Background
Ambient fine particular matter (PM
2.5) pollution has become an important environmental and public health problem and draws great concerns worldwide for its contribution to global disease burden [
1]. In the past decades, China has experienced rapid economic growth, urbanization, and industrialization that cause severe PM
2.5 pollution, which makes it one of the most ambient PM
2.5-polluted countries [
2]. Even though China issued the Air Pollution Prevention and Control Action Plan in 2013 to reduce the air pollution level, the annual average PM
2.5 concentration in 2017 (47.0 μg/m
3) was still higher than the World Health Organization (WHO) guideline level (10 μg/m
3, 2005) [
3].
Respiratory diseases mainly including acute upper respiratory infections, influenza and pneumonia, chronic obstructive pulmonary disease (COPD), and asthma are common diseases and pose a serious threat to health. For instance, chronic respiratory diseases are among the primary causes of morbidity and mortality worldwide [
4]. PM
2.5 could go through the respiratory tract and accumulate in the lung parenchyma which cause respiratory diseases such as acute lower respiratory infections and chronic obstructive pulmonary disease [
5].
Epidemiological studies have demonstrated that exposure to short-time ambient PM
2.5 is associated with increased mortality of respiratory diseases, and each 10 μg/m
3 increment in ambient PM
2.5 level is significantly associated with 0.5~2.0% increased risk of respiratory mortality [
6,
7]. However, there are only a few studies that explored the association between ambient PM
2.5 exposure and respiratory mortality in the elderly, and the results are conflict [
8,
9]. Under the background of population aging, studies in the elderly have important implications as aging has become a significant social and public health challenge. In addition, studies exploring the effect of ambient PM
2.5 exposure on the years of life lost (YLL) from respiratory diseases are scarce. Compared with mortality, the disease burden indicator of YLL is more comprehensive for bringing in life expectancy at death into consideration [
10].
Although the modifications of traditional demographic characteristics in the health effects of PM
2.5 exposure are frequently analyzed, the impact of birth season is rarely taken into consideration. However, birth season is frequently applied as a proxy for a wide range of environmental and other factors exposed in prenatal and early postnatal life such as air pollution, sun exposure, and nutritional status [
11,
12]. These factors in the early life may influence the health outcome in later life. Previous studies have indicated the associations between birth season and mortality [
13,
14]. Thus, whether birth season plays a role on YLL from respiratory diseases in the elderly related to ambient PM
2.5 exposure remains an interesting topic to be explored.
This study aims to assess the impact of exposure to ambient PM2.5 on the disease burden of respiratory diseases by the indicator of YLL in the elderly in Ningbo, China, and to explore whether the modification effect of birth season exists. The results will provide scientific evidence for ambient PM2.5 control and susceptible population protection.
Discussion
In this study, YLL from respiratory diseases was estimated to explore the relationship of ambient PM2.5 exposure and the disease burden in the elderly population in Ningbo, China. Especially, a significant association between ambient PM2.5 exposure and YLL from respiratory diseases in the elderly individuals was found. Furthermore, birth season played a potential modification effect on the relationship that YLL from respiratory diseases related to ambient PM2.5 exposure was higher in the elderly who were born in winter than those born in other seasons.
Ambient PM
2.5 pollution is a great challenge to public health, both in developed and developing countries [
24,
25]. Short-term exposure to low-concentration PM
2.5 is associated with increased mortality as well, which raises concerns about PM
2.5 pollution [
25]. In China, the concentration of PM
2.5 is respectively lower in south areas than in north areas, and higher in winter compared with other three seasons [
26]. The annual average concentration in our research site located in the Yangtze River Delta was lower than the mean concentration of whole China and showed the highest level in winter.
Our results showed that an increase of 10 μg/m
3 in ambient PM
2.5 was associated with 1.06% (95% CI 0.17%, 1.95%) increase in mortality of respiratory diseases, and this result was consistent with previous studies [
6,
7]. Furthermore, relatively stronger effect of exposure to ambient PM
2.5 on mortality of respiratory diseases in the elderly was observed in our study. Li et al. found that each 10 μg/m
3 increment in PM
2.5 was related to an increase of 0.51% (95% CI 0.30%, 0.73%) in respiratory mortality in all-ages [
27]. Sui et al. found that each 10 μg/m
3 in PM
2.5 increased death risk of respiratory diseases by 0.90% (95% CI 0.23%, 1.57%) [
8]. Compared with the former two studies in China, Ningbo showed relatively lower mean PM
2.5 concentration (45.5 μg/m
3 in Ningbo, 84.9 μg/m
3 in Beijing, 66.18 μg/m
3 in Hefei) and higher increased mortality risk. The reason may be that the elderly is more vulnerable to the effect of PM
2.5 pollution on respiratory diseases [
9].
To our knowledge, the effect of exposure to ambient PM
2.5 on YLL from respiratory disease in the elderly was evaluated for the first time. Per 10 μg/m
3 PM
2.5 increase was significantly associated with an increase of 1.61 (95% CI 0.12, 3.10) years in YLL from respiratory diseases in the elderly. Li et al. found that each 10 μg/m
3 increase in ambient PM
2.5 was related to 0.98 (95% CI 0.42, 1.54) years increment in YLL from COPD in the elderly (≥ 75 years), and COPD is one of the major types of respiratory diseases [
28]. Since the mean PM
2.5 concentration of the two studies is similar (45.5 μg/m
3 VS 49.58 μg/m
3), it indicates that COPD may account for major percentages in YLL from respiratory diseases. This recommends us to explore the association of ambient PM
2.5 with YLL from other respiratory diseases such as asthma, which is a common respiratory disease as well.
Birth season has been demonstrated to be a risk factor of population mortality in previous studies. Zhang et al. found that women born in spring and summer had a higher cardiovascular mortality than women born in the autumn [
14]. Our results provided new insights that birth season may increase the disease burden of respiratory diseases in the elderly. As a unique birth season, winter had a potential modification effect on YLL from respiratory diseases related to ambient PM
2.5 exposure. Similar effect has not been discovered in other research up to now.
Chronic respiratory diseases account for an essential part of respiratory diseases and were the third leading cause of death next to cardiovascular diseases and neoplasms in 2017 [
4]. According to the Development Origins of Health and Disease, environmental factors work during the phase of developmental plasticity to change the susceptibility of the individuals to the noncommunicable diseases in later life [
29]. Early life encompassing prenatal and early childhood has the potential to act as a window of sensitivity to ambient PM
2.5 exposure, which increases the risk of chronic respiratory diseases [
30].
Lung development lasts for a long time from utero to early adulthood, and the first year after birth is greatly important to substantial structural development of lung [
31]. The developing lung is highly susceptible to ambient pollutants [
32]. Gauderman et al. showed that exposure to PM
2.5 was associated with delayed development of lung function in a prospective study [
33]. In our study, the ambient PM
2.5 concentration was highest in winter, followed by spring, in Ningbo, China. Compared with other seasons, people spend more time indoors in winter for the relatively lower temperature outdoors. Research found that ambient air was the major source of indoor PM
2.5 in winter in the Yangtze River Delta [
34]. Besides, increased indoor activity and decreased ventilation frequency contributed to high indoor PM
2.5 level in winter. Thus, those born in winter are exposed to higher PM
2.5 pollution level in their early life, and may induce irreversible damage to their lung development which influences their sensibility to respiratory diseases in later life.
Cold temperature is another possible risk factor of respiratory mortality in later life for those born in winter. Research found that cold weather contributed to the incidence of respiratory diseases [
35]. On the one hand, low temperature is beneficial for bacteria to survive in water droplets [
36]. Especially, newborns show evident differences to the adult in immune function, which increase their susceptibility to respiratory diseases [
37]. On the other hand, cold weather and corresponding holidays in China limit accessibility to medical facilities and may delay the treat time. Adverse respiratory events in early life would be related to susceptibility to air pollution in later life.
This study has the following strengths. First, birth season indicates PM
2.5 exposure level in early life may influence lung development in the later life. Nevertheless, the impact of birth season in the respiratory health effects associated with ambient PM
2.5 is rarely explored. To the best of our knowledge, our study analyzed the potential modification effect of birth season on YLL from respiratory diseases related to ambient PM
2.5 exposure for the first time. Second, the effects of PM
2.5 on the respiratory health in the elderly were investigated in our study. With the continuous population aging trend, it is of great public importance to explore the risk factor of respiratory diseases in the elderly. Third, YLL was calculated to assess the disease burden resulting from respiratory diseases in our study. YLL comprises life expectancy at death and assigns different weights to deaths occurred at various ages [
19]. Thus, YLL is more accurate for quantifying premature deaths than mortality and reflects the disease burden of PM
2.5 exposure better. From the public health implication prospect, YLL is more informative for resource allocation and policy-making [
21].
However, some limitations existed in our study. First, our study site was a single city located in South China and had relatively lower ambient PM
2.5 concentration compared with other Chinses cities. Thus, the extrapolation of the results to other geographical areas should be taken with caution. Second, personal PM
2.5 exposure was substituted by fixed monitoring data due to data availability. However, this would cause measurement bias that the effect may step toward null [
38]. Third, the components of PM
2.5 associated with respiratory diseases were not considered in our study. Considering the toxicity of different components of PM
2.5 varied, it is meaningful to explore the effect of PM
2.5 components on YLL from respiratory diseases in the elderly if data are available.
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