively (in single-pollutant models) [18]. Recently, a study about the relationship between CO and daily mortality in three Chinese cities (Shanghai, Anshan, Taiyuan) estimated that 1 mg/m³ increase of CO was associated with 2.89 percent (95% CI: 1.68, 4.11) increase of nonaccidental mortality in the single-pollutant model [19]. Another meta-analysis of Chinese studies estimated that 10 µg/m³ increase in PM₁₀ corresponded to a 0.44 percent (95% CI, 0.13, 0.76) increase in nonaccidental mortality [20]. There were several time series studies for association between air pollution and daily mortality in Beijing before the 2008 Olympics, Zhang et al examined the association between air pollution and daily mortality in an urban district using data from 2003 to 2008 and found that 10 µg/m³ increase of NO₂ was associate with 0.271 percent (95% CI: 0.086, 0.457) increase for cardiovascular mortality and 0.947 percent (95% CI: 0.759, 1.135) increase for respiratory mortality. As for PM₁₀, the corresponding figures were 0.164 percent (95% CI: 0.144, 0.184) and 0.101 percent (95% CI: 0.057, 0.145), respectively [7]. Yang et al investigated the relationship between air pollutants and cardiovascular mortality in Beijing using data from January 1, 2003 to December 31, 2003 and found that 10 µg/m³ increase of NO₂ was associate with 0.404 percent (95% CI: 0.1, 0.8) increase for cardiovascular mortality and the corresponding figure was 0.381 percent (95% CI: 0.2, 0.6) for PM₁₀ [21].

In order to compare our results with those in other studies, we also converting our percent increase of daily mortality associated with an IQR increase of pollutant concentrations into 10 µg/m³ increase for NO₂ and PM₁₀ and 1 mg/m³ increase for CO. Using two-day moving average of the air pollutants concentrations, an increase of 1 mg/m³ of CO was associated with a 3.14 percent (95% CI: 2.45, 3.83) increase in nonaccidental mortality, and the corresponding numbers were 1.25 percent (95% CI: 0.99, 1.53) for 10 µg/m³ increase of NO₂ and 0.25 percent (95% CI: 0.17, 0.33) for 10 µg/m³ increase of PM₁₀, respectively. For cardiovascular mortality, increase of 1 mg/m³ of CO was associated with a 3.54 percent (95% CI: 2.59, 4.49) increase, and the corresponding numbers were 1.30 percent (95% CI: 0.92, 1.68) for 10 µg/m³ increase of NO₂ and 0.24 percent (95% CI: 0.12, 0.35) for 10 µg/m³ increase of PM₁₀, respectively. For respiratory mortality, an increase of 1 mg/m³ of CO was associated with a 2.94 percent (95% CI: 0.84, 5.03) increase, and the corresponding numbers were 0.89 percent (95% CI: 0.05, 1.72) for 10 µg/m³ increase of NO₂ and 0.29 percent (95% CI: 0.03, 0.54) for 10 µg/m³ increase of PM₁₀, respectively_. Our estimates on effects of gaseous pollutants (CO and NO₂) in ambient air seemed higher than results of studies conducted before 2008 in Beijing and studies in multi-city analysis and meta analysis [7], [18], [19], [21], [22]. There are two possible explanations, automobiles became a major source of air pollutants in Beijing in the past several years. Compared with industrial emissions, automobile emission was much closer to the zones of human activities and was more likely to be inhaled. Health consequences of automobile derived from air pollutants in Beijing could be more severe and should be further investigated. Further, Beijing is also undergoing substantial changes in sociodemographic and health conditions such as rapidly aging, urbanization, lifestyles, disease spectrum, and so on, many of which may affect population susceptibility to the adverse effects of air pollution [23]. As estimated, PM₁₀ concentration in Beijing is generally comparable with that in a few years ago [21] and in multi-city analysis [20] and meta-analysis [18] worldwide. Health effects of PM₁₀ may be related to its composites and the residents’ sensitivity to air pollution. Components of particular mass were classified into five categories in previous studies, i.e., secondary inorganic aerosol, sea salt, heavy metal, mineral matter and construction dust [24]. For Beijing, contribution of these components to PM_2.1 accounted for 23.9%, 6.5%, 1.1%, 13.3% and 4.0%, whereas for 12.0%, 9.5%, 0.8%, 37.1% and 7.8%, to PM_2.1-9, respectively. The estimated contribution of anthropogenic sources to PM_2.1 in Beijing was 3.5 times as that of nature sources whereas the same ratio was 0.6 for PM_2.1-9. Contribution of secondary components to PM_2.1 was equivalent to that of primary emissions, suggesting that the precursors emitted from coal combustion and vehicle exhaust should be controlled in target areas. In contrast, the ratio of primary emissions to secondary particulate matters in PM_2.1-9 was up to 5, indicating measures are required to reduce dust from construction areas [24]. When it comes to susceptibility, the APHENA study of European and North American cities have already confirmed that the elderly and unemployed individuals were at higher risk of short-term PM exposure [25]. And obesity and active smoking have been newly recognized as a possible susceptible factors. Two cohort studies have shown that a greater body mass index enhances the susceptibility for PM-induced cardiovascular mortality, at least in women [26], [27].

In our analysis, we found associations of air pollutants and nonaccidental and cardiovascular mortalities were stronger in women than in men, which is consistent with other reports [28]. The fact that few Chinese women smoke might have further contributed to the effect of air pollutants on mortalities in China. And one study showed that effects of air pollution was greater in nonsmokers than smokers [29].

As in many other studies [30]–[32], we also found the elderly people were more vulnerable to the adverse effects of air pollution (Table 4), who have often had chronic diseases that put them be at higher risk of harmful health effects due to ambient air pollution exposure [33]. Obviously, there was an overlap between elderly people and patients with cardiopulmonary diseases. We referred to the Beijing Municipal Statistical Yearbook−2007 and Beijing Municipal Statistical Yearbook−2010 and found that the cardiovascular morality rates were 129.82 per 100,000 in 2007 and 156.97 per 100,000 in 2010, and the respiratory mortality rates were 58.83 per 100,000 in 2007 and 60.92 per 100,000 in 2010, respectively, indicating an increasing trend of death from chronic diseases [34], [35]. According to Beijing Statistical Bureau (2012), people over 65 years old accounted for 14.1% of the permanent residents in Beijing [36], which means that a considerable number of residents in Beijing tend to be at increased risk for air-pollution-related health effects.

Currently the most popular were single air pollutant model without considering the inner link between different air pollutants, which could lead to in certain limitations when using the single model in explanation of the results. If a multiple air pollutants were directly fitted into the model, collinearity could inevitable confound the model due to the nonindependence between different air pollutants, thus leading to the instability of the model. In order to solve the problem, principal component analysis was adopted to adjust the collinearity of different air pollutants in the multiple air pollutants model [14]. It is a multivariate method that combines three air pollutant indexes by means of an appropriate linear model, and then it generated an independent and specific composite latent variable (Principle component) with extracted variation information of original independent variable.

Sensitivity analysis of the model suggests that cumulative effects (multiday effects from lag01 to lag04) were generally larger and more sensitive than lag effects (single day effects from lag1 to lag4).

Our analysis also has several limitations. Like in most time-series studies, we used averages of monitoring stations as a surrogate for exposures to air pollution. Further, we did not have individual level information, and therefore were unable to evaluate other potential effect modifiers such as obesity and smoking. Finally, our data on weather parameters were extracted from only one monitoring station and may not accurately reflect the status of the whole city.

In summary, in our time-serious analysis, we found that ambient air pollutants were strongly associated with nonaccidental and cardiopulmonary mortalities in Beijing during 2009∼2010. Further, women and the elderly were more vulnerable to the adverse effects of ambient air pollution. These updated data in Beijing may help authorities there formulate relevant policies to improve its air environment and health status of residents.