1. Introduction
Fresh and breathable air is critical to sustaining life forms. Unfortunately, air pollution has become a worldwide concern, impacting climate and human health and increasing morbidity [1–3]. In Asia, non-exhaust emissions have increased significantly over the past 50 years, resulting in almost 92% of the population breathing polluted air, which accounts for 70% of air pollution related deaths in this region [4]. The World Health Organization (WHO) evaluates that PM2.5 “cause about 8% of lung cancer deaths, 5% of cardiopulmonary deaths and about 3% of respiratory infection deaths” [5]. The presence of air pollution has an impact on climate change, leading to an anticipated rise in global temperatures between 1980–1999 and 2090–2099 by 1.8–4.0°C based on various emission scenarios [6]. The pollution scenario is a grave concern in developing countries due to overpopulation, rapid urbanization, industrialization, and deforestation [7–9]. Air pollution comes from two main sources: ‘non-anthropogenic sources’ like volcanic eruptions, bushfires, and radioactive decay, and ‘anthropogenic sources’ which can further be categorized into stationary sources (different factories, power plants, industries, agricultural practices) and mobile sources (transportation-related emissions) [10,11]. Transportation-related sources are further divided into exhaust and non-exhaust emissions [12]. ‘Exhaust emission’ indicates vehicles’ emissions powered by fossil fuels (petrol or diesel) due to incomplete combustion. However, ‘non-exhaust emission’ refers to particles emitted from brake wear, tire wear, road wear, and even resuspended road dust [13–15]. Though vehicular exhaust emissions are primary contributors to particulate matter (PM) pollution, sometimes, they can be surpassed by non-exhaust emissions [16, 17]. PM is a complex mixture of solid and liquid particles suspended in the air [18]. PM can be classified into ‘coarse PM’, ‘fine PM’, and ‘ultrafine PM’ based on particle size, which has been associated with deposition in the respiratory tract [19–21]. Coarse PM (PM10 with an aerodynamic diameter ≤ 10 μm) is mostly generated from natural or commercial sources (construction sites, industries, mines). ‘Fine PM’ (PM2.5 with an aerodynamic diameter ≤ 2.5 μm) and ‘ultrafine PM’ (PM0.1 with an aerodynamic diameter ≤ 0.1 μm) are generated from traffic-related sources [18, 22]. Non-exhaust emissions mainly consist of coarse PM and a considerable amount of PM2.5 [23]. Coarse particles can penetrate the upper respiratory tracts, whereas fine particles can reach up to the deeper region of the lungs and generate high oxidative stress [24]. Ultrafine PM can even enter the blood and reach different organs [25]. Both coarse and fine PM can cause several cardiovascular and respiratory diseases, asthma, and chronic obstructive pulmonary disease (COPD). Moreover, coarse PM can increase the mortality rate, while prenatal exposure to fine PM increases ‘autism spectrum disorder’ (ASD) risks [26, 27]. Exposure to PM for both short and long duration can induce morbidity and mortality from carcinogenicity, pulmonary and cardiac diseases [28].
Polycyclic aromatic hydrocarbons (PAHs) or heavy metals bound with PM can cause chronic diseases and toxicity [12]. Inhabitants, especially children, have significant health risks due to continuous exposure, as it can enter through inhalation, ingestion, and dermal contact [29, 30]. PM-bound heavy metals can accumulate in the fatty tissues or circulatory systems, affecting the normal functioning of the central nervous system and inducing carcinogenic, mutagenic, and teratogenic effects [31, 32]. Especially high concentration of Pb affects the brain tissues and the central nervous system and increases cardiovascular-related mortality and morbidity [33, 34]. On the contrary, Cd, As, and Cr have high carcinogenic risks [35, 36].
Non-exhaust PM poses a higher health risk than exhaust PM, causing DNA damage, lung inflammation, and premature mortality [26, 37–38]. Brake and tire wear PM have higher oxidative potential generating Reactive Oxygen Species (ROS) [39–41]. Studded tires and brake wear particles can induce toxicity and mitochondrial depolarization, having a higher oxidative potential than diesel engines and other non-tailpipe sources [12, 42]. Brake wear particles with high Cu concentration can reduce cell viability, disrupt mitochondrial membrane integrity, modulate cardiac rhythm, increase supraventricular beats’ frequency, and induce a prothrombotic response in non-smoker males (23–30 years) [43]. Braking events further convert Sb2S3 into Sb2O3, a potential human lung carcinogen [44]. Fig. 1 illustrates particulate emission from different exhaust and non-exhaust vehicular sources and their health impacts.
Researchers have recently focused on non-exhaust PM due to its contribution to overall particulate pollution and potential health effects. The implementation of new exhaust emission regulations has resulted in a rise in non-exhaust emissions, which now exceed the amount of exhaust emissions. Although non-exhaust particles may be coarser, they can still cause serious health risks by irritating the respiratory system and causing inflammation. Therefore, it is crucial to quantify and understand the impact of non-exhaust PM to determine how road design, urban planning, and alternative modes of transportation could decrease overall particulate pollution. However, there is currently a lack of studies exploring non-exhaust emissions and their health impacts in Asian countries. Of this interest, this review aims to fill this gap by shedding light on particulate pollution from non-exhaust emissions, specifically in the Asian context, to assess the current status and its health consequences, encouraging future researchers to plan mitigation measures to reduce air pollution, improve air quality, and provide a roadmap for better possibilities for environmental sustainability.
2. Non-Exhaust Emissions: Sources, Chemical Composition, and Emission Factors
Since the dawn of civilization, humans have striven to create objects that meet their needs. However, these activities increase pollution, leaving detrimental impacts on the atmosphere. Exhaust and non-exhaust vehicular emissions are significant contributors to environmental pollution, especially particulate pollution [45].
2.1. Sources
The sources of non-exhaust emissions can be categorized as follows.
2.1.1. Brake wear
Brake wear particles are abraded due to the frictional contact between a rotating disc/drum and a brake pad during braking events. Automobiles employ two types of brakes: disc and drum brakes. Disc brakes use flat pads, pressing against a circular metal rotor, while drum brakes use curved shoes, pressing inside a rotating cylinder [13]. Front brakes provide approximately 70% of the total braking power and wear down faster than rear brakes [46]. Vehicle brakes experience frequent and intense abrasion, generating high temperatures that degrade linings and rotors, producing micron-sized particles [13, 47]. The emissions rates depend on the vehicle’s weight, deceleration rate, contact pressure, rotor temperatures, sliding speed, and material composition of brake discs and pads [48, 49].
2.1.2. Tire wear
Tire wears are the particles aroused from frictional events between tires treads and road surfaces. Tire comprises various polymers and fillers [50]. Primary components are black carbon, fibers, steel, polyaromatic hydrocarbons, and other organic and inorganic compounds, and the core part is composed of polymers [51]. Tire particles’ emissions depend on various factors such as climatic, road and driving conditions [48]. Coarse particles are mostly generated through mechanical friction and shear, whereas fine particles are produced by excess heating [50].
2.1.3. Road surface wear
Road wear is regarded as airborne particles (primary and secondary PM) discharged from road surfaces built of asphalt or concrete; sand, cement, or coarse aggregates are the most common materials used to make road surfaces [52]. Porto et al. [53] pointed out that various polymers and fillers like silica, clay, and waxes can be blended with road surfaces. Higher temperatures can cause to release particles that subsequently scatter in the air. Friction between the tire and the road surface abrades the road surface, which in turn releases airborne particles [50].
2.1.4. Resuspended road dust
Resuspended road dust refers to particles on paved surfaces that become airborne due to wind turbulence, tire shear, and vehicular movement. These particles can be composed of wear particles from brakes, tires, and the road surface itself. In recent decades, road dust has emerged as a significant source of pollution, affecting air quality [50]. Climatic factors influence road dust sources significantly; studded tires are the significant contributors in a cold region, whereas tire and brake wear particles are the dominant components in urbanized areas [54, 55].
2.2. Chemical Composition
2.2.1. Brake wear
Brake wear particles are emitted from the brake pad and brake disc, which contain various inorganic metals such as Cu, Zn, Fe, Ti, and Pb, as well as barium sulfate, sulfate silicate, carbon fibers, and graphite [48, 50]. Chemical composition largely varies from the original brake lining material. Sb has been detected as a potential marker of brake wear particles due to its significant concentration in braking materials [48]. Along with Sb, Ba, Cu, and Fe are also dominant in brake wear [56]. However, Grigoratos & Martini [13] noticed that modern braking linings have significantly reduced lead concentration.
2.2.2. Tire wear
Tire wear particles consist of metal components as well as organic components like rubber copolymer, tars, soot, and organotin [45]. The main constituents of tires are rubbers, processing oil, fillers, reinforcements, additives, and vulcanization agents. The properties of particles differ significantly across different types of vehicles. For instance, truck tires comprise 80% natural rubber material, whereas the amount is 15% in passenger cars. The differences in component properties can be markers for identifying tire wear particles [12]. Inorganic components like Si, Ca, Ti, Cu, Al, K, Fe, Pb, and Mg are also enriched in tire particles. Gustafsson et al. [57] pointed out that Zn is a potential marker of tires, particularly friction ones, due to the significant concentration of Zn.
2.2.3. Road surface wear
Due to the complex characteristics of road surface particles, chemical composition determination is quite difficult. Road surface particles contain cement, bitumen, asphalt, resin, and inorganic metals like Ca, Si, K, and Fe [45, 48]. The road surface wear mainly consists of small mineral fragments dominated by concrete and asphalt [48]. Particles from various exhaust and non-exhaust sources are deposited on the road surface and resuspended by wind or vehicular turbulence, making distinguishing road surface particles from the mixture challenging. Moreover, particles from different exhaust and non-exhaust sources are deposited on the road surface and resuspended by wind or vehicular turbulence, making it difficult to distinguish road surface particles from the mixture and determine their chemical composition [50].
2.2.4. Resuspended road dust
Road dust can be either natural or anthropogenic in origin and its composition can be influenced by a variety of factors such as weather, location, road structure and maintenance, vehicle traffic, and driving conditions [58, 59]. The predominant material in road dust is crustal, but it can also contain high concentrations of toxic heavy metals such as Pb, Cu, As, Sb, Cd, Ni, and Zn, as well as brake and tire wear particles. The ‘X-ray diffraction’ (XRD) mineralogical characterization also disclosed that alkali feld-spars, quartz, clay minerals, and carbonate are present in roadside dust. Crustal enrichment factor (CEF) is used to distinguish anthropogenic particles from natural crustal sources [48, 50]. However, further research is needed to distinguish between road surface wear particles and dispersed road dust, and isotopic tracers may be a useful tool in identifying particle sources.
2.3. Emission Factors
Emission factors are the amount of a specific pollutant released per unit distance traveled or energy/fuel consumed. It is an effective tool for researchers and regulatory bodies to quantify pollutant emissions [60].
2.3.1. Brake wear
Brake wear particles can be emitted at varying rates, depending on driving behaviour, such as the frequency and intensity of braking [61]. Almost 16–55% of the total non-exhaust PM10 emissions are contributed by brake wear particles. These PM emissions are higher on urban roads, where braking events occur more frequently [13]. 86% of the released airborne brake particles were PM10, 63% were PM2.5, and 33% were PM0.1 (Fig. 2). European Environment Agency [52] Emission Inventory Guidebook measures these emissions in milligrams per kilometer traveled annually, ranging between 2.9–8.1 (PM10), 2.1–5.5 (PM2.5), and 1.2–3.1 (PM0.1) mg km−1 vehicle−1 respectively in light weighted vehicles. On the contrary, heavy-duty vehicles’ emissions range from 0–80 mg km-1 vehicle-1 for PM10 and 0–15 mg km-1 vehicle-1 for PM2.5. Heavy-duty vehicles emit approximately ten times more tire wear particles than light weighted vehicles and passenger cars [52].
2.3.2. Tire wear
Compared to brake wear, tire wear emits significantly lower PM10 (5–30%) and PM2.5 (4–7%) [47] (Fig. 2). However, less than 10 percent of tire wear is emitted as PM10 under usual driving conditions. The amount of tire wear emissions is influenced by speed, tire type, road surface, and driving conditions [47, 62]. Both direct measurements and modeling techniques have been developed to determine the emission factor of tire wear. Tire types are a significant driving factor in the generation of dust particles. Winter tires (128 mg km−1 vehicle−1) produce significantly higher dust compared to their summer (3.8 mg km−1 vehicle−1) counterparts due to increasing ground friction during winter [63, 64]. Studded tires with metal studs emit significantly higher PM10. Therefore, some countries restrict their use during certain times of the year or on certain roads. For instance, the studded tire is legal in Sweden only from October to April [45, 47].
2.3.3. Road surface wear
European Monitoring and Evaluation Programme and European Environment Agency employ the methodology of Klimont et al. [65] to estimate road surface wear’s emission factors. Klimont et al. [65] addressed the challenge of measuring particle emissions from road surface wear by subtracting factors such as tire wear, brake wear, and re-suspension from the total non-exhaust emissions. However, their study presents some limitations due to the high uncertainty of the values and the limited information obtained [50].
2.3.4. Resuspended road dust
The unusual chemical properties of resuspended road dust can influence emission factors. The composition of dispersed road dust, which includes tire and brake wear particles and road surface wear, makes it challenging to determine emission factors. In order to estimate these factors, direct measurements and modeling techniques are employed. The United States Environmental Protection Agency (USEPA) established a protocol for collecting road dust from paved and unpaved roads recommending a site-specific silt-loading collection [59]. Road dust dispersion is the predominant non-exhaust PM emitting source. The resuspension of road dust accounts for 28–59% and 9–56% of non-exhaust PM10 and PM2.5 emissions relatively [47, 59] (Fig. 2). PM10 emission factor ranges from 49 (asphalt-based road), 330 (cobbled-stoned road), 187–733 (heavy-duty vehicles) and 33–131 (light-duty vehicles) mg km−1 vehicle−1 [66, 67]. Higher driving speeds increase the concentration of particles, particularly for studded and non-studded tires; however, some studies have found that faster driving speeds can reduce dust emissions [57, 66, 68].
3. Particulate Pollution: The Exhaust Non-Exhaust Ratio
Although exhaust emissions are the primary PM source, non-exhaust sources can sometimes contribute more due to varying characteristics, numbers, and ages of traffic-related sources. For instance, China’s non-exhaust PM emissions were seven times higher than exhaust emissions [16]. In Delhi, a megacity representative of developing countries like India, non-exhaust emissions were found to be almost six times higher than exhaust emissions [69]. Exhaust sources mainly emit fine PM comprised of hydrocarbons, while non-exhausts emit coarse PM comprised of various heavy metals [56, 62,70–71]. As a result, both exhaust and non-exhaust sources generate different types of secondary PM. Exhaust emissions contain volatile organic compounds (VOCs) that can react with sunlight to produce organic secondary PM, while non-exhaust sources mainly generate inorganic secondary PM [23, 72, 73].
Though exhaust pollution is improving gradually, it differs in different regions [23]. For instance, exhaust emission generates 20–30% PM2.5 per year in Delhi, whereas the amount is 10.7% (PM10) and 16.8% (PM2.5) in China [74, 75]. The global status of non-exhaust emissions is also relatively scary. Tire wear emissions are 1000 times worse than conventional car exhaust emissions (4.5 milligrams/kilometer), generating approximately 5.8 grams/kilometer [76].
Furthermore, tire wear significantly contributes to microplastic emissions, which can adversely affect human health: 30 percent in Germany and more than 50 percent in Denmark [77]. Moreover, 3–7% of PM2.5 is generated from tire wear and its components, which resulted in 2.7 and 2.9 million death in 2017 and 2019, respectively [78, 79]. Almost 1.3 million tons of tire wear in Europe is generated annually [77, 80]. In 2019, the UK government’s ‘Air Quality Expert Group’ reported that non-exhaust emissions from road wear accounted for 60% and 73% of PM2.5 and PM10, respectively [76]. Amato et al. [81] described that 10% of PM10 is generated due to road dust resuspension.
A recent study by OECD [49] indicated that if demand for electric vehicles increases by 4%, non-exhaust emissions will increase by up to 53.5% by 2030, which could increase PM emissions by 52.4%. Compared to internal combustion engine vehicles, light-weight electric vehicles emit PM10 and PM2.5 in lower amounts, with reductions of 18–19% and 11–13%, respectively. However, electric vehicles emit 4–7% less PM10 and 3–8% more PM2.5 than regular vehicles [82, 83] (Fig. 1). Non-exhaust emissions are identified as the leading source of pollution that generates 60% and 73% of PM2.5 and PM10, respectively, according to a report by Emissions Analytics [76] (Fig. 1).
Emission trends suggested that non-exhaust sources are expected to contribute 80–90% of particulate pollution by 2020, up from 50% in 2000 [84]. In Germany, PM2.5 emissions from traffic sources increased from 25% in 2000 to 70% by the end of 2020 [85]. Even vehicles with zero exhaust can still contribute to fine and ultrafine particulate matter emissions, indicating non-exhaust emissions will continue to be a potential source of particulate matter in the future, responsible for almost 90% of total emissions by the end of 2020 [84, 86–87].
4. Prevalence of Respiratory and Cardiovascular Diseases in Asian Countries: The Developed, Developing, and Underdeveloped Agglomerations
Asia is a diverse and vast continent, with over one-third of the world’s population residing here [88]. Asia Population [89] reported that the population of Asia is 4.7 billion, which accounts for 60 percent of the global population, with China and India having the most inhabitants at 31.69 and 29.36%, respectively. The Maldives is the least populated country, with 99 percent of its land being water. The population of Asia is projected to reach 5.3 billion by 2050, presenting opportunities and challenges for sustainable development and resource management. As Hubert H. Humphrey famously said, “Asia is rich in people, rich in culture, and rich in resources. It is also rich in trouble,” acknowledging the continent’s diversity and richness while highlighting the challenges that come with high population density, such as resource depletion, environmental degradation, and political instability.
Since 1960, the Asian continent has been experiencing growth, although the growth rate varies across countries [90]. The continent comprises the most developed, developing, and underdeveloped countries. Hong Kong, Japan, South Korea, and Taiwan are the world’s most developed countries [88, 91]; however, it also contains underdeveloped countries like Bangladesh, Nepal, Afghanistan, Cambodia, Bhutan, and Myanmar, which the United Nations categorizes based on factors like the Human Development Index (HDI) and national income (GDP) [92]. Moreover, India, a developing nation because India’s GDP and HDI are still at developing levels, is also on the same continent [93]. The diversity in development levels across the Asian continent is attributed to various factors such as historical, political, and economic differences among countries.
Asian countries with worse air quality share a common trait: most are ‘developing’ or ‘underdeveloped’ due to lacking pollution knowledge and enough budget to implement a pollution control strategy [88]. Surprisingly, an advanced country like Japan is ranked 92nd in ‘118 most polluted cities’ worldwide [94]. However, advanced countries like Japan, Hong Kong, and Taiwan can prevent the level of air pollution efficiently with the help of better technologies [88]. In developed countries, the concentrations of PM2.5 reduced from 15.27 to 14.91 μg/m3, whereas it significantly increased from 19.91 to 21.57 μg/m3 in developing countries [95].
Asia is the largest and most populous continent, with diverse ethnic, linguistic, and religious groups [96, 97]. HUGO Pan-Asian SNP Consortium [98] found that relatedness is higher among individuals from the same linguistic or ethnic groups, despite significant gene flow. South Asians comprise about 25% of the global population but account for 60% of all heart disease patients due to genetic and lifestyle factors like a high-fat diet and lack of physical activity. South Asians also had higher cardiovascular risks and coronary artery disease (CAD) due to higher lipoprotein levels than other ethnic groups [99,100]. Volgman et al. [101] also reported occurrence of ‘atherosclerotic cardiovascular disease’ (ASCVD) was higher among South Asians than among the East Asians group; exposure to ambient PM can exacerbate these risks, highlighting the need for targeted interventions and policies to reduce exposure and increase awareness and screening for ASCVD in this group. Moreover, Further research is needed to understand these disparities and develop interventions to reduce cardiovascular disease burden in these populations.
5. Non-Exhaust Pollution Scenario in Asian Countries
Non-exhaust pollution scenario differs in different Asian regions. Despite stricter ‘air quality regulations’, many air pollution occurrences have been noticed in Asian countries in the past decades, both eastern and southern sides [30, 102]. IQAir [103] reported, “Countries and regions in East Asia, Southeast Asia, and South Asia suffered from the highest annual average PM2.5 concentration weighted by population”. Non-exhaust pollution scenario in different cities of Asia is documented in this section (Table 1). Fig. 3 represents the total number of non-exhaust-related publications in Asian countries.
5.1. Bangladesh
Bangladesh has the worst air quality in the world. ‘Dhaka’, the capital of Bangladesh, is situated in the middle of the country; almost 20 million people live here. As the population continuously increases, vehicles also increase, contributing significantly to air pollution [104]. ‘Bangladesh road transport authority’ [105] reported that the number of cars had tripled, whereas the number of auto-rickshaws increased sevenfold in the last 14 years in Dhaka. Begum et al. [106] found that ‘road dust’ is a potential non-exhaust source. Haque et al. [107] investigated potential health risks posed by toxic metals (Pb, As, Hg, Cr, and Cd) found in road dust from Dhaka, which revealed that the mean concentration levels of Pb, Hg, and Cd were significantly higher than their corresponding background values, suggesting that exposure to these toxic metals may pose health risks to humans. Begum et al. [106] reported that particulate pollution from all other sources, including road dust increased in six years (2002 to 2008–2009). Alarmingly, the concentration of fine particles is higher than the Bangladesh National Ambient Air Quality Standard (BNAQS) of 15 μg/m3, indicating a severe problem.
5.2. China
According to WHO, almost 83 percent of Chinese residents breathe polluted air [108]. According to the Ministry of Environmental Protection, 7.8% of the total car in China cross the minimum national standard, which is one of the probable reasons for air pollution [109]. Non-exhaust emissions significantly contributed to coarse & fine PM, with 65.6% and 29.1% respectively. In contrast, exhaust sources only contributed 10.4% and 20.7% to these pollutants. Among non-exhaust sources, road dust resuspension was the highest contributor to coarse PM, accounting for 29.6%. Meanwhile, road-tire abrasion dominated fine PM, accounting for 12.3% of its mass [110]. Zhang et al. [111] analyzed non-exhaust emissions in tunnels of four megacities in China, revealing that non-exhaust sources emit both coarse and fine particles. Road dust is the leading non-exhaust emissions source; road dust contributes 54–69% of total PM10, and 11–34% of total PM2.5, whereas tire particles emit 4–7% PM10 and 3–10% PM2.5 and brake wear emit 1–3% PM10 and 0.4–5% PM2.5 (Fig. 2). The amount of particulate emission from non-exhaust sources is significantly higher in the whole country, with the amount of particulate emission varying based on road surface and gradient [111]. Nanjing, an urbanizing and developing city, also experienced poor air quality and increased air pollution due to the increasing vehicles and anthropogenic and industrial activities. Compared to exhaust, non-exhaust pollution became prominent because of the increasing quantity of electric vehicles. The obtained result also revealed road dust as a potential non-exhaust source [112].
5.3. India
Non-exhaust emissions are the dominant contributor to PM2.5 emissions in both urban and rural areas. In urban areas, the annual PM2.5 emissions are contributed to by exhaust emissions (15%), tire wear (4%), brake wear (3%), road surface wear (3%), and resuspended road dust (75%). Meanwhile, rural areas have slightly different percentages: exhaust (8%), tire (1%), brake (0.4%), road (1%), and resuspended road dust (89%) [113]. Delhi is one of the most polluted megacities in India [114]. Though CNG-driven vehicles can reduce exhaust emissions, they cannot minimize particulate emissions due to non-exhaust emissions. So, understanding these emissions characteristics is crucial to reduce PM emissions [115]. While many studies have estimated the exhaust pollution scenario, few have analyzed non-exhaust pollution. Nagpure et al. [116] found that resuspended road dust was the major contributor to non-exhaust emissions in Delhi, consisting of 91–92% road dust, 7% brake particles, and 2% tire wear (Fig. 2). PM10 emissions gradually increased from 1991 to 2001, indicating projection will be three times higher than in 2011. Another study by Singh et al. [115] revealed that non-exhaust pollution was almost six times higher than exhaust pollution in Delhi. Obtained results were helpful in planning for pollution control strategy.
5.4. Indonesia
Very few studies analyzed non-exhaust pollution in the country. Huboyo et al. [117] analyzed particulate emissions in Semarang; the source of this particulate pollution is both exhaust and non-exhaust traffic emissions. Different survey methods revealed that the concentration of pollutants was significant in the afternoon compared to the morning due to an increase in traffic-related vehicles, indicating that air pollutants projection will be increased in the future due to an increase in vehicle numbers.
5.5. Japan
IQAir reported that Japan had ranked 92nd in 118 most polluted cities worldwide. The report found that industrial activities, trans-boundary effects, and expanding number of vehicles are potential air pollution sources [94]. Non-exhaust sources are one of the potential contributors to PM emission; the concentrations of tire wear in PM10 are significantly lower, representing 0.84% of total PM consisting of tire wear particles [118] (Fig. 2). Ijima et al. [119] reported that almost 30 percent of total brake abrasion dust can be directly emitted as PM2.5, whereas 90% emit as ultrafine particles and 74–92% emit as ambient particulate. Generally, road dust particles contribute to a higher amount of coarse PM than fine particles in urban and urbanizing areas [120]. These studies help to develop an action plan to reduce non-exhaust emissions across the city.
5.6. South Korea
In Korea, PM2.5 is a major responsible factor in poor air quality. Non-exhaust sources contribute as much as exhaust emissions to particulate pollution here. The Korean Ministry of Environment’s Clean Air Policy Support System (CAPSS) has acknowledged the need for further research on non-exhaust emissions due to the lack of available research and a reliable method for measuring vehicle emissions [121]. Kwak et al. [122] analyzed non-exhaust pollution in an asphalt-based pavement in Hwaseong. The result indicates that road wear particles contribute 14–35 times higher PM10 and PM2.5 emissions than tire wear particles. Uhm et al. [123] found that dust, non-combustion sources, and vehicular non-exhaust emission are potential contributors to particulate pollution in Seoul. The study revealed that among all the exhaust and non-exhaust sources, road transport and street dust are potential contributors to PM2.5, which can contribute 26 and 22 percent, respectively; however, the PM showed a decreasing trend (almost 16%) in 2020 as compared to 2019 due to the lockdown during COVID-19.
5.7. Malaysia
Air quality has been worsening in Malaysia in recent years due to massive urbanization along with transboundary pollution. Malaysia is ranked in Southeast Asia’s top three polluted countries [124]. Wahab et al. [125] found that street dust is a potential non-exhaust source in Kuala Lumpur, a rapidly growing metropolitan area, in terms of population and number of vehicles. The study revealed that road dust contains toxic metals in high concentrations and prolonged exposure can cause potential risks. According to a study conducted by Elhadi et al. in [126], the dominant sources of PM10 in the area were found to be vehicle exhaust, brake and tire wear, industrial emissions, re-suspension of dust, and oil combustion based on the results of principal component analysis and cluster analysis. The study also found that increased trace metal levels associated with PM due to vehicular exhaust and non-exhaust emissions. Therefore, different mitigation strategies should be implemented to reduce this in the future.
5.8. Mongolia
Mongolia has recently emerged as one of the most heavily polluted countries in the world, particularly during winter, due to rapid urbanization, industrialization, and an increasing number of vehicles, coupled with transboundary effects. The capital city, Ulaanbaatar alone, houses nearly half of the country’s population and frequently tops the list of cities with the worst air quality worldwide [127, 128]. Despite the worsening air quality situation, research on non-exhaust emissions in Mongolia remains insufficient, making it difficult to understand the characteristics of particulate pollution from non-exhaust sources. However, Hopke et al. [120] identified street dust as one of the major contributors to non-exhaust pollution. Li et al. [129] also found that road dust is a significant cause of environmental pollution and can indicate the presence of heavy metal contamination due to atmospheric deposition. Further research is necessary to gain a deeper understanding of non-exhaust pollution scenarios and develop effective mitigation strategies.
5.9. Nepal
Nepal is a beautiful Asian country with amazing scenic beauty and rich cultural heritage. However, beneath the scenic beauty lies a significant concern that plagues the nation - air pollution. The concentration of PM2.5 in the air is almost seven times higher than the WHO’s standard, making it a leading cause of death and disability in Nepal [130]. Vehicular emissions, rapid industrialization, and urbanization are Nepal’s primary causes of air pollution. Although research on non-exhaust emissions is limited, Shakya et al. [131] found that soil dust is the second major source of PM, contributing to 33 and 42% of the measured chemical composition of PM2.5 and PM10, respectively, especially during the monsoon season.
5.10. Pakistan
Pakistan is listed among the topmost polluted countries worldwide, with a higher pollution level than WHO standards [132]. Lahore, the second largest city in Pakistan, is affected by resuspended road dust. Dust, especially industrial dust and resuspended road dust, contributes 18.2% and 4.6% of total PM10 [133]. The study also revealed that the concentrations of PM10 ranged from 254 to 555 μg/m3, with an average of 406 ± 87 μg/m3, which was much higher than the standard set by WHO, indicating an alarming condition. Another study reported that PM concentration was much higher than standards set by WHO (daily 50 μg/m3), US EPA (150 μg/m3), and PAK EPA (150 μg/m3) in Faisalabad, the third largest city in Pakistan [134]. Anthropogenic activities such as industrialization, transportation, and agriculture are key contributors to the deterioration of air quality, leading to a significant increase in concerns about particulate matter in Pakistan. In addition, transboundary (China-Pak border) effects significantly increased the pollution level. Therefore, proper strategies should be implemented to reduce the pollution level.
5.11. Sri Lanka
According to Ileperuma [135], the majority of cities in Sri Lanka are experiencing high levels of air pollution. Kandy, the rapidly urbanizing area of Sri Lanka, has experienced poor air quality due to their expanding vehicle numbers and poor maintenance. Here, the dominant pollutants are PM and PAHs; the average PM10 concentration in the study area was 129 μg/m3, which can be increased to 221 μg/m3, representing a significant threat [136]. Abayalath et al. [137] also assessed non-exhaust pollution in Kandy, which revealed that street dust is an essential contributor to particulate pollution, especially coarse particles. The concentration of PM10 is 156.415±66.567 μg/m3, much higher than the WHO standards (daily 50 μg/m3). Taking this into account, effective control strategies should be implemented to mitigate non-exhaust PM emissions to address the issue of pollution. Hence, it is imperative to take steps in managing and regulating non-exhaust emissions.
5.12. Thailand
Air pollution has become a severe concern in Thailand, especially in winter. Phetrawech and Thepanond [138] found high levels of particulate pollution in Saraburi, potentially from both exhaust and non-exhaust sources. The study revealed that resuspended road dust is the dominant non-exhaust emissions contributor, particularly to the total concentrations of PM10. The daily concentration of PM10 is higher than 120 μg/m3, much higher than the daily PM emission standards set by WHO. Along with road dust, brake and tire particles and road surface wear also contribute to this PM emission. Control strategies should be implemented to reduce the alarming PM emissions.
5.13. Vietnam
Vietnam is listed in the top 10 polluted countries around the globe [139]. Traffic-related sources (both exhaust and non-exhaust) are significant contributors to air pollution. Nguyen et al. [140] assessed particulate pollution in Hanoi, one of Vietnam’s most polluted cities. The findings of the study revealed that the amount of PM2.5 was much higher than the daily PM emission standard set by WHO. The study also revealed that the amount of particulate pollution was reduced during the lockdown, indicating the lockdown’s efficiency in reducing pollution. More research is needed, however, to completely understand the potential sources of non-exhaust emissions contributing to ambient PM and implement effective strategies accordingly.
6. Health Consequences in Asian Countries
Over the last 50 years, pollutant emissions from various sources have significantly increased in Asia, leading to almost 92% of the population being exposed to polluted air, bringing deleterious health consequences [141, 142]. State of Global Air [143] reported that four Asian countries, India, Bangladesh, Pakistan, and Nepal exposed to the highest PM2.5 concentration in 2019, highlighting air pollution as the fourth leading risk factor. Almost 1.7 million deaths from lung cancer and chronic heart and lung diseases were attributable to long-term air pollution exposure in India in 2019; in China, the number of premature deaths was almost 1.8 billion.
The health impacts of non-exhaust pollution in Asian countries are documented in this section (Table 2).
6.1. Bangladesh
Bangladesh is the world’s most polluted country, with air pollution shortening life expectancy by 6.7 to 8 years [144]. Previous studies estimated the health risks of tracer elements, heavy metals, or PAHs bound with PM, present in roadside dust of Dhaka, the capital of Bangladesh [32, 104, 145–147]. The accumulation of trace elements in road dust within urban areas has emerged as a noteworthy issue of public health. However, the concentration of heavy metals was significantly elevated during the rainy season [148]. Rahman et al. [32] found that the amount of chromium (Cr) in street dust was higher than the safe level; thus, long exposure might develop neurological disorders, particularly in children. Sultan et al. [146] also found a high concentration of Cr, which increased the carcinogenic risk in Dhaka. Obtained results indicate that carcinogenic and non-carcinogenic risks, and developmental and neurological disorders, are expected to increase due to prolonged exposure to road dust. Another study revealed that local inhabitants may be at risk for developing cancer over the course of a lifetime due to continual exposure to toxic metals found in road dust [107].
6.2. China
Road traffic regulations have reduced exhaust emissions, but non-exhaust particle emissions from tire and brake wear, road surface abrasion, and traffic turbulence remain uncontrolled [149]. Toxic metals in fine PM are a serious threat to human health due to their non-degradable nature and ability to accumulate in the body, despite their small fraction in PM [150]. Non-exhaust health impacts were performed in different regions of China, like Beijing, Xian, Nanjing, Shenzhen, etc. [112, 151–153]. Xian has a densely packed population of almost 12 million people and 3 million cars [154]. In Xian, Li et al. [152] found that dust containing trace metals (Cr, As, and Pb) increased non-cancer risks in children, while some heavy metals (As, Cr, and Ni) increased cancer risks in adults. Twelve meters tall city wall surrounding the city traps pollutants and poses significant health risks to pedestrians. Similar results were also discovered by [Huang et al. [150], revealing that the carcinogenic risks associated with Cr and As were above the EPA threshold value, with levels almost 19 and 6 times higher than the threshold value for adults, respectively. In Nanjing, an urban sprawl, non-carcinogenic risks from metals in urban street dust were 7.5 times higher in children, indicating an alarming condition [112].
6.3. India
India, particularly Kolkata, is facing severe air pollution due to rapid urbanization and an increasing number of vehicles. According to the World Health Organization [155], Kolkata is the second most polluted megacity in the world. A recent study by Kolakkandi et al. [156] revealed that heavy metals in street dust in Kolkata pose a high risk to the health of juveniles, who are more susceptible to cancer than adults, indicating the urgent need for proper air cleaning programs in the area. India State-Level Disease Burden Initiative, Air Pollution Collaborators, found that air pollution was attributed to 17.8 percent of total deaths in India, with particulate pollution increasing the premature death rates by 115 percent from 1990 to 2019. Gutikunda and Goel [157] also examined the health impacts of PM in Delhi, the country’s capital revealing that premature mortality and asthma attack was increased due to elevated level of PM. In addition, heavy metal concentrations were comparatively higher in fine fractions than in coarse fractions. The pollution load index value of road dust containing PM10 and PM2.5 has adverse health impacts [158]. Heavy metals like cadmium, lead, and chromium can harm human health by affecting reproduction, development and causing cardio-, haemo-, and immunotoxicity [159]. These findings help to develop a proper plan for reducing emissions and improving residents’ health.
6.4. Indonesia
Indonesia’s air quality has degraded significantly over the past two decades. The average Indonesian can expect to lose 1.2 years of life expectancy due to air pollution levels that do not meet WHO guidelines; however, in most polluted areas like Jakarta, air pollution may reduce life expectancy by two years [160]. Very few studies analyzed non-exhaust pollution in this country. Wijaya et al. [161] examined the health impacts of road dust in central Jakarta, one of the most administrative cities, rapidly growing from 2010 to 2020; almost 1 million residents live here as per the 2020 census [162]. Wijaya et al. [161] also reported that long-term exposures to high concentrations of toxic metals in street dust can significantly affect residents’ health.
6.5. Japan
In Japan, non-exhaust sources increase carcinogenic risks, respiratory and cardiovascular diseases. Previous studies revealed that particulate pollution is attributed to many health issues among local people [163, 164]. In addition, the presence of transition metal components in PM2.5 can cause inflammation in the respiratory system and lead to the development of cardiovascular and respiratory illnesses [165]. Zhang et al. [164] performed the health risks from PM-bound heavy metals in road dust in Kitakyushu. As some toxic metals, such as As, Pb, Cr, etc., present in high concentrations in the sampling area, local children were more prone to develop chronic symptoms. The study’s findings also revealed that children faced high cancer risks and moderate non-cancer risks compared to adults. Khanal et al. [163] also found that road dust can potentially develop carcinogenic risks suggesting that non-exhaust particles, such as brake, tire, and road surface wear, and resuspended road dust may be contributed more significantly to roadside air PM concentrations. Therefore, it is necessary to develop control policies to mitigate pollution to improve the health of local people.
6.6. South Korea
South Korean people have experienced higher risks of air pollution. U.S. Centers for Disease Control and Prevention reported that PM2.5 has been associated with various diseases such as heart disease, cancer, and low birth weight. The association between PM and Parkinson’s and other neurological disorders was also hypothesized [166]. Exposure to non-exhaust emissions and background air pollution can lead to various health issues, ranging from eye irritation to lung and heart problems [121]. Shin [121] disclosed that the risk to the population depends on their exposure to a threshold of PM10 at 100 μg/m3 and their ability to recover. For instance, resident drivers experience health issues in Seoul when the patch on the road has at least 144 μg/m3. Jeong and Ra [167] assessed the health effects of toxic elements found in street dust in Busan, the largest port city in South Korea. The study found that due to runoff, toxic elements are present in Busan’s street dust and further deposited in coastal areas [168]. Jeong & Ra [167] found that toxic elements are generated by brake abrasion and tire wear, and the high concentrations of Sb, Cu, Zn, Cd, and Pb have increased health issues. Children are more susceptible to carcinogenic risks as compared to adults, and oral ingestion is the primary exposure pathway. However, the carcinogenic risk is more prone to near tire repair shops and industrial areas. Another study assessed non-exhaust health impacts in Seoul; PM rising occurrence is often noticed in the city, especially in winter [169]. The findings of the study also indicated direct relation between increased PM concentration and health hazards in pedestrians.
6.7. Malaysia
Air pollution has become a serious concern over the past decades in Malaysia, contributing to respiratory illnesses being the second leading cause of death (14.8%), followed by cardiovascular diseases (8%) in the country [170]. Rawang is a rapidly urbanizing area with almost 12.3% urbanizing rate per year, and the increasing number of vehicles leads to increased pollution levels [171, 172]. Praveena [173] examined health risks from road dust, revealing a high chance of cancer and non-cancer risks for local people due to exposure to road dust. Furthermore, particulate pollution increases respiratory diseases in outside workers compared to inside workers [174]. During the periods of the southwest and northeast monsoons in Klang Valley, Malaysia, the levels of As, a trace metal, were found to exceed the recommended guidelines set by both the USEPA and WHO, indicating a potential threat to the health and well-being of individuals residing in the area, and highlighting the need for further investigation and implementation of measures to mitigate the presence of As in the environment Elhadi et al. in [126].
6.8. Mongolia
Dr. Sergey Diordista, a representative of WHO in Mongolia, said, “Air pollution has become one of the most challenging issues in Mongolia”. Each year 132 out of 100,000 people die from air pollution in Mongolia, compared to only 92 out of 100,000 globally [175]. In Ulaanbaatar, 29% of deaths from lung and heart disease and 40% from lung cancer happened due to air pollution [175]. However, information regarding non-exhaust emissions in Mongolia is limited, making it challenging to understand particle pollution fully. Chonokhuu et al. [176] examined the health impacts of heavy metals in street dust, disclosing that road dust had potential non-carcinogenic risks, which made the scenario more threatening. According to Li et al. [129], ingestion is the primary exposure route to metals in road dust in the non-carcinogenic risk assessment. Among the metals, Mn, Cr, Pb, and As are significant contributors to non-cancer risks in children and adults. Children are more vulnerable than adults. However, the carcinogenic risk assessment also showed that Cr is the main contributor to cancer risks in the Bayan Obo Mining Region, with risks 2–3 times higher than other metals. Therefore, it is critical to perform additional research on Mongolia’s non-exhaust emissions to create efficient policies and methods to reduce particle pollution.
6.9. Nepal
Nepal, particularly Kathmandu, is facing a major environmental health issue due to particulate pollution, which has transformed the city from the ‘city of temples’ to the ‘city of pollution’ [177]. This severe air pollution has led to serious illnesses and dangerous airborne infections. People suffer from several chronic and respiratory infectious diseases linked to air pollution, which is deemed a silent killer. On average, air pollution shortens life expectancy by four years in Nepal. In the Outer Terai, where half of Nepal’s population resides, individuals’ average life expectancy is predicted to decrease by over six years. Meanwhile, residents of Kathmandu are expected to lose an average of three years [178]. Despite the severity of the issue, Nepal still lacks non-exhaust related studies. However, Raj & Ram [179] discovered high Pb, Cd, and Hg concentrations in road dust, indicating potential carcinogenic risks for roadside people. Therefore, Nepal must conduct more studies on non-exhaust-related pollutants to understand the risks posed to its citizens completely.
6.10. Pakistan
Air pollution has become a life-threatening factor in Pakistan, a rapidly growing country in the Asian continent. Out of 153 million premature deaths, 11 million were noticed in Pakistan [180]. Ullah et al. [181] reported that air pollution not only affected human health but also changed the psychology and behavior of the residents. Response to air pollution varies among local people due to genetic variation, exposure time, the health status of the individual, and air pollution level [182, 183]. Qadeer et al. [184] found high carcinogenic risks for local people from road dust due to toxic metals in Lahore and Faisalabad urban areas; children in this area are 4–5 times more susceptible to non-carcinogenic risks. However, the COVID-19 lockdown significantly reduced pollution levels [185]. The reduction in pollution during the lockdown also underscores the potential benefits of implementing sustainable practices and policies to mitigate environmental degradation.
6.11. Sri Lanka
Air quality and pollution management are still significant challenges in Sri Lankan cities. Several studies determined that particulate pollution negatively affects the health of local people [186, 187]. Priyankara et al. [187] reported that increased PM exposure was directly linked with hospitalization cases due to asthma and respiratory diseases. The study also found that people living in urban areas with high levels of air pollution were at a higher risk of developing respiratory illnesses. Furthermore, the authors found that senior people (65+), especially males, were more affected. Another study reported that children of roadside schools had a higher possibility of developing respiratory diseases than children of schools situated in rural areas due to higher levels of pollutants near the roadside [186]. Nandasena et al. [188] also found that lung cancer, respiratory symptoms, and low birth weight were associated with ambient air pollution exposures.
6.12. Vietnam
Particulate pollution has become a severe issue in Vietnam as the level of PM2.5 is 5–10-fold more than WHO standards [189]. These fine particulates enter the lungs and cardiovascular system, resulting in diseases like heart diseases, respiratory infections, lung cancer, stroke, and chronic obstructive pulmonary disorders. Lelieveld et al. [190] also reported that the premature death rate due to exposure to PM was higher in Vietnam. Mai Luong et al. [191] found that hospitalization cases of children (aged less than five years) increased due to exposure to PM2.5 in Ho Chi Minth city. The study also revealed that male children were more affected than female children, and lower respiratory infections also increased due to particulate pollution. According to IHME [192], the percentage of death due to lower respiratory infection has increased in Vietnam in the last ten years (2007–2017) because of poor air quality. So, strategies should be developed to minimize air pollution and ensure the health of local people.
7. Technological Solutions and Innovations
Traffic-related sources are significant air pollution contributors; current regulations aim to exhaust and non-exhaust emissions from the brake, tire, road surface wear, and road dust resuspension. Although electric vehicles can minimize tailpipe emissions, they increase brake and tire particles emission due to the heavy-weight batteries [50]. There are various potential prevention strategies to address this issue, including modification of tires wear, replacing rubber with suitable rubber materials, improving brakes with reduced wear emitted properties, modifying the composition of brakes and tires to limit heavy metal uses, improving road surface, managing vehicles condition by reducing overall mass, and driving pattern brakes, road pavements can be adapted as potential prevention strategies [12]. The regenerative braking system also effectively reduced brake wear particle emissions by up to 60–90% [50]. Improving and maintenance of road surfaces along with high vacuum sweeping can also go a long way in reducing resuspended road dust up to a certain limit. Additionally, managing vehicle conditions and implementing traffic signal regulations can significantly reduce the negative impact of traffic emissions on the environment, as high-speed increase the generation of brakes and road surface particles and ensures road dust resuspension [81]. Technological solutions to limit non-exhaust emissions are still in the developing phase and requires further improvement to abate the emerging threat. Fig. 4 illustrates different mitigation strategies for non-exhaust particulate emissions.
8. Research Gap and Limitations
After comparing different studies, we found that there has been a conspicuous lack of research on non-exhaust emissions, leaving a considerable void in our understanding of the diverse facets of these emissions and their effects on the environment, climate, and human health. Particularly in Asian countries like Russia, Kazakhstan, Kyrgyzstan, Tajikistan, and Turkmenistan, there has been a notable absence of research on non-exhaust emissions, despite their severe effects. Along with that, further research should be done on the health risks posed by non-exhaust particles. In addition, researchers only work towards the phenomenon of standardization of emission factors. However, temporal and spatial variation of the emission factors are still lacking; environmental, road and driving conditions should also be taken into account. Although very few studies reported the mitigation strategies, comprehensive research still lacks the ability to reduce non-exhaust PM pollution. Moreover, distinguishing between direct and resuspended emissions can be challenging due to varied sources requiring further investigations. By doing so, we can take the necessary steps to improve air quality and safeguard public health.
9. Conclusions and Future Prospects
Non-exhaust pollution and associated health consequences are increasing in most Asian continents, the largest, most populous, and most heterogeneous continent; the reason is likely multi-factorial, including vehicles and road characteristics and lacking regulations implementation. Studies have shown that poor air quality in many Asian countries is linked to non-exhaust sources, and exposure to these pollutants has been associated with increased risks of respiratory and cardiovascular diseases and cancer, particularly among children. One major concern is the presence of PM-bound heavy metals and PAH in road dust, which can significantly increase adverse health effects. In addition, non-exhaust sources also contribute to the generation of microscopic plastic particles, which pose a significant threat to the environment. Most Asian countries had experienced poor air quality; the concentration of pollutants was much higher than WHO’s standard. Despite the worsening air quality situation, research on non-exhaust emissions in Malaysia, Mongolia, Indonesia, and Nepal remains insufficient, making it difficult to understand the characteristics of particulate pollution from non-exhaust sources. Therefore, it is crucial to do further research on non-exhaust emissions and develop and implement regulations and policies to control non-exhaust particulate pollution and raise public awareness of the associated health risks. Sustainable modes of transportation such as walking, cycling, and public transit should be prioritized in urban planning and transportation policies to reduce overall vehicular emissions. Furthermore, individuals can make a big difference by limiting car use and promoting renewable energy. We have to work together to create a greener and healthier future.
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