The low-cost and easy-to-use nature of rapidly developed PM_2.5 sensors provide an opportunity to bring breakthroughs in PM_2.5 research to resource-limited countries in Southeast Asia (SEA). This review provides an evaluation of the currently available literature and identifies research priorities in applying low-cost sensors (LCS) in PM_2.5 environmental and health research in SEA. The research priority is an outcome of a series of participatory workshops under the umbrella of the International Global Atmospheric Chemistry Project–Monsoon Asia and Oceania Networking Group (IGAC–MANGO). A literature review and research prioritization are conducted with a transdisciplinary perspective of providing useful scientific evidence in assisting authorities in formulating targeted strategies to reduce severe PM_2.5 pollution and health risks in this region. The PM_2.5 research gaps that could be filled by LCS application are identified in five categories: source evaluation, especially for the distinctive sources in the SEA countries; hot spot investigation; peak exposure assessment; exposure–health evaluation on acute health impacts; and short-term standards. The affordability of LCS, methodology transferability, international collaboration, and stakeholder engagement are keys to success in such transdisciplinary PM_2.5 research. Unique contributions to the international science community and challenges with LCS application in PM_2.5 research in SEA are also discussed.

Particulate matter (PM) emitted from incense burning is a serious problem because of its effects on human health and the environment, especially on the air quality of Southeast Asia. This study investigated the PM1 and PM2.5 concentrations in two temples in Vietnam and Taiwan, focusing on events (i.e., new moon and full moon day) and non-event days using PM sensors. Moreover, PM2.5 samples were collected to determine organic carbon (OC) and elemental carbon (EC) concentrations. The PM1 and PM2.5 concentrations in Vietnam (22.7 ± 18.7 and 36.5 ± 33.9 µg m-3, respectively) were lower than those at the Taiwan temple (74.5 ± 53.4 and 97.0 ± 65.4 µg m-3, respectively). The incense burning activity in the Taiwan temple occurred inside the temple while the incense burner at Vietnam’s temple was located outdoors. During the event days, PM1 and PM2.5 concentrations were about two times greater than on non-event days, highlighting the impact of incense burning on the PM concentrations. By breaking the study area into control and exposure sites, we found that the people inside the temple experienced 2–5 times higher PM concentrations as compared to those in the ambient air. The PM1/PM2.5 ratio was 0.6 ± 0.1 for the Vietnam temple, which was lower than the Taiwan value (0.7 ± 0.1). The OC and EC concentrations in the Taiwan temple were 129.40 ± 97.68 and 1.16 ± 2.31 µg m-3, respectively, significantly greater than those found in the Vietnam temple (OC = 27.70 ± 7.66 µg m-3, EC = 1.63 ± 0.93 µg m-3). Furthermore, the OC/EC ratio in the Vietnam temple (20.24) was similar to those reported from incense and biomass in previous research. The increase in religious activities was the major factor leading to the enhancement of the air pollutant levels surrounding the study areas. This study provides valuable information on PM and carbonaceous aerosols emitted from incense burning in Southeast Asia megacities.

The in-traffic microenvironment can enhance personal exposure to fine particulate matter (PM). With this study, we aimed to calibrate a DustTrak instrument (DustTrak 8533 DRX Aerosol Monitor, TSI Incorporated, Shoreview, MN, USA) and low-cost sensors (AS-LUNG-P sensors) and then assess inhalation exposure to PM_2.5 and PM₁ for different commuters in central areas of Ho Chi Minh City (HCM). The DustTrak instrument and low-cost sensors were calibrated using a gravimetric method under side-by-side conditions. Relationships between the DustTrak signals and PM concentrations measured by the gravimetric method were identified using simple linear regression models for PM_2.5 (R² = 0.998, p-value < 0.05) and PM₁ (R² = 0.989, p-value < 0.05). Meanwhile, PM concentrations determined by the AS-LUNG-P sensors and the gravimetric method were correlated using two-segmented linear regressions. To obtain the corresponding two-segment regression equations, the response of the AS-LUNG-P sensors was compared with the corrected DustTrak data. The coefficient of variation (CV) evaluated for all sensors was smaller than 10%, indicating that the data were applicable for particle assessment. For inhalation exposure assessment, the results showed that commuters using open transport modes, such as bikes, motorbikes, and walking, were exposed to more PM than those using closed transport modes (e.g., cars). Specifically, the bicyclists had the highest inhaled doses of PM among the open transport groups. PM exposure levels in the morning were higher than in the afternoon. Additionally, exposure levels to PM concentrations rapidly increased when passing through intersections of major roads and moderately decreased when using surgical facemasks.

The coronavirus disease 2019 (COVID-19) measures during 2020−2021 may lead to a downward trend of fine particulate matter (PM2.5) concentrations in Ho Chi Minh City (HCMC), which is one of the most air-polluted cities in Vietnam. This study aimed to compare PM2.5 variations during the COVID-19 period (2020−2021) with a short-term trend of PM2.5 (2016−2019) in HCMC in conjunction with meteorological conditions. Five fixed-site locations were chosen to monitor PM2.5 concentrations by using low-cost particulate matter sensors (Purple Air II-SD) in five urban districts of HCMC from December 2019 to March 2021. This study also collected hourly PM2.5 concentrations from the United States Consulate General HCMC and meteorological variables (i.e., wind speed, wind direction, air temperature, and relative humidity) from the weather station at Tan Son Nhat International Airport, 2016–2021. PM2.5 concentrations of the COVID-19 period in HCMC were notably less than those of the 2016−2019 period by 18%. However, about half of the days during this period still had daily PM2.5 concentrations exceeding the World Health Organization's standard of 15 µg m-3. Besides the impacts of COVID-19 restrictions on the PM2.5 decrease, noticeable co-effects of meteorological variables on the decline of PM2.5 in HCMC were also observed in the rainy season and with the west-south-west monsoon wind direction. Wind speed and air temperature substantially contributed to PM2.5 reductions in Bayesian multiple linear regression models. In conclusion, the large decrease of PM2.5 concentrations during the unprecedented period of COVID-19 pandemic gave us a better chance to more fully comprehend the PM2.5 pollution status in HCMC for future reference.

Abnormal emergence of epidemics has been believed to be one of the most significant effects of climate change on human health, especially in the tropical zone. Hand-foot-mouth disease (HFMD) which possibly related to climatic changes has emerged in Vietnam since 2003. Ho Chi Minh City (HCMC) is particularly a Southern city with the highest cases and mortality numbers of HFMD in the whole country. Therefore, we conducted a retrospective observational study to analyse the association between climate variations and HFMD in HCMC. HFMD and meteorological data from 2010 to 2014 were collected from The Preventive Medicine Centre of HCMC and Sub-Institute of Hydrometeorology and Environment of South Vietnam. The research used time-series analysis and Poisson regression model to analyse the effects of climate fluctuations on HFMD, adjusting for seasonal and trend effects, lag effects and degrees of freedom. The research findings showed that climate variables (average temperature, average humidity, minimum humidity, precipitation, and the Southern Oscillation Index (SOI) daily data) had a significant influence on HFMD. Noticeably, the effects of climate variations on HFMD were considerable in children aged 1-3 and 3-5 years. The maximum lag effects were 3 weeks for temperature and humidity, 4 weeks for precipitation, and 6 weeks for SOI. Specifically, the relative risk of HFMD increased by 7.2 % for every 1°C increase in temperature, 6.75 % for every 1 % increase in humidity, 6.32 % for every 5 mm increase in precipitation, 4.42 % for every 5 units increase in SOI. The study provided a quantitative evidence that the increase of HFMD was significantly affected by the increase of climate variations. Therefore, the study provided the foundation for developing theory that is used in models and, ultimately, for making forecasts about future impacts of HFMD associated with climatic changes.