1 Introduction

Korea’s aquaculture is defined as artificially cultivating and harvesting aquatic animals and plants (including the production of fish seeds), and using fishing ships and fishing gears or installing and operating aquaculture facilities (Ministry of Oceans & Fisheries, 2022). Over the past decade (2010–2020), the occupational injury and illness rate in aquaculture was 2 to 3 times higher than that of the overall Korean industry (Ministry of Employment and Labor, 2022), but most previous researches on aquaculture of Korea have focused on factors such as efficiency and profitability of the farming operations (Baek, 2020; Park, 2012).

Aquaculture workers can be exposed various harmful factors (Cavalli et al., 2019; Ngajilo & Jeebhay, 2019), especially biological hazard such as microorganisms that proliferate due to feed, fish, and high relative humidity (Sherson et al., 1989; Yajima & Kurokura, 2008).

In aquaculture, various biological hazards including pathogenic microorganisms can exist in the culture water and the fish being cultured (Cole et al., 2009; Durborow, 1999; Fry et al., 2019; Swinker et al., 2002). When workers in contact with contaminated water, these biological hazard can infect workers through their skin (Cavalli et al., 2020). However, workers in aquaculture may also be exposed to biological hazards in form of aerosols in the air, even if they do not have direct contact with water or organisms (Gołaś et al., 2022).

Bioaerosols are very small airborne particles (0.001 ~ 100 μm) that can contain biologically generated and living organisms from plants/animals (Georgakopoulos et al., 2009), including viruses, bacteria, and fungi (Mandal & Brandl, 2011). Exposure to bioaerosols can lead to occupational diseases such as hypersensitive respiratory diseases like asthma, rhinitis, and bronchitis (Hu et al., 1997; Su et al., 2001).

Several studies on biological hazards or bioaerosol exposure for workers in aquaculture have been conducted (Sherson et al., 1989; Yajima & Kurokura, 2008), but in Korea, most studies have focused on consumers (Kim et al., 2008; Na & Jeong, 2020). Therefore, the purpose of this study is to evaluate the level of biological hazard exposure and to provide fundamental data for the health management of aquaculture workers in Korea.

2 Method

2.1 Subject

As shown in Fig. 1, ten aquaculture farms in areas where Korean aquaculture is concentrated were selected for representation (Statistics Korea, 2022). The farms are located in Gyeongsangnam-do, Jeju, and Jeollanam-do, and there are eight land water tank seawater aquaculture farms and two land water tank freshwater aquaculture farms. Some example pictures of farms are shown in Fig. 2. Each farm is aquaculturing fishes such as flatfish, rockfish, and eel. An overview of the farms is presented in Table 1. On-site evaluation was conducted in July to August 2022.

Fig. 1
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Overview of investigation area

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Fig. 2
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Example picture of aquaculture farm in Korea (Left: farm A, right: farm G)

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Table 1 Overview of aquaculture farms
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2.2 Measurement

Airborne samples for trapping airborne bacteria and fungi were collected using a six-stage viable particulate cascade impactor (TE-10–800, Tisch Environmental Inc., USA). The range of aerodynamic diameter at each stage was: stage 1 (> 7.0 μm), stage 2 (4.7–7.0 μm), stage 3 (3.3–4.7 μm), stage 4 (2.1–3.3 μm), stage 5 (1.1–2.1 μm), and stage 6 (0.65–1.1 μm). For sampling, culture media such as Tryptic Soy Agar (TSA) and Sabouraud Dextrose Agar (SDA) were used to trap airborne bacteria and fungi, respectively. A sterilized media was placed in each stage of the equipment for the collection of respective size of particle. The exterior of the collection equipment was sterilized with 70% ethyl alcohol in order to prevent contamination. The media after collecting air sample was immediately sealed with a parafilm for laboratory use as soon as it was taken out from the equipment and it was directly transported to microbial analysis laboratory where it is cultured before observation.

Samplings were conducted in the working area, resting area, feed making area, and outdoor. The working area is where the main tasks take place, consisting of corridors and fish tanks for culturing fish. The resting area is where the workers take a rest or eat some snacks. The feed making area is where make feed by frozen ingredient and feed making machine. In the working area, samplings were taken near the water tank, as close as possible to the surface of the water. In the resting area, samplings were taken at the center of the room, and in the feed making area, samplings were taken at a location where workers usually stand while working. In the outdoor, samplings were taken at a location which is 10 m away from the entrance of building. The impaction sampler was placed at a height of 1.2–1.5 m above the ground, and each of the samples was collected for 5 min at the flow rate of 28.3 ℓ/min. Samples were taken in three iterations from 2:00 PM to 4:00 PM, with a 30-min interval between each sampling.

The collected airborne microorganisms were cultured for 48–72 h at a temperature range of 15–30 °C, and their concentration was determined using a plate count method. In addition, the temperature and relative humidity of inside the aquaculture farms and outdoors, which affect the concentration of bioaerosols, were measured using a WBGT meter (hs-32, Metrosonics, USA). For outdoor temperature and relative humidity measurements, the meter was placed in a place exposed to direct sunlight and all measurements were recorded after stabilizing the sensor for about 3 min.

All the cultured airborne bacteria were identified to the level of genera according to Bergey’s manual, and after Gram staining, a biochemical test was conducted to identify genera with an automatic identification system—VITEK (Model VITEK 32 system, bioMerieux Inc., France). Airborne fungi were observed under an optical microscope at 100 × or 400 × magnification to assess the colony's shape and color, vegetative hyphae, sexual and asexual reproductive organs, and the spore's color and shape. Genera were identified according to the taxonomic method of Ainsworth and Baron.

2.3 Data analysis

Frequency analysis of data was dealt with SPSS ver 26.0 (IBM, USA). To calculate the average concentration of airborne bacteria and fungi, we combined all concentrations of six-stage impactor. After combining, we calculate the average combined concentration.

3 Results

3.1 Bioaerosol level of aquaculture farm

The levels of total airborne bacteria and fungi in ten aquaculture farms are shown in Figs. 3 and 4. For airborne bacteria, the highest measured value was 979.6 (± 252.2) CFU/m3 in working area of farm G, while the lowest was 24.6 (± 17.5) CFU/m3 in resting area farm F. The average concentration for the ten farms was 436.2 (± 298.8) CFU/m3 in working area, 100.8 (± 77.7) CFU/m3 in resting area, and 297.2 (± 227.1) CFU/m3 in feed making area. For airborne fungi, the highest measured value was 2,341.0 (± 1,424.5) CFU/m3 in working area of farm H, while the lowest was 10.5 (± 2.3) CFU/m3 in resting area of farm A. The average concentration for the ten farms was 528.9 (± 820.9) CFU/m3 in working area, 92.0 (± 135.5) CFU/m3 in resting area, and 333.8 (± 523.1) CFU/m3 in feed making area.

Fig. 3
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Airborne bacteria measurement results

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Fig. 4
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Airborne fungi measurement results

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The results of temperature and relative humidity measurements are shown in Table 2. The temperature near the water tanks ranged from a maximum of 28.3 °C to a minimum of 25.5 °C, while the outdoor temperature ranged from a maximum of 31.5 °C to a minimum of 23.4 °C. The relative humidity near the water tanks ranged from a maximum of 93% to a minimum of 84%, while the outdoor relative humidity ranged from a maximum of 83% to a minimum of 77%.

Table 2 Temperature and relative humidity measurement results
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3.2 Size distribution of bioaerosol of aquaculture farm

Size distribution of airborne bacteria and fungi captured in a Korean aquaculture farm using the Andersen cascade sampler (six stages) is shown in Fig. 5. For airborne bacteria, stage 2 (4.7–7.0 μm) had the highest proportion at 23.2%, while for airborne fungi, stage 1 (> 7.0 μm) accounted for the highest proportion at 39.3%. As for the respirable size range (below 5.0 μm, stage 3 ~ stage 6), 57.0% of airborne bacteria and 51.5% of airborne fungi were classified as respirable sizes.

Fig. 5
figure 5

Size distribution of airborne bacteria and airborne fungi of aquaculture in Korea

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3.3 Identification of bioaerosol of aquaculture farm

The concentration and identification of airborne bacteria in a Korean aquaculture farm are shown in Table 3. The measurement results indicated that Staphylococcus spp. accounted for the highest proportion, at 56.3% and 57.2%, in both seawater and freshwater aquaculture farms, respectively. Following that, Micrococcus spp. (19.8% and 15.2%), Corynebacterium spp. (9.3% and 12.1%), and Bacillus spp. (7.7% and 7.2%) were observed in descending order. Enterococcus spp., Streptococcus spp., Enterobacteriaceae spp., and E. coli spp., among others, accounted for less than 2%. Klebsiella spp. was not detected in either seawater or freshwater aquaculture farms. The trends were generally similar for respirable size airborne bacteria as well.

Table 3 Identification and level of airborne bacteria of aquaculture in Korea
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Table 4 is regarding the concentration and identification results of airborne fungi in a Korean aquaculture. The measurement results indicated that Cladosporium spp. accounted for the highest proportion, at 33.0% and 32.4%, in both seawater and freshwater aquaculture farms, respectively. Following that, Penicillium spp. (23.9% and 22.3%), Aspergillus spp. (16.8% and 16.0%), and Alternaria spp. (12.9% and 17.1%) were observed in descending order. The trends were generally similar for respirable size airborne fungi as well.

Table 4 Identification and level of airborne fungi of aquaculture in Korea
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4 Discussion

Current indoor biological hazard management standards of the Korean Ministry of Environment set the maximum allowable concentration of total airborne bacteria at 800 CFU/m3 and total airborne fungi at 500 CFU/m3. Compared to these standards, both working area of farm G and H exceeded the limits for airborne bacteria and fungi. In particular, the concentration of airborne fungi was four times higher than the management standard.

In all aquaculture farms, the concentration of bioaerosol appeared higher in the order of working area–feed making area–resting area. The reason for feed making area having a lower concentration of bioaerosol than working area is thought to be the existence of fish and water (Gołaś et al., 2022), low temperature with seasonal variation. Feed making area has low temperature compared to working area for frozen feed processes, which makes it difficult for bacteria and fungi to proliferate. Additionally, this study was conducted in summer season; this temperature difference would have been more extreme. And also it is believed that the fewer numbers of workers also contribute to this difference.

Farm G and H showed much higher levels of airborne bacteria and fungi than other farms, and this can be attributed to the following reasons. First, it is the influence of the climate. Jeju Island, where farms G and H are located, is characterized by hot and humid summers due to the influence of the Southeast Asian monsoon (Bae & Nam, 2020). In addition, on the day of measurement, G and H farms recorded higher temperatures and relative humidity compared to other farms. High humidity is a factor that increases the population of bacteria and fungi (Arundel et al., 1986). Therefore, it is estimated that high temperatures and humidity have affected the concentration of biological hazards in farms G and H. However, the concentration of airborne bacteria and fungi in farms E and F, which have similar temperatures and relative humidity levels in the same area of Jeju Island, is lower, indicating that the influence of the climate is not absolute.

Second, the shape of the tanks may be a factor. In farms A ~ F and I, J, the water surface of the tanks was located at chest height. In contrast, in farms G and H, the water surface of the tanks was located at foot level. According to previous study, the concentration of bioaerosol decreases as one moves further away from the water surface (Li et al., 2013; Yang et al., 2019); however, our results showed a different pattern. Therefore, it is believed that there may be other causes here.

Third, working pattern is a factor. In comparison to other fish farms, farms G and H had more work done during measurements and had more moisture on the ground. Human activity is the main factor of indoor airborne bacteria and fungi (Buttner & Stetzenbach, 1993; Hospodsky et al., 2012). Moreover, when transferring fish to other tanks, a large amount of water spills out, which can increase the concentration of bioaerosol. It is believed that this is the main factor of high concentration of bioaerosol in farms G and H.

Therefore, it is considered that particularly high bioaerosol concentration in farms G and H was most influenced by the quantity and form of work. This also indicates that under certain conditions, aquaculture workers may be exposed to very high concentrations of airborne bacteria and fungi.

Upon examining the size distribution characteristics, it was observed that airborne bacteria exhibited a relatively even distribution across different sizes, whereas fungi were concentrated in stage 1 (> 7.0 μm). This is attributed to the morphological difference where fungi generally have larger particle sizes compared to bacteria. This finding aligns with previous studies reporting that the average particle size of fungi is larger than that of bacteria (Ghosh et al., 2015; Pastuszka et al., 2000; Qian et al., 2012).

However, in the respirable size range (below 5.0 μm, stage 3 ~ stage 6), both airborne bacteria and fungi showed proportions exceeding 50%. Respirable bioaerosols are easily suspended in the air and can readily enter the human body, potentially causing infectious diseases (Kim et al., 2009; Liang et al., 2020; Pastuszka et al., 2000; Reponen et al., 1994). In this study, more than half of airborne bacteria and fungi in the aquaculture farm were in the respirable size range, so extra caution should be taken regarding infections caused by small-sized bioaerosols.

On the other hand, humidity is one of the factors that influence the size of bioaerosols (Pasanen et al., 1991; Reponen et al., 1996). However, in the case of fungi, it has been reported that most fungal spores in the size range of 0.1 to 10 μm have low hygroscopicity and do not have a significant impact on respiratory deposition (Kesavan et al., 2017; Pan et al., 2021; Walls et al., 2017). Aquaculture farms maintain a higher relative humidity compared to the outdoor environment, which can lead to the generation of larger-sized bioaerosols. However, the elevated relative humidity can also increase the opportunity for biological hazards, particularly the growth of fungi (Arundel et al., 1986).

Based on the identification results of airborne bacteria, Staphylococcus spp., Micrococcus spp., Corynebacterium spp., and Bacillus spp. were determined to be the dominant species, accounting for over 90% of the total. These findings are similar to previous studies conducted in other industries such as hospitals (Kim et al., 2010), child day-care centers (Aydogdu et al., 2010), and poultry farms (Plewa & Lonc, 2011). Among them, Staphylococcus spp., which had the highest prevalence in this study, has been identified as a major biological hazard in aquaculture (Pridgeon & Klesius, 2012).

Regarding airborne fungi, Cladosporium spp., Penicillium spp., Aspergillus spp., and Alternaria spp. were identified as dominant species, accounting for 86.6% and 87.8%, in both seawater and freshwater aquaculture farms, respectively. These results are similar to findings from outdoor air studies (Khan et al., 1999) and general households (de Ana et al., 2006). However, when comparing the results of Ziaee et al., (2018) with our study, the order of dominant species differs. This difference can be attributed to factors such as humid air in the aquaculture farms and distinction in climate between the measuring locations. Additionally, in our study, the category Unidentified exhibited a higher proportion, at 13.4% and 12.2%, compared to airborne bacteria. This can be attributed to the preferential classification of the four dominant species and the influence of mold introduced from external sources due to the coastal location of the aquaculture farms.

Both airborne bacteria and fungi exhibited I/O ratios (indoor-outdoor ratios) exceeding 1, and for fungi, the maximum ratio reached 8.0. These values are generally higher compared to feedstuff-manufacturing factories (3.1 ~ 6.4) (Kim et al., 2009), hospitals (Kim et al., 2010), child day-care centers (0.4 ~ 4.7) (Aydogdu et al., 2010), and general households (0.71 ~ 9.15) (Ye et al., 2021). This suggests that the indoor environment of the aquaculture farms is contaminated with airborne bacteria and fungi at higher concentrations than the outdoor environment.

The limitations of this study are as follows. First, the number of aquaculture farms surveyed was insufficient. The farms are not enough to represent the aquaculture industry in Korea, and consideration should be given to various fish species. Second, this study was conducted in short period in summer. To gather more insight about the bioaerosol exposure of aquaculture workers, it is needed to conduct seasonal measurement and comparisons. Finally, type of work was not distinguished, which the amount of bioaerosol emitted may depend on the type and load of work being done.

However, this study is the first to conduct field assessments of bioaerosol exposure among aquaculture farms in Korea. Therefore, future research that supplements the limitations based on the results of this study will be necessary.

5 Conclusion

Bioaerosol exposure assessment for Korean aquaculture workers was conducted. The measurement results showed that two out of the ten surveyed aquaculture farms had total airborne bacteria and fungi concentrations that exceeded the Korean management standard. The causes of only these two farms exceeding the indoor management standards of airborne bacteria and fungi, i.e., 800 CFU/m3 and 500 CFU/m3, are not clear. So, future works will have to reveal these causes. Additionally, concentrations of airborne bacteria and fungi at levels between 10 and 50% of the management standard were measured in most of the surveyed farms, apart from the two that exceeded the standard. In size of bioaerosol, respirable size proportions were over 50%, both airborne bacteria and fungi. So, more extra caution is needed for small-sized bioaerosols. The identification results revealed the presence of dominant species similar to other industries in previous. However, the aquaculture farms exhibited a higher I/O ratio compared to other industries. Therefore, based on this study, more in-depth research on bioaerosol exposure in aquaculture workers should be conducted, taking into account additional factors such as the shape of the water tanks, species of farmed fish, and types of work.