Industrialization has led to increased animal density in enclosed production buildings, resulting in high concentrations of viable and non-viable bacteria and fungi, as well as metabolites in bioaerosols [21]. The poultry industry has been found to pose a significant global health risk due to microbiological contamination [73]. Farm facilities housing multiple animals promote complex mixtures of microorganisms in bioaerosols, including dust-containing feathers, skin fragments, feces, feed particles, microorganisms, and chemicals [74]. Long shifts in manufacturing plants have become common, resulting in workers inhaling complex bioaerosols, which can pose several health hazards in agricultural environments [21]. This situation has prompted increased studies on occupational health. Bioaerosols from farms can also pose health risks to nearby residents [53,74], highlighting the importance of research on human health, environmental impact, and the One Health approach to address these concerns. Broilers and laying hens are susceptible to bacterial and viral infections of the upper respiratory tract, as indicated by several studies [38,51,62,70]. The transmission of pathogens can occur through inhalation, close contact with infected animals, feces, litter, or contaminated objects, and inadequate biosecurity controls can result in significant economic losses [74]. As international trade expands, food safety concerns regarding the rapid spread of foodborne pathogens through the global food chain are increasing [73].

Moreover, environmental health concerns arise from the utilization of animal by-products, such as poultry manure and litter, in agriculture. Repeated use of these by-products as manure can lead to the accumulation of contaminants in agricultural soils, potentially increasing their bioavailability and toxicity in the environment [74]. Air sampling has been widely used to characterize occupational exposure to fungi, but it is important to consider the appropriate sampling period and the influence of variables such as ventilation and building features. Passive sampling methods, such as settled dust assessment, have been shown to be more reliable for collecting contamination over a longer period of time. Broiler manure and animal bedding have been identified as the primary sources of indoor air microbial contamination in the poultry industry [76,77,78,79,80,81,82,83].

It is recommended to use a multiapproach sampling protocol for a more comprehensive understanding of microbial contamination. While culture-based methods have been primarily used for microbial characterization, culture-independent methods such as cloning approaches and quantitative real-time PCR have shown to be suitable for various bioaerosol measurements. Molecular tools, such as whole-genome sequencing, could provide more information on the biodiversity of microorganisms in these environments. Overall, these findings highlight the importance of considering various sampling methods and assays in the assessment of indoor microbial contamination in the poultry industry [60,61]. Studies on bioaerosols in poultry production are limited and identifying all organisms, both viable and non-culturable, is important for characterizing bioaerosols in these facilities [60]. Inhalation exposure to non-viable microorganism components such as endotoxins and mycotoxins may cause health hazards, so evaluating non-viable components may be useful for assessing pulmonary disease risk. Microbial assessment of poultry farms shows the presence of numerous microbes, including zoonotic pathogens, which can act as transport agents of airborne diseases [49,61]. Despite the growing threat of fungal infections to human health, there are fewer studies conducted on fungi (and also viruses) compared to bacteria, and this lack of attention and resources makes it challenging to determine the precise burden of fungal infections and to encourage policy and programmatic action [75].

Due to the extensive use of antibiotics in the livestock industry, these facilities are significant sources of antibiotic resistance genes (ARGs). Therefore, multidrug-resistant bacterial pathogens may be transmitted through the inhalation of bioaerosols [55]. This explains the frequent screening for bacterial antimicrobial susceptibility by several studies [29,37,43,45,49,53,63,69,71,72,75,77].

Several potentially pathogenic bacteria have been identified [24,30,75,77]. The potential dispersal pattern and distance of airborne bacteria and ARGs from these animal sources remain unknown [53]. However, it is important to note that clinically significant multidrug-resistant bacteria Staphylococcus sp. [53,75], E. coli [29], Campylobacter jejuni [71], among others, belonging to the WHO priority pathogens list of antibiotic-resistant bacteria (2017), were isolated from poultry farms.

Recently, the World Health Organization (WHO) published the first fungal priority pathogens list [79], listing 19 groups of human fungal pathogens associated with a high risk of mortality or morbidity. This formal recognition by the WHO highlights an important group of infections, which has been perennially neglected in terms of the awareness and research funding needed [80].

Regarding fungal assessment, despite the low number of studies (14 out of 58, 24%), several fungi comprising the critical priority group of the WHO list (2022), namely, A. fumigatus, were frequent in indoor air samples [32,36,47], along with Candida albicans [32]. Regarding the high-priority group, Fusarium sp. [32,38,47], the order Mucorales [40,47], and Candida tropicalis [36] were also some of the ones reported.

Concerning microbial components, endotoxin, a major component of the outer membrane of Gram-negative bacteria, poses a serious health risk [34]. Endotoxins found in airborne organic dust have been linked to respiratory disease in both humans and animals [34].

Regarding mycotoxins, some of the literature already evidenced occupational exposure in animal production facilities [40]. In fact, fungal species recognized as mycotoxin producers were reported in some of the selected studies [32,36,42,78]. Even though only one of the selected studies performed mycotoxin assessment, the obtained results are enough to hypothesize that workers in these settings may be at a higher risk of Aspergillus mycotoxicosis. Indeed, elevated concentrations of A. flavus and A. versicolor were recovered through environmental sampling. Additionally, through human biomonitoring, analysis of mycotoxins and/or their metabolites in blood and urine evidence detectable levels of the carcinogenic mycotoxin AFB1 [40].

Briefly, to mitigate and decrease such pollutants it is crucial to establish international standards for what constitutes good microbiological indicators from environmental samples, which could be used to guide risk reduction decisions and create effective incentives for people to follow such guidance, which have already been suggested [81].

Globally, temperature rises due to climate change have various impacts on ecosystems, human health, animal health, and food production, which also affect AMR [81].

The emergence of resistant fungal strains in occupational exposure scenarios has already been demonstrated [82,83]. Indeed, temperature increases may influence the susceptibility of pathogens (bacteria, fungi, and parasites) in chicken environments [84]. Thus, as in the case of bacteria, antifungal resistance should be addressed in further research [85,86]. Additionally, it is crucial to investigate the effects of heat stress on poultry production to formulate various effective mitigation strategies to reduce significant production losses [84].

The prevalent airborne microorganisms in animal production buildings are not well characterized in terms of quantity, composition, and risk group. Identification and quantification would be useful for determining the causative agents and performing risk assessments [27].

The poultry industry must be sustainable, and it needs to produce more with less, while benefiting all [87]. The sector must improve human, animal, and environmental health and welfare. Implementing a comprehensive and coordinated One Health approach that incorporates exposure assessment can help tackle threats to health and ecosystems [81], ensuring priority areas for action in order to mitigate microbial exposure, promoting a safe environment for workers and animals in poultry facilities, along with less environmental impact.

Overall, these findings highlight the need for improved biosecurity measures and environmental management practices to ensure animal health, food safety, and environmental sustainability in the poultry industry.