Sanitation workers are essential to maintaining public health and infrastructure. Yet, their occupational exposure to bioaerosols, including microbial agents, remains insufficiently studied, particularly in the context of evolving public health infrastructures such as increased waste sorting and separate sewerage systems, which may alter the nature and levels of these exposures. 

While it is well-established that regular contact with waste and sewage may pose health risks, the understanding of exposure pathways, microbial compositions, and associated health outcomes remains limited. These knowledge gaps highlight the need for new and focused research.

Objective
Bioaerosols are complex mixtures of airborne particles of organic origin, including fragments of plant, animal, and microbial matter, or even whole viable microorganisms. Occupational exposure to bioaerosols has been associated with respiratory diseases, but their complex composition makes toxicity assessment challenging, and official occupational exposure limits are lacking.

The objectives of this PhD thesis were therefore to address these key gaps by investigating the microbial exposure levels and associated health outcomes among three distinct groups of sanitation workers: waste collectors, wastewater treatment plant workers, and combination vacuum-jet truck operators.

Methods and Results
The studies examined microbial exposure pathways and sources, the composition of airborne microbial communities, as well as potential correlations with inflammatory and lung damage biomarkers in workers’ serum. By combining conventional occupational exposure methodologies with advanced technologies, the three studies provided both traditional exposure characterizations and novel insights into microbial exposures. Workers’ personal exposures to bioaerosols were measured for whole workdays in all three studies.

• In the first study, an often-overlooked pathway of exposure was described, namely secondary exposure, as exemplified by the accumulation of microorganisms on waste collectors’ work clothes and their subsequent resuspension into the air within waste truck cabs. The findings revealed that fungi, specifically molds, were more prone to resuspension compared to bacteria. Additionally, the study demonstrated that great quantities of microorganisms, including pathogenic and allergenic species, accumulated on work clothes. These results highlighted the importance of implementing good hygiene practices for workers regularly exposed to elevated levels of microorganisms.
• The second study addressed an understudied aspect in the scientific literature, which is task-specific microbial exposure levels. Workers were grouped by their main tasks to assess bioaerosol exposures across four wastewater treatment plants during two seasons. The findings revealed that most workers involved in wastewater treatment processes or sewer system maintenance were exposed to significantly higher microbial levels compared to office workers, though some variability was observed in the exposure levels among workers performing the same tasks. Microbial community compositions of personal exposures were characterized through DNA sequencing and culturomics, uncovering the presence of pathogens and certain task-specific differences. Furthermore, a significant positive correlation between C-reactive protein, an inflammatory biomarker in workers' serum, and bacterial exposure levels was found. These findings are important because job titles alone may not accurately reflect workers’ exposures.
• As sewerage infrastructure in Denmark is being adapted to manage increased rainfall due to climate change, the occupational exposures faced by sewer maintenance workers may also change. Among these workers are a previously unstudied group—operators of combination vacuum-jet (jet-vac) trucks—who use high-pressure water jets and vacuum systems to maintain sewers. High-pressure cleaning and sewage work can lead to the generation of hazardous bioaerosols, making it critical to examine the biological risks faced by these workers. In the third study, jet-vac operators were grouped by the type of sewers they serviced—those conducting wastewater, stormwater, or a combination of both. The results showed moderate microbial exposure levels, with serum levels of Serum Amyloid A—an inflammatory biomarker measured in the workers’ serum—differing significantly between exposure groups.

Collectively, the findings in this Ph.D. project provide insights into the occupational risks faced by sanitation workers and provide new knowledge to inform exposure reduction strategies and improve worker protection