1. Introduction
Air pollution has become a major environmental risk to global public health and has considerably increased the world’s disease burden. The World Health Organization (WHO) first reported the negative effects of air pollution on human health in a technical report published in 1958 [
1]. According to data released in 2016 based on the WHO’s new air quality model [
2], approximately 92% of the world’s population resides in places with air quality below the recommended standards of the WHO [
3]. In addition, more than 90% of air pollution-related deaths occur in low- and middle-income countries, most of which are in Asia and Africa [
4], highlighting the severity of air pollution in these countries.
Numerous studies have reported the adverse effects of air pollution on human health [
5,
6,
7,
8], including an increased mortality risk [
9]. According to the WHO, one-third of stroke, lung cancer, and heart disease cases are caused by air pollution. According to the 2017 Global Burden of Disease [
10], 4.1 million individuals worldwide die prematurely because of air pollution every year. Air pollution is the fourth leading cause of the disease burden in China [
11,
12]. Therefore, the public must be made aware of the potential health effects of air pollution. On the basis of health and pollutant toxicity impact assessments and a scientific literature review, the WHO developed a new air quality guideline [
13] to limit air pollution levels and protect the public’s health.
In 1999, the US Environmental Protection Agency proposed the first air quality index (AQI) for reporting, on a daily basis, the air cleanliness or pollution level and its impact on health. Many countries have adopted the AQI to report daily air quality data because it is an easy-to-understand indicator of air quality; however, methods used for the evaluation of the AQI differ among countries [
14]. Currently, the AQI is widely used to evaluate environmental air quality, release risk information in a timely manner, and recommend certain behavioral measures to mitigate the short-term health risks of air pollution.
The AQI reflects only the level of the individual pollutant with the highest subindex and does not consider the possible synergistic effects of simultaneous exposure to multiple pollutants and differences in the characteristics of the exposure–response relationship among various air pollutants in different countries or regions [
14]. Therefore, the AQI does not reflect the no-threshold dose–response relationship between air pollutants and health risks [
15,
16,
17]. To overcome the shortcomings of the AQI, a study conducted in Canada first proposed and developed the air quality health index (AQHI), which is used to estimate the combined health effects of multiple air pollutants and evaluate air quality [
18]. Since 30 December 2013, Hong Kong has been officially reporting the AQHI to inform people of the short-term health effects of air pollution. Mason et al. [
19] were the first to evaluate the AQHI and its relationships with specific respiratory diseases (respiratory tract infection, asthma, chronic obstructive pulmonary disease, and pneumonia) in Hong Kong. They discovered a 14% decline in hospital admissions for respiratory infection immediately after the Hong Kong government started reporting the AQHI. The findings of an age-specific analysis revealed significant decreases in hospitalizations for respiratory tract infection and pneumonia in children. In contrast to the AQI, the AQHI provides information on the short-term combined health effects of multiple air pollutants. Thus, the AQHI can be used to provide guidance on outdoor activities for the general population and specifically for those susceptible to air pollution [
20]. Several time-series or case-crossover studies have provided strong evidences for the short-term health effects of air pollution [
21,
22]. Different individuals respond differently to air pollution even when exposed to identical levels of pollution. Thus, investigating the adverse health effects of air pollution and identifying factors that increase susceptibility to these health effects are critical for elucidating the mechanisms underlying the adverse health effects of air pollution and lowering health risks in targeted populations.
Because factors, such as season, time, geographical location, and weather changes, affect air quality, determining the environmental air pollution level at a specific time and location and effectively conveying information on the health effects of that air pollution to the public are critical topics in epidemiological research. Because of differences in the relationship between air pollution and health risks among differing regions, the AQHI of one country or city cannot be directly applied to another country or city. To ensure that the AQHI accurately reflects the impact of air quality on public health and can be used to effectively guide the health behavior of local residents, the link between local health and air pollution data should be established. Moreover, these data should be used to develop a local exposure–response model, construct a localized AQHI in response to regionally complex and changeable air pollutants, and accurately and reasonably assess air pollution. These activities can facilitate the construction of a reasonable evaluation index for providing public health recommendations.
Some cities in China (e.g., Hong Kong, Shanghai, Tianjin, and Guangzhou) employ an AQHI [
17,
19,
23,
24,
25,
26]. The air pollution in Beijing, the capital of China, has become severe due to the rapid development of China’s economy, and this pollution has exacerbated citizens’ health problems. Although scholars [
27,
28,
29] have established an AQHI for Beijing, most have considered only two to three pollutants and investigated their health effects; factors increasing the susceptibility of individuals to the health effects of air pollution have not been identified. Because of limitations in monitoring data (prior to 2010), studies have used data on particulate matter (PM) with an aerodynamic diameter of ≤10 μm (PM
10), but ignored PM with an aerodynamic diameter of ≤2.5 μm (PM
2.5), which is more harmful to humans. Thus, identifying factors that increase the susceptibility of individuals to the health effects of air pollution is crucial in investigating the health risks of air pollution and minimizing these health risks in targeted populations. In the present study, the health and air pollution data of Beijing in recent years were employed to construct an AQHI for Beijing and analyze the susceptibility of various populations.
4. Conclusions
In this study, an AQHI for Beijing was successfully constructed using a GAM and environmental, meteorological, and health data from 2018 to 2020. Factors increasing the susceptibility of various populations to the health effects of air pollution were determined. The results revealed that even if exposed to an identical level of air pollution, different populations responded differently. Older adults, women, and those with respiratory disease were discovered to be more susceptible to the short-term health effects of air pollution. For these susceptible groups, the S-AQHI for different populations (men, women, those with lung cancer and chronic respiratory disease, those aged >75 years, and those aged ≤75 years) was established. The results indicated that the AQHI and S-AQHI are not different in terms of changes in the percentage of daily mortality of various specific populations. This indicates the universality of the AQHI in predicting health risks and demonstrates that the construction of the S-AQHI for different age groups, genders, and diseases is unnecessary. Moreover, we discovered that the exposure–response relationship between the AQHI and total mortality risk of citizens is linear. AQHI is correlated with AQI, and the AQHI can be used for air quality and health impact assessments. Each IQR increase in the AQHI and AQI results in an increase of 1.894% and 1.029%, respectively, in total daily mortality, indicating that the AQHI is more capable than the AQI in predicting the daily mortality of citizens. This study constructed an accurate and reliable Beijing AQHI for evaluating the short-term health effects of air pollution in Beijing. In this study, health risk assessments for different populations and susceptible populations were conducted in the study area. The findings serve as a scientific reference for the revision and proposal of environmental air quality standards and policies and can be used to guide citizens and susceptible populations in adopting health-related behaviors.