Assessment of the pulmonary toxicity of particulate matter emitted during haze (air pollution) events in Chiang Mai, Thailand using an in vitro macrophage model and transgenic zebrafish as alternatives to rodents

Chiang Mai (Thailand) experiences severe haze (air) pollution in the dry season (December – April) each year mainly due to local and regional biomass burning (e.g. agricultural land). A major component of haze is airborne particulate matter (PM). During haze events, biomass burning is likely to be the dominant source of PM emissions, and at other times emissions from traffic dominate. The hazard of traffic derived PM has been extensively investigated previously but there are uncertainties regarding the toxicity of PM emitted from biomass burning. The toxicity of PM10 samples collected during and after haze events in Chiang Mai in 2020 was compared in J774.1 macrophages as they are responsible for the clearance of inhaled particles from the lung. Diesel exhaust particles (DEP) and ultrafine carbon black (ufCB) were included as benchmark particles as they have been commonly used as a surrogate for PM in experimental studies. Cytotoxicity was evaluated 24h post exposure at concentrations of 3.9-125µg/ml. Cytokine production (TNF-α, IL-6, IL-1β, MIP-2) was assessed and cell morphology visualised using light and scanning electron microscopy. The hydrodynamic diameter, zeta potential and endotoxin content of all particles was assessed as well as the metal content of PM samples to identify what physico-chemical properties of the PM samples drove their toxicity. Pilot studies were performed in non-protected life stages of zebrafish to investigate the developmental toxicity of PM and to assess whether PM could activate an inflammatory response. All particles induced a concentration dependent decrease in J774 cell viability and increased TNFα and MIP-2 cytokine production. Only PM samples stimulated IL-6 production and only non-haze PM stimulated IL-1β production. No change in IL-10 production was detected for any particle. PM samples and DEP caused vacuole formation in cells, but the mechanism underlying this is unknown. The concentrations of endotoxin and metals were highest in non-haze PM, which may explain why it induced the greatest inflammatory response. No developmental toxicity was observed in zebrafish, but PM was able to activate an inflammatory response (as indicated by an increase in macrophage and neutrophil accumulation in a tail fin injury model). As non-haze PM was more toxic than haze PM in our in vitro model, our results indicate that the source of PM emissions can influence its toxic potency and more specifically that PM emitted from biomass burning may be less toxic than PM emitted from traffic. Currently, PM regulations in Thailand and internationally that aim to protect human health are based on mass concentrations of PM without consideration of the source of PM emissions. A greater understanding of whether the toxicity of PM emitted from different sources varies will provide an evidence base that informs the introduction of appropriate policy intervention strategies to improve the health of the population of Thailand, as well as other countries.