Composting plants are regarded as one of the important sources of environmental bioaerosols. However, limitations in the size distribution of airborne bacteria have prevented our comprehensive understanding of their risk to human health and their dispersal behavior. In this study, different sizes of airborne bacteria were collected using an eight-stage impactor from a full-scale composting facility. Size-related abundance and communities of airborne bacteria as well as human pathogenic bacteria (HPB) were investigated using 16S rRNA gene sequencing coupled with droplet digital PCR. Our results indicate that the bacterial concentrations from the eight stages were approximately 10⁴–10⁵copies/m³. Although no statistical correlation was detected between the particle size and the Shannon index, the influence of size on bacterial lineages was observed in both composting and packaging areas. For airborne bacteria from different stages, the dominant phyla were Firmicutes, Proteobacteria, and Actinobacteria, and the dominant genera was Bacillus. Seven out of eight HPB with a small geometric mean aerodynamic diameter had a high concentration in composting areas. Based on diameters of 2.42 to 5.09 µm, most HPB in the composting areas were expected to be deposited on the bronchus and secondary bronchus. However, in the packaging areas, the deposition of HPB (diameters 3.70 to 8.96 µm) occurred in the upper part of the respiratory tract. Our results on the size distribution, abundance, and diversity of these bacteria offer important information for the systematic evaluation of bacterial pathogenicity and the potential health impacts on workers in composting plants and the surrounding residents.

堆肥厂被认为是环境生物气溶胶的重要来源之一。但是，空气传播细菌的大小分布有限，使我们无法全面了解其对人体健康的危害及其传播行为。在这项研究中，使用八级撞击器从大型堆肥设施中收集了不同大小的空气传播细菌。使用16S rRNA基因测序与液滴数字PCR结合，研究了空气传播细菌以及人类病原菌（HPB）的大小相关丰度和群落。我们的结果表明，八个阶段的细菌浓度约为10 ⁴ –10 ⁵拷贝/ m ³。尽管在粒径和香农指数之间未发现统计相关性，但在堆肥区和包装区均观察到粒径对细菌谱系的影响。对于不同阶段的空气传播细菌，优势菌群是Firmicutes，Proteobacteria和Actinobacteria，优势菌属是芽孢杆菌。几何平均空气动力学直径小的HPB中有八分之七在堆肥区中浓度很高。基于直径为2.42至5.09 µm，堆肥区的大多数HPB预计会沉积在支气管和次级支气管上。但是，在包装区域，HPB的沉积（直径3.70至8.96 µm）发生在呼吸道的上部。我们对这些细菌的大小分布，丰度和多样性的研究结果为系统评估细菌的致病性以及对堆肥厂工人及其周围居民的潜在健康影响提供了重要信息。