Summary

Biological degreasing stations, also called bioremediating parts-washing systems, biological parts washers or simply biowashers, use a degreasing agent containing bacteria that break down fats, oils and greases (FOG) by mineralization. The manufacturers of these agents claim the microorganisms used are harmless, since they are classified as Risk Group 1 according to the four-group infection risk ranking system. Some researchers, however, identified a number of Risk Group 2 bacteria (moderate individual risk, low community risk), such as Pseudomonas aeruginosa, in biological degreasing station solutions, but no metrological data were available for assessing the occupational risk of exposure through inhalation. The purpose of this study was to provide such data.

Five biological degreasing stations were monitored for a year. Bioaerosols were sampled every two months using an Andersen single-stage impactor and three-piece polystyrene filter cassettes. Sterile tubes were used to collect 50-mL samples of degreasing fluid from the biological degreasing stations. In addition, for each biological degreasing station, a 50-mL sample of unused degreaser was collected straight from its container at the first visit. The samples were used to count and identify culturable bacteria, either directly by incubation of the agar medium from the Andersen impactors or using 200-µL smears of extracts from the polycarbonate filter or the liquid samples. Several methods were used to identify the bacteria: Gram staining, catalase test, oxidase test, MicroScan plate reading, fatty acid profile analysis and mass spectrometry analysis.

The year-long monitoring of liquids from the five biological degreasing stations demonstrated that culturable microorganism concentrations ranged from 3.6 x 10⁴ to 2.6 x 10⁷ CFU/mL. Sixty species of bacteria classified as Risk Group 1 or Risk Group 2 were identified, including Gram-positive as well as Gram-negative bacteria. Several bacteria genera were found, including Bacillus, Pseudomonas, Citrobacter, Burkholderia, Staphylococcus and Stenotrophomonas, though only the species Bacillus subtilis was found in the unused solutions for all five biological degreasing stations. In other words, the biological degreasing stations were rapidly colonized by exogenous microorganisms such as Pseudomonas aeruginosa. The main risk with skin contact is wound infection or accidental ingestion—should the mouth come in contact with the hand or with a contaminated object, for example. Strict personal hygiene measures, including wearing gloves and hand-washing before and after using the biological degreasing station, are therefore necessary.

This study shows that workers using a biological degreasing station have very low exposure to bioaerosols. While recommended intervention levels for occupational exposure to bioaerosols are around 10⁴ CFU/m³, the average ambient concentrations measured during this study were all below 480 CFU/m³. Moreover, use of an air blower to dry parts degreased in the biological degreasing stations did not significantly increase worker exposure to culturable microorganisms. No respiratory protection is therefore recommended during biological degreasing station use.