The escalation of air pollution caused by particulate matter is a noteworthy factor contributing to the deterioration of air quality, which is increasingly becoming an urgent concern in various regions internationally ^[1]. Particulate matter with a diameter of less than 2.5 mm is commonly referred to as fine particulate matter or PM_2.5. The particles possess the ability to penetrate deeply into the lungs and bloodstream owing to their microscopic size, thereby leading to a diverse range of detrimental impacts on human health ^[1]. The study conducted by Pope et al. ^[2] in the United States found that prolonged exposure to PM_2.5 pollution, even at concentrations lower than the current air quality regulations set by the federal government, was associated with an increased risk of mortality due to lung cancer. A recent study conducted by Liu et al. ^[3] has established a correlation between inhaling PM_2.5 pollution and increased susceptibility to stroke in China. In addition, Carey et al. ^[4] found that prolonged exposure to PM_2.5 in the United Kingdom was associated with an increased likelihood of mortality resulting from cardiovascular disease. At the same time, Atkinson et al. ^[5] found a positive correlation between short-term exposure to PM₁₀ and an increased probability of hospitalization due to cardiovascular disease in London.

In Southeast Asia, the issue of air pollution caused by biomass burning is a major concern for both public health and the environment, with significant consequences, as noted by Amnuaylojaroen et al. [6,7]^[6][7]. Biomass burning refers to combusting organic matter, such as trees and crop residues, for agricultural purposes. In mainland Southeast Asia, biomass burning is a common practice. It is extensively used in rural areas for a variety of functions, including land clearance, cooking, and heating ^[8]. Keywood et al. ^[9] proposed that the release of fumes and particulate matter resulting from biomass burning substantially impacts air quality, human health, and ecology. Several studies have shown that biomass combustion is a significant contributor to increasing levels of fine particulate matter (PM_2.5) in mainland Southeast Asia, resulting in air pollution. Also, PM_2.5 concentrations in mainland Southeast Asia tend to increase in the future ^[10]. During the dry season, biomass-burning emissions were shown to be responsible for as much as 70% of PM_2.5 concentrations. Furthermore, the emissions from biomass burning have major effects on the environment, including climatic changes and ecological deterioration ^[11]. In comparison, Nhung et al. ^[12] discovered that inhaling PM_2.5 from biomass burning relates to increased sensitivity to respiratory and cardiovascular disorders in rural Vietnamese households. Moreover, exposure to PM_2.5 is associated with a 69% and 29% increase in the likelihood of hospitalization for respiratory and cardiovascular disorders, respectively.

2. Air Quality in Mainland Southeast Asia

Ramanathan et al. [24]^[13] have extensively discussed the topic of air pollution in Southeast Asia, particularly over the continent, for several decades. PM_2.5 is the most important air pollutant contributing to air quality in this region [25]^[14]. The PM_2.5 issue in this region, similar to other global regions, can be broadly categorized into two types: anthropogenic sources, which comprise emissions from industrial activities and transportation, and biomass burning emissions, which involve uncontrolled combustion of agricultural waste and forest fires. Amnuaylojaroen et al. [25]^[14] and Duc et al. [26]^[15] have reported the observation of pollutant peak periods, particularly with respect to particulate matter, during the dry season, specifically from February to April. According to Amnuaylojaroen et al. ^[8], emissions from biomass burning, notably those resulting from agricultural and forest fires, have a considerable influence on PM_2.5 pollution in the region. In addition, particulate matter (PM) released through biomass combustion has a significant impact on the atmosphere, both directly and indirectly, by altering the level of atmospheric radiation [27]^[16]. According to Bond et al. [28]^[17], biomass burning is a substantial source of fine particulate matter (PM_2.5), accounting for 41% and 74% of the world’s total primary black carbon and organic carbon emissions, respectively [29,30]^[18][19]. The emission data plot of PM_2.5 is presented in Figure 1, which encompasses both anthropogenic sources from the Emissions Database for Global Atmospheric Research version 6 (EDGARv6) [19]^[20] and natural sources from the Global Fire Emissions Database version 3 (GFED3) [20]^[21]. The EDGAR and GFED databases provide comprehensive information on emissions, including national amounts and grid maps at a resolution of 0.1 and 0.25 degrees for EDGAR and GFED, respectively, on a global scale. The data are available for various timeframes, from yearly to monthly and even hourly intervals. The data presented illustrate the annual distribution of emissions resulting from biomass burning in mainland Southeast Asia during the years 2000 and 2010. The research findings indicate that there is a significant rise in biomass burning emissions in the mainland Southeast Asia area in comparison to anthropogenic emissions. Compared to the year 2000, there was a significant rise in biomass burning emissions up to 0.0208 Tg, with a specific focus on Myanmar, Laos, and the northern part of Thailand. As stated above, biomass burning is the major source of PM, notably PM_2.5, that contributes to air quality problems in this region. Biomass burning in Southeast Asia is substantially related to fire emissions [31]^[22]. In recent decades, there has been a significant rise in biomass burning caused by fires in multiple countries across Southeast Asia [32]^[23]. The data from the VIIRS Moderate Imaging Spectroradiometer (MODIS) [33]^[24] provide information on the number of fire hotspot locations in various countries, including Thailand, Laos, Myanmar, Cambodia, Vietnam, and Malaysia, for the years 2000, 2010, and 2020 (Figure 2). Based on data obtained in the year 2000, it was observed that Cambodia had the highest number of hotspots, which amounted to 21,077, while Vietnam had the lowest number of hotspots, with a count of 2293. During the subsequent two decades, there was a notable variation in the number of hotspots identified in multiple countries. Myanmar experienced a significant increase in hotspots, with a surge from 15,992 in 2000 to 91,388 in 2010, indicating a remarkable growth of 471%. Nonetheless, there was a notable reduction in the rate of expansion of the aforementioned phenomenon during the subsequent decade. As of the year 2020, the number of hotspots had solely experienced an increase to a total of 66,989. The number of hotspots in Laos experienced a substantial escalation, demonstrating a noteworthy increase of 2383% from 2363 in the year 2000 to 58,684 in the year 2010. The area being analyzed experienced a significant deceleration in the rate of expansion of hotspots over the subsequent decade, similar to that observed in Myanmar. There was a slight increase in the number of hotspots, reaching a total of 47,331 by 2020. Through a period of two decades, Thailand demonstrated a relatively consistent number of hotspots, experiencing a slight decrease from 33,368 in the year 2010 to 30,234 in the year 2020. The number of hotspots in Vietnam endured a significant increase from 2293 in 2000 to 17,821 in 2020. Malaysia demonstrated the lowest count of hotspots in contrast to the remaining five countries during a period of two decades. Although biomass burning is relatively associated with fire emissions [25]^[14], meteorological factors, i.e., precipitation, also play a key role in controlling air quality problems in Southeast Asia. Vadrevu et al. [34]^[25] revealed the association between fires and precipitation in Southeast Asian countries, finding that precipitation had a high negative correlation with fire numbers. Furthermore, recent research indicates that the majority of the fires in this region are caused by humans. Forests are set on fire for a variety of reasons, including inducing the growth of new grass for grazing, clearing the land for farming, such as cutting and igniting, obtaining minor forest goods such as honey, palatable, and foliage, hunting wild animals, and residue from crops burning in agricultural areas [35,36,37,38]^{[26][27][28][29]}. According to the MERRA reanalysis data, levels of PM_2.5, ranging from 16 to 22 µg/m³, were detected in the year 2000 in many countries, such as Thailand, Myanmar, and Laos. The years of 2010 and 2020 witnessed a significant rise in PM_2.5 levels, which ranged from 22 to 40 µg/m³. A reduction in the PM_2.5 concentration was observed in several regions of Myanmar, Thailand, and Laos in 2020. The spatial distribution of Aerosol Optical Depth (AOD) showed a similarity to the distribution of PM_2.5 concentration. The AOD demonstrated an increasing pattern in various countries, such as Myanmar, Laos, and Thailand, between 2010 and 2020, as opposed to 2000. The levels of PM_2.5 concentration in this region were attributed to the increase in hotspots and biomass-burning emissions. The distribution of air quality, especially during the dry season, is dominated by the Asian winter monsoon [25]^[14]. From November to March, the Asian winter monsoon circulates air from the Asian continent to the ocean. In March, the winds reached the northern region of Thailand via two primary paths. The primary channel is distinguished by winds traveling from eastern Asia into Laos and the northern part of Thailand, while the additional channel is distinguished by winds moving from Myanmar and entering the northern part of Thailand from the northwest. These circulation patterns transport and distribute trace gases and aerosols released by biomass burning in this region [25]^[14]. As previously stated, there has been a discernible trend of increasing concern regarding the issue of air quality degradation caused by particulate matter over the last few decades. Meanwhile, there is a tendency for an increased risk of this issue in the future. Based on recent reports, there is a projected exacerbation of air quality issues related to particulate matter in Southeast Asia. For example, Nguyen et al. [40]^[30] conducted a study utilizing a numerical model to analyze the effects of climate and emission changes on mixing. On average, it was projected that PM_2.5 concentrations would increase between 10 and 21% and 20 and 28% during the dry and wet seasons, respectively, across four specified countries under the RCP4.5 and RCP8.5 scenarios. The findings of the simulation suggest that the fluctuation in PM_2.5 concentrations is primarily influenced by the emission trend, with climate change also exerting a significant impact. Amnuaylojaroen et al. ^[10] reported that alterations in climatic parameters and emissions have led to an increase in PM_2.5 concentrations across northern Peninsular Southeast Asia, ranging from (+1) to (+35) μg/m³ during the dry season. Lee et al. [41]^[31] have reported that the elevated concentration of PM_2.5 in Southeast Asia is not solely attributed to biomass burning emissions but also to anthropogenic emissions from sectors such as ship emissions, coal consumption, and biofuel consumption, which are projected to contribute to a 26% and 8.6% increase in PM_2.5 concentration in the region, respectively.

3. Health Effect of Poor Air Quality

The deteriorating air quality in Southeast Asia over the past few decades has had a direct impact on the health of individuals residing in this region ^[6]. According to the Global Burden of Disease Study (GBD) 2019 [42]^[32], chronic respiratory diseases were responsible for a significant number of deaths in mainland Southeast Asia between 1990 and 2019. The present study provides an assessment of mortality rates linked to chronic respiratory illnesses, such as chronic obstructive pulmonary disease, asthma, and other respiratory diseases, in mainland Southeast Asia, spanning the period from 1990 to 2019. There was an increase in the number of deaths attributed to chronic respiratory diseases in Cambodia from 12,756 in 1990 to 14,413 in 2019. Likewise, it can be observed that in Laos, there has been a rise in the mortality rate from 2096 in 1990 to 3179 in 2019. The mortality rate in Malaysia has exhibited an upward trend, with a rise from 6565 in 1990 to 7754 in 2019. The mortality rate in Myanmar has exhibited an upward trend, with fatalities rising from 38,962 in 1990 to 59,782 in 2019. The mortality rate in the Philippines has shown an upward trend, with a rise from 35,858 in 1990 to 51,186 in 2019. Between 1990 and 2019, there was an increase in the number of deaths in Thailand from 29,630 to 36,750, and in Vietnam, the number of deaths rose from 36,698 to 48,568 during the same period. The report by GBD found that Cambodia incurred 238,290 disability-adjusted life years (DALYs) in 1990, which increased to 306,245 DALYs in 2019. In 1990, Laos, Malaysia, and Myanmar incurred 29,232, 139,703, and 947,801 disability-adjusted life years (DALYs), respectively. In 2019, these countries experienced an increase in DALYs, with Laos, Malaysia, and Myanmar reporting 46,364, 182,303, and 1,448,635 DALYs, respectively. In 1990, the quantity of disability-adjusted life years (DALYs) recorded in Thailand was 676,741, which subsequently rose to 794,748 by 2019. The incidence of disability-adjusted life years (DALYs) in Vietnam increased from 1,117,224 in 1990 to 1,658,316 in 2019. Within the designated temporal scope, a significant number of years of life lost (YLLs) in mainland Southeast Asia were attributed to chronic respiratory diseases, including but not limited to chronic obstructive pulmonary disease and asthma, among other respiratory ailments. In 1990, Cambodia documented approximately 80,051 years of life lost (YLLs), an amount that rose to 101,328 YLLs in 2019. The YLLs in Laos were 10,271 in 1990 and increased to 15,156 in 2019. The YLLs in Malaysia were 42,112 in 1990 and increased to 51,715 in 2019. The number of years of life lost (YLLs) in Myanmar experienced an increase from 318,056 in 1990 to 492,813 in 2019. In 1990, Thailand experienced 272,644 years of life lost (YLLs), which subsequently rose to 328,846 in 2019. Likewise, it can be observed that in Vietnam, the years of life lost (YLLs) amounted to 323,631 in 1990 and exhibited an increase to 455,831 by 2019. Within the designated temporal parameters, chronic respiratory ailments exerted a substantial influence on the overall health of the populace residing in mainland Southeast Asia, leading to a noteworthy quantity of fatalities as well as a significant burden of disability-adjusted life years (DALYs) and years of life lost (YLLs). This highlights the necessity for continuous endeavors to alleviate air pollution and other hazardous elements linked with these ailments in this region. The mortality count arising from chronic respiratory ailments, namely chronic obstructive pulmonary disease, asthma, and other respiratory diseases, in Southeast Asia was found to display gender-based variations, as per the results of the Global Burden of Disease Study (GBD) 2000 [42]^[32]. The displayed fatality count for males and females within the specified region during 2000 was ascertained. In the context of Cambodia, the number of male deaths was recorded at 1661, while the number of female deaths was documented at 1530. In contrast, Laos reported significantly lower numbers, with 229 deaths among males and 199 among females. The recorded deaths in Malaysia were 3217 among males and 2457 among females. Based on the data, there were 13,487 reported deaths among males and 10,586 reported deaths among females in Myanmar. The recorded number of deaths in Thailand was 13,051 for males and 9638 for females. The mortality rate for males in Vietnam was 16,262, while for females, it was 13,491. According to available data, the number of fatalities in Cambodia in 2010 was approximately 2405 for males and 2208 for females. In Laos, the recorded count of male fatalities was 300, while the count of female fatalities was 259. The nation of Malaysia documented 4486 male deaths and 3318 female deaths. According to available data, the number of male fatalities in Myanmar was 20,222, whereas the number of female fatalities was recorded as 16,093. The number of recorded deaths for males in Thailand was 15,384, whereas, for females, it was 11,348. The number of fatalities documented in Vietnam was 21,477 for males and 17,624 for females. The evaluation of mortality rates for males and females in the region during the year 2019 is presently underway. According to the mortality statistics of Cambodia, there were 4327 documented deaths among males and 3958 documented deaths among females. In Laos, the number of recorded deaths among males was 448, while the number of recorded deaths among females was 387. The recorded deaths in Malaysia were 6491 for males and 4851 for females. From the currently available data, the mortality rate in Myanmar reveals that there were 30,569 deaths among males and 24,429 deaths among females. The recorded deaths in the Philippines were 33,448 among males and 27,390 among females. The recorded number of deaths among males in Thailand was 23,284, whereas the recorded number of deaths among females was 16,903. The recorded deaths in Vietnam were 34,286 among males and 28,175 among females. In the study by Baptista et al. [43]^[33], PM_2.5 is strongly positively linked with chronic respiratory illness mortality, implying that increased particulate matter exposure increases mortality. Many Asian countries have experienced increased air pollution from industry and motor vehicle emissions in the recent decade [44]^[34]. Pollution is also caused by crop waste burning and bushfires in different Asian countries [45,46]^[35][36]. Furthermore, there was a rise in the incidence of asthma exposure to air pollution in the group of studies conducted on Asian populations investigated in the US-based Health Effects Institute (HEI) report [47]^[37], with a risk score of >1 and 2, enhancing a possibility for air contaminants to drive up the incidence of asthma [44]^[34].