PolyU researchers reveal hidden health risks from urban air microbes, mapping their sources, pathways and health impacts

Air pollution poses a widespread threat to human health, particularly due to its strong link to respiratory diseases. Airborne microbes, including bacteria, fungi, viruses, and cellular debris, are estimated to account for approximately 25% of atmospheric particulate matter (PM), some of which are pathogenic to humans. Researchers at The Hong Kong Polytechnic University have conducted a groundbreaking investigation into the sources, composition and health-related toxicity of these microbial particles, revealing concerning connections between air pollution and its impact on human health. 

The contribution of microbial components and their sources to the bioactivity of airborne fine particulate matter (PM2.5) remains unclear. To address this research gap, Prof. JIN Ling Nathanael, Assistant Professor of the PolyU Department of Civil and Environmental Engineering and Department of Health Technology and Informatics, and Prof. Polly Hang Mei Leung, Professor of the PolyU Department of Health Technology and Informatics, along with their jointly supervised PhD student, Ms Jinyan YU,  and other well-known international scholars, have systematically assessed bacterial endotoxin in PM2.5 and traced its association with inflammatory response of bronchial epithelial cells.The research, titled, “Disproportionately higher contribution of endotoxin to PM_2.5 bioactivity than its mass share highlights the need to identify low-concentration, high-potency components” was published in Environmental Science & Technology.

After regularly collecting airborne PM2.5 samples, the research team examined their components to identify those responsible for triggering the production of inflammation-related proteins in bronchial epithelial cells. Endotoxins are found to account for up to 17% of the inflammation response, despite making up less than 0.0001% of PM2.5’s total mass. They exhibit low concentration and high potency characteristics. Notably, endotoxins show the highest toxicity-to-mass contribution ratio among all PM2.5 components with known related data. These finding suggest that reducing PM2.5 toxicity may not require a proportional reduction in its overall mass, and identifying and controlling high potency components should be the priority. 

The research assessed the toxic effects of inhalable microbial components, revealing that bacteria, particularly Gram-negative species, dominated the atmospheric microbial community. Endotoxin, a structural component of Gram-negative bacterial cell walls, was identified as a key contributor. Notably, the sources of coastal site Gram-negative bacteria mainly originate from the natural environment, but their sources in the urban area increasingly shifts to anthropogenic contributions, including those from the built environment, sewage treatment and human activity. 

Prof. JIN said, “Accurately identifying toxic components and sources is key to effective air quality management and health protection. We link endotoxin toxicity to its bacterial origin through microbial source tracking of its Gram-negative bacterial producers. As major pollution sources such as industrial and vehicular emissions decline due to global clean-air initiatives, previously overlooked high potency components will become increasingly importance in health risk management.”

Ms YU noted, “This study provides a novel method to assess the role of microbial compounds in PM2.5-induced human immune response. It lays the foundation for identifying and measuring various toxic substances in air pollution.” 

Enhancing public health protection requires an integrated framework that links air pollutants, its sources and health risks. In another recent study, Prof. JIN and Dr Franklin Wang Ngai CHOW, Research Assistant Professor of the PolyU Department of Health Technology and Informatics, along with their jointly supervised Postdoctoral Fellow Dr Chunlan FAN and PhD student Mr Tian CHEN, and other well-known international scholars, have focused on a group of airborne fungi called Candida.  They are the largest genus of yeasts, which can cause infections ranging from mild to life-threatening, found in respirable suspended particulate (PM10) from urban areas. The research titled, “Public health implications of airborne Candida: viability, drug resistance, and genetic links to clinical strains,” were published in Environmental Science & Technology Letters. 

Candida species are classified by the World Health Organisation (WHO) as priority pathogens due to their server health impacts, drawing global attention on potential health risk. The team identified multidrug-resistant Candida parapsilosis in urban air and revealed its close genetic links to clinical strains from infected individuals. This suggests that people may be exposed to drug-resistant fungi through inhalation or skin contact, raising concerns that urban pollutants may promote antifungal resistance. The research highlights the urgent need to recognise urban air as a significant medium for the spread of antifungal-resistant strains. 

The research also revealed that Candida species are seasonally prevalent in urban ambient air. Viable Candida were detected in air of anthropogenic settings such as wastewater treatment facilities, healthcare environments and ventilation systems of residential buildings. Notably, Candida parapsilosis showed consistent abundance throughout the year, highlighting its strong environmental resilience and widespread occurrence in urban areas. It was also identified as the most dominant Candida and most antifungal-resistant species. 

Significantly, the research provides a systematic investigation into how airborne Candida may spread in the community, including how it is carried, transmitted, and causes infection. Prof. JIN said, “The spread of drug-resistant fungi in both environmental and clinical settings, alongside a growing at-risk population, highlights antifungal resistance as a critical global environmental health issue. Moving forward, it is important to identify urban-specific reservoirs, investigate conditions that promote resistance, and model airborne transmission pathways.”