Introduction
Urbanization and industrialization have been major contributing factors to the ongoing change in global climate, with increased air pollution and poor air quality being some of the main consequences thereof. As the global climate and air quality deteriorates, exposure to air pollution remains a fundamental concern to public health.
In recent years, developing countries in Asia like India and China have embarked on major urbanization and industrialization and consequently have been struggling immensely with severe air pollution. Various studies and health-related agencies have presented evidence that reveals ambient air pollution continues to increase daily particularly in such developing countries, resulting in various health effects and fatalities on the general population. In 2008 approximately 1.3 million deaths were attributed to ambient (outdoor) air pollution exposure, and tripled to approximately 3.7 million deaths in 2012; directly stating that more than 50% of air pollution-related deaths (7 million) globally was due to outdoor air pollution only [
1,
2]. The World Health Organization also estimated that in 2016 ambient air pollution was responsible for 4.2 million premature deaths globally [
3].
Outdoor air pollution in China is undoubtedly a severe environmental and health concern; however, explicit details about ambient air pollution patterns in China, its similarities or differences from other countries and associated adverse health effects in rural areas and small cities are unknown. Some studies have reported the relationship between various air pollutants and health effects, with respiratory diseases (morbidity and mortality) remaining on the forefront in many Chinese cities and globally [
4‐
7]. The respiratory system is directly and easily exposed to the external environment, making it more susceptible to the effects and influences of the surrounding environment. Subsequently, air pollution has been widely acknowledged as a major influence and exacerbating factor on various respiratory diseases such as lung cancer, bronchitis, chronic obstructive pulmonary diseases, pneumonia, asthma, and influenza. Globally, respiratory diseases remain in the top three leading causes of non-communicable diseases (NCD’s) mortality, and therefore it is highly essential to identify related risk factors and appropriate preventive measures clearly. In China, chronic respiratory diseases remained in the top five leading causes of mortality in 2016, accounting for 9% (approximately 870,000 deaths) of the total NCD deaths (9259,000) [
8]. Despite the continuous effort and significant improvements in ambient air quality in China over the past decade, NCD’s were responsible for approximately 89% of all total mortality in 2016, a figure slightly higher than the global proportion of 60% for NCD’s [
8,
9]. With respiratory diseases continuing to be on the rise, the World Health Organization predicts by 2030 respiratory diseases will be the leading cause of morbidity and mortality [
10].
Although strong epidemiological evidence linking exposure to ambient air pollutants and respiratory mortality exists in China (particularly in the metropolis), an urgent need for experimental studies to adequately describe the associations and mechanisms that triggers this health outcome in non-metropolis cities like Xi’an remains. With short-term exposure to ambient air pollution having a significant public health impact that can no longer be neglected, as well as the fact that the complex nature of ambient air pollutants and health effects varies by location; the objective of this study was to assess a potential association between exposure to ambient air pollutants and respiratory mortality in Xi’an.
Discussion
In recent times, the formation of air pollution control guidelines and policy concerning health have been primarily based on epidemiological studies which reveal the adverse effects of air pollution on health. However, the struggle of pinpointing certain air pollutants to specific health outcomes remains. As it has been established, air pollution is a mixture of several pollutants, and these pollutants correlate either positively or negatively. The high correlation of air pollutants results in collinearity, making it difficult to determine the independent effect of a single pollutant. Most studies have faulted in properly adjusting for collinearity and thereby do not evaluate the accurate independent effect of each pollutant. In this study, keen attention was devoted to adjusting for collinearity using the principle component method. Results showed an increase in the effect estimates for all pollutants after adjusting for collinearity.
The current study aimed to explore a possible association between short term exposure to ambient air pollution and respiratory mortality in Xi’an. The results showed that daily ambient air pollutants had clear seasonal differences with PM
2.5 and SO
2 levels higher during the cold seasons. This seasonal trend validates the continued reliance on coal combustion for energy source particularly during winter, in addition to the increase of motor vehicles resulting in elevated traffic emissions in Xi’an [
23,
24]. In contrast, O
3 was higher during the warm seasons; this trend is valid given that photochemical reaction from which O
3 is given off occurs more in the warm seasons due to the presence of sunlight (radiation). The hot and windy nature of the warm seasons also results in further suspension and dispersion of O
3 [
25]. The varying temporal trends between the studied ambient air pollutants may be due to different emission sources and possible confounding meteorological parameters such as temperature and relative humidity [
26]. The role of temperature in aggravating mortality as well as the effects of air pollutants on health has been noted in some studies [
27‐
29]. A study by Middleton et al. revealed that ozone was responsible for respiratory hospital admissions in Cyprus during the warm season [
30]. Of the three assessed ambient air pollutants, only PM
2.5 exceeded the annual average of the National Ambient Air Quality Standard (
GB3095–2012) in China [
22]. In this study, respiratory mortality was generally higher during winter and lower during summer.
Globally, respiratory diseases are on the rise, and various epidemiological studies report that short-term exposure to ambient air pollution causes adverse health effects, including an increase in respiratory mortality [
5,
7,
31]. The three pollutants observed in this study showed positive associations with respiratory mortality. The effect estimates were consistently high before and after adjustment for collinearity by principal component analysis, with ozone having the highest estimates. However, only PM
2.5 and SO
2 associations with respiratory mortality were statistically significant (
P < 0.05). From both associations, the effect estimates of SO
2 was higher than PM
2.5.
In comparison to studies carried out in other cities, our results had similar trends. A study done in the Pearl River Delta Region [
32] associated exposure to PM
2.5 and respiratory mortality with estimates of 1.68(1.00, 2.37), while ours was 1.313(1.032, 1.708). Another study in Wuhan [
33] associated exposure to SO
2 and respiratory mortality with estimates of 1.9(0.2, 3.6) covering a more extensive effect range than ours 1.402(0.827, 2.854). Although the association for O
3 in our study was not statistically significant in the general analysis, the relative risk was however high in the single model and after adjusting for collinearity. Quite a few studies have also reported high effect estimates of O
3 concerning respiratory mortality. A study in Shenyang [
34] reported a high and statistically significant effect estimate of 4.7(0.00, 9.9). The non-significance of O
3 may be attributed to several factors including concentration, duration of exposure and susceptibility of the population [
35].
We investigated the lag relationship of ambient air pollution and respiratory mortality using a single day lag (0–4) and cumulative lag (01–04). For all observed pollutants, the effect estimates of the cumulative lag days were slightly larger than those of the single day lags, which suggests that accumulated exposure of the air pollutants increases the risk of respiratory mortality, a phenomenon that was noted in other studies as well [
36]. Of all the pollutants, PM
2.5 observed more statistically significant lags than other air pollutants. Its best lag noted was lag 2 [1.003(1.000, 1.006)], which was statistically significant and remained unchanged after adjusting for collinearity by principal component analysis; indicating that the confounding effect of other pollutants did not influence the lag effects of PM
2.5. Other studies have also reported statistically significant single model lag effects (precisely lag 2) on respiratory mortality [
19,
37‐
39]. SO
2 observed its best lag at lag 4, with a slight decrease in lag effect estimate after the adjustment for collinearity. Although the relative risk for O
3 multi-pollutant model increased when compared to the single pollutant model, no statistically significant lag effect was observed for ozone in the general lag analysis. Above all, sensitivity analysis of this study showed that effects due to lag exists, further proving that the temporal aspect of air pollution on health outcomes should be a critical factor in the analysis of environmental stressors and health.
Gender and age have been noted as effect modifiers in health assessments and have somewhat been standardized as proper means of population stratification. In this study, subgroup analysis revealed that every 10 μg/m3 increase of PM2.5 concentration was significantly associated with risk of respiratory mortality in the male, female and ≤ 64 years subgroups, no statistically significant association was noted for the ≥ 65 years subgroup. The male subgroup showed a slightly higher relative risk with a narrow confidence interval, as compared to the female’s which was lower with a wider confidence interval. This suggests the effect in the male subgroup was stronger and more precise than the female subgroup. SO2 was statistically associated with respiratory mortality in the male subgroup only. Similar trends of higher relative risk for males and ≤ 64 years and lower estimates for females and ≥ 65 years was also noted for SO2. Although some studies have reported stronger associations of these pollutants in females than males and some consider the ≥ 65 years subgroup more vulnerable than the ≤ 64 years, the differences in these results could be attributed to location, pollution concentration, population size as well as other underlying health factors associated with the population.
To observe the aspect of delayed effects within the population, the lag-response relationship for the subgroups was analyzed. The graphical presentation of the relationship (Fig.
2) showed that the effects of each pollutant in regard to each subgroup did not change or fluctuate greatly in the single day lags. The effects of the single day lag also maintained a narrow interval. In contrast, the cumulative lag showed changes in effects per lag and had a wider interval. The lag-response analysis also revealed a statistically significant association of O
3 and females at lag 0 [0.964(0.938, 0.991)], which was the only significant association (
P = 0.0087) noted for ozone in this study. Most studies have reported associations between ozone and females; a meta-analysis by Bell et al. [
40] revealed that mortality due to ozone exposure was noted more in females [1.12(0.62, 1.63)] than males [0.73(0.40, 1.07)].
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.