Numerous research studies in the literature analyze the impact of construction sites on air quality. The case study in Qingyuan, China, found that the average daily concentration of total suspended particulates (TSP), namely PM10 and PM2.5, surrounding a construction site increased by 42.24%, 19.76%, and 16.27%, respectively [
14]. The large diameter particulate matter is the most prominent pollutant caused by the construction site and mainly contributed to the city’s dust [
15,
16]. In another case study of Germany’s construction sites, Faber et al. (2015) found that the PM10 emissions from building construction sites contribute to 44% of the total PM10 emissions from total construction activities in Germany.
Hence, construction sites result in many air pollutants and damage to air quality. Still, the specific influential factor related to the construction site on air quality is much more complicated. For instance, pollutant emissions can be caused by various mechanical processes, including transporting and handling bulk materials, drilling, sawing, milling, compacting, and grading the ground [
17]. Additionally, the transportation of construction vehicles on the surrounding dirty and unpaved temporary roads may lead to the same order of magnitude or even higher than those caused by other construction site activities [
14,
17,
18]. For instance, 6% of NOx pollutants and 10% of traffic-related PM emissions in Germany have resulted from mobile construction vehicles [
19]. In terms of the mechanical and thermal building process, the combustion exhaust of machinery will increase particle and trace gas emissions from construction sites [
20]. In addition, influential factors from the external environment will also affect the air quality in the vicinity of construction sites. Wind, humidity, and temperature can affect pollutant emissions. Existing scholarly research found that the pollutant emissions caused by construction sites have an imperatively positive correlation with wind speed, relative humidity, and that they have a weakly correlativity with temperature [
21]. According to Araujo’s research [
22], weather conditions could also have a potential effect, but the correlation cannot be further proved due to limitations of the size and complexity of the construction sites studied. Two factors of ‘air pollutant diffusion’ and ‘air quality parameters’ are discussed below.
2.1. Air Pollutant Diffusion
The diffusion law is an imperative aspect of research on building construction dust [
14]. Different air diffusion methods directly impact the recording data of various climate stations in Hangzhou. Firstly, studies about diffusion law found that pollutant dispersion occurs from construction sites, and that the pollutant concentration decays at increasing distances or proximities [
23]. In other similar studies of pollutant diffusion, Hitchins et al. [
24] determined the PM concentration at an increasing distance at two sides of a road in Australia. They found that PM2.5 and ultrafine particles can decay to around 50% of the maximum, occurring at 100–150 m from the road. Additionally, according to the concentration results in weekly traffic conditions in Italy, it is reported by Buonanno et al. [
25] that PM10 concentration would decrease exponentially away from the freeway. Secondly, wind can change the way of dispersion and affect the decay rate [
23]. According to the measurement results of monitoring construction sites by Azarmi et al. [
23], the concentration level of PM2.5 and PM10 increase when wind direction is from construction sites to the monitoring station. Another case study of Azarmi and Kumar [
23] in Haywards Heath in West Sussex, United Kingdom, indicated that the particular matters emitted by the demolition process in construction sites are much more significant in the downwind direction and decreased logarithmically with downwind distance.
All research studies considering the concentration of pollutants have defined locations because construction sites’ location and the surrounding environment data pose some uncertainties. For example, in Nakada and Urban’s case study of air quality in São Paulo State, Brazil [
9], the decrease in pollutant concentration in the main urban road was lower than in the other analyzed areas. Their result is probably due to the effects of transportation that was connected with several highways. Furthermore, the height of construction would also have an impact, because it determined the emission source. Based on a numerical simulation of dust dispersion at the urban building construction site by Wen [
26], the influence of construction dust from pollution sources in a high position on surrounding air quality is much lower than that from pollution sources in a low position. This fact indicates that the advantage of height is conducive to the rapid dilution of dust and reduces air pollution in the surroundings.
2.2. Air Quality Parameters
Gabriele et al. drew attention to the fact that the risk of infection and death by the COVID-19 pandemic could be associated with long-term exposure to air pollutants [
7]. For instance, for every 1 μm/m
3 above the mean, the infection ratio increased by 2.7% for NO
2 and 3.0% for PM10 [
27]. Therefore, proper selection of air parameters is the imperative precondition of a comprehensive and precise correlative mathematical model. The air quality index (AQI) is selected here to indicate the overall urban air quality, which is widely used by governmental environmental protection agencies and scientists worldwide [
28]. The AQI refers to a combination number representing the total air quality and pollutant concentration [
29]. It is calculated by the concentrations of each main category of pollutants. Other air quality parameters are selected based on the ‘Air Pollutant Guidelines’ provided by WHO, involving particulate matter (PM), ozone (O
3), nitrogen dioxide (NO
2), and sulfur dioxide (SO
2) [
1,
30].
Particulate matter (PM) is a common proxy indicator for air pollution [
31]. PM10 (
) and PM2.5 (
) are selected as air quality indicators in this research, as they are two kinds of respirable particulate matter air pollutants mainly related to construction work. More importantly, they pose a formidable public health threat in cardiovascular and respiratory disease, as well as in cancers [
32,
33], leading to approximately 4.2 million premature deaths worldwide in 2016 [
31]. In this situation, WHO air quality guidelines provide strict and detailed guide values and interim targets for PM2.5 and PM10. Current research results indicate that the PM2.5 and PM10 mainly generated by dust at the constructions site would impact air quality to a certain degree [
12,
15,
16,
17,
33,
34,
35]. In terms of PM2.5, it normally suspends in the air for a longer time and has a longer and worse influence on air quality than PM10 due to its small diameter [
32]. Furthermore, PM2.5 has a seasonal characteristic due to its seasonal impact of gardens and farms in the city [
36]. In a case study of construction sites and projects in London, the PM10 concentration in the working period was about 2.2-fold higher when compared with the non-working period [
16]. However, according to the analysis result of case studies in Qingyuan city, the impact was limited in a range [
34]. The fact that PM10 concentration exceeds the limit is mainly caused by the external atmospheric environment rather than the construction site itself (ibid). Particulate matter is more special and crucial during the course of the COVID-19 pandemic as it is likely related to the infection ratio. Indeed, PM10 is suggested as an indicator with relevance to the majority of the epidemiological data [
30]. To a greater extent, PM10 has been an independent predictor of the spatial spread of COVID-19 [
27]. For instance, Gabriele et al. [
7] analyzed the correlations between atmospheric pollutant concentration and spatial-temporal distribution of cases and deaths. Their studies found that PM2.5 and PM10 had a higher non-linear correlation than NO
2 and other air parameters.
Furthermore, ozone (O
3), nitrogen dioxide (NO
2), and sulfur dioxide (SO
2) are air pollutants that can have a marked effect on human health [
31]. Their effects include nose and throat irritation, lung inflammation, ischemic stroke, the triggering of asthma, etc. [
30,
31,
37,
38,
39,
40,
41]. The sources of these three air pollutants are complicated to identify. It is said that they are partly relative to the construction project, but mainly result from the combustion of fuels in industry and vehicles [
30,
31,
42]. In the combined impact of building sites on air quality, these air quality parameters (i.e., air quality index, PM10, PM2.5, O
3, NO
2, and SO
2) would be selected in this research to provide a comprehensive and accurate assessment of air quality.
Many scholars and institutions have regulated the guideline value or the limit value of the pollutant concentrations. These are shown in
Table S1 of Supplementary Materials, indicating China’s ambient air quality standard, published by the Ministry of Environmental Protection (MEP) in China [
43]. Generally, most Chinese cities should be within the level-1 concentration limit, including Hangzhou city. In special cases, it can exceed the level-1 limit to some extent, but it cannot exceed the level-2 limit. Therefore, the level-1 and level-2 concentration limits are the primary and secondary references, respectively.
In addition, WHO has defined the guidelines and the interim target of air quality parameters in the second edition of their report on ‘air quality guidelines for Europe’ [
44]. The detailed values are shown in
Table S2. The guideline value represents the health effect that poses lower or no risk to the public. As highlighted by WHO [
31], the interim target combines the observation in the studies on long-term health effects and the necessity of urban development. Based on the existing literature review on the topic, the impact of construction sites on air quality and pollutant emissions are complicated and related to multiple dimensional factors. Due to the limitation of the effect’s extent, construction sites usually affect the air quality by environmental and constructional factors. In the same environment and construction activity situation, a proper micro-management plan of the construction sites (i.e., on numbers and locations) can help the local governments make better decisions, especially during COVID-19 and similar disruptive events.