Contamination of surgical mask during aerosol-producing dental treatments

Dental health care professionals are exposed to numerous risk factors [1]. Mostly important are transmissions of infectious agents such as bacteria and viruses [2], but also the exposure to solvents, nanoparticles, and other substances can occur [3].

The transfer of microorganisms does not necessarily result in a risk to dental professionals. Infection and manifestation of an associated disease depend on the pathogenicity of the bacteria, the amount of them transmitted, and the current immune status of the contaminated person [1, 4]. Infectious agents can be transferred directly from the patient to the dental staff, from the dentist to the patient, and from patient to patient usually by establishing infection pathways via the staff or via (hollow) instruments, clothings, or the dental units [1, 5‐7]. The highest risk for the transmission of pathogens occurs by direct blood to blood contact. However, the most common and most intensively investigated source of infection in the dental practice is aerosol. Aerosols are released in numerous dental treatment modalities [8]. These are cleaning of oral surfaces with air-water spray, preparation of carious and non-carious tooth substances with high-speed handpieces, supra- and subgingival cleaning of biofilm contaminated tooth surfaces with ultrasonic devices or powder-water spray, and endodontic and oral surgery using ultrasonic instruments [9]. These aerosols may contain microorganisms from the oral cavity or from biofilms of the dental unit, as well as blood droplets and blood-borne viruses [1, 2, 6, 10, 11]. The treatment of numerous patients per day exposes dental staff to a high frequency to this contaminative agent [12].

To protect staff and patients, numerous unspecific protective measures are applied. Dental units, surfaces, and instruments are disinfected and sterilized, aerosols are reduced by suction, and staff disinfect their hands and wear protective clothing and specific protective equipment. Also rinsing of the oral cavity using chlorhexidine or other disinfectants prior to treatment is a protective measure [4]. The protective equipment for dental staff consists of gloves, goggles, and surgical masks. For surgical interventions, hair cover and gowns are also used. Clear recommendations have been formulated for hand disinfection and the use of sterile or non-sterile gloves. These recommendations are currently formulated at the level of state-associated boards [13]. However, studies on the compliance with infection control practices in dentistry do not show very encouraging results. Hand disinfection is frequently not performed correctly before dental work or after removing gloves as defined by the guidelines [14].

In contrast to gloves, the correct use of face masks has so far not been a major issue in guidelines. There are only a few recommendations [15] and very few systematic studies on their correct use. Previous studies have focused mainly on the safety of dental health care workers against high-risk pathogens [16]. The contamination potential of surgical masks themselves was investigated in only one study. The authors have found microorganisms accumulate on the outer surface of the surgical mask when masks were used more than 2 h [17]. However, the method of usage described in this study differs fundamentally from the application in dentistry. In dentistry, (I) the surgical face mask is usually worn for shorter periods of time; (II) moreover, it is used on nearly every patient, and (III) the face mask is contaminated with microbial aerosols during almost every treatment. It has not yet been investigated if microorganisms from oral biofilms released in aerosols during dental treatment survive on the surface of face masks. Similarly, it is unclear if it is possible to transfer microorganisms from a contaminated face mask to other surfaces. The objectives of this study were therefore (I) to investigate the microbial contamination of the face mask during different aerosol-producing dental treatment modalities and (II) to analyze if touching of the contaminated face mask had a contamination potential.

Two important findings were obtained through this study. First, we proved that microorganisms survive on surgical masks as used during dental work for a timeframe of at least about 30 min. Secondly, we demonstrated that viable microorganisms can be recovered by the hands from the contaminated masks and transferred to a susceptible surface. The study has thus proven a contamination pathway that has not been described so far. From the results of this study, it can be assumed that there is a risk of infection from a contaminated surgical mask via the hands to any other surfaces or to humans in dental facilities.

We showed clearly in our study that the hands could be contaminated by touching a contaminated face mask. This might result in a contamination of the dentist’s hand with microorganisms from the patient treated before with a relevant risk for the next patient or the therapeutist itself. As a consequence, face masks must be discarded after each patient. In addition, the hands must be disinfected after touching of the face mask, e.g., if the mask is removed after an aerosol-producing therapeutical procedure to talk to the patient and replaced for the next therapeutical step.

Within this study, aerosol-releasing treatment techniques were applied which are used typically several times in daily dentistry. The usual measure to prevent the dissemination of aerosols, the high-volume evacuation, was also applied. The evacuation proved not to be able to achieve a complete prevention of contamination of the outer surface of the surgical masks. This result confirms former studies on the exposure of the dentist to fine particles or microorganisms despite using the high-volume evacuation [18]. This underlines the importance of the mask for the protection of dental staff. Remarkably, the different treatment modalities showed no significant differences in microbial contamination of the mask and the transfer of microorganisms from the mask to the sterile gloves. The surgical mask seems to provide excellent conditions for the survival of oral or dermal bacteria. This could be caused by the humidity of the exhaled air of the mask wearer. Moreover, the temperature of the exhaled air, which is almost body temperature, is perfect for the replication of many bacteria. In a previous study [17], it was found that when wearing surgical masks for more than 2 h, an increasing number of microorganisms from the environment or from the oral cavity and respiratory system of the mask wearer accumulate. In the present study, sterile masks were worn for 30 min. No viable microorganisms could be detected in these control experiments. Although this result is in line with another study [17], it cannot be ruled out that, in addition to the patient’s own microorganisms, microorganisms from the dentist’s oral cavity may also reach the outer layer of the mask through the face mask. However, this limitation is not relevant for the conclusions of this study.

The majority of the microorganisms found in our study were typical bacteria of the oral or dermal microbiome. The species with the highest prevalence in this study was S. epidermidis. It was detected on two-thirds of the studied surgical masks and gloves (Table 1). One-fifth of masks and gloves showed contamination with other Staphylococci spp. The species M. luteus, R. dentocariosa, S. oralis, and Bacillus spp. were each detected on more than ten masks and gloves.

Although the contamination scores for the masks and the sterile gloves that touched the masks were significantly lower than that for gloves worn during dental work, a similar microbiological spectrum was identified.

The sampling technique and microbiological methodology used in the study had strengths and limitations. One strength of the cultivation of microorganisms on BHI agar is the exclusive detection of viable microorganisms. Non-viable bacteria are not able to cause infections. The selected agar, like any other microbiological substrate, has selective properties but is usually employed to detect the vast majority of fast-growing species. Obligate anaerobic microorganisms are not recorded, nor are various slowly growing bacteria. For MALDI TOF MS diagnostics, only colonies with different phenotypes were selected. This might led to an underestimation of the microbial spectrum. In cases of dense microbial growth, several areas were analyzed. However, competitive growth may also have reduced the detected microbial spectrum. The technique of microbiological sample collection may also underestimate contamination and microbial diversity on masks and gloves. The surgical mask is made of air-permeable material. The contact sample on agar probably only detects the microbes adhering to the surface. The gloves on the agar plates with an inner diameter of 90 mm could also not be completely brought into contact with the culture medium. In summary, the total numbers of viable microorganisms on both the mask and the gloves must be expected to be higher in real-life settings than in our study.

Some specific bacteria that were detected on masks and transmitted via the hands to agar plates will be discussed in the following. Each of the coagulase-negative staphylococci such as S. epidermidis or even S. aureus is potentially multi-resistant bacterial species. The prevalence of S. aureus was low in this study and lower than found in a previous study [19]. This species was only detected in three samples. Reasons for this could be that only patients who did not report general diseases were enrolled in this study and dental staff were informed and were highly compliant with the hygienic standards in our clinic. Reports are available in the literature showing a higher prevalence of S. aureus in dental students than in control groups [20, 21], but other studies revealed lower carriage rates [16]. Nevertheless, this species naturally represents a risk pathogen and in this study, a possible transmission path for S. aureus was demonstrated.

The most frequently isolated microorganism in this study was S. epidermidis. It was detected in 22 of the 50 microbiological samples obtained from gloves touching the surgical mask. This high detection frequency is in line with the results of other studies [22]. Staphylococcus epidermidis is the most common member of coagulase-negative staphylococci on human epithelial surfaces and it has to be regarded as an important nosocomial pathogen [23].

The other detected microorganisms such as S. capitis, S. oralis, M. luteus, or R. dentocariosa and all others are oral or dermal bacteria of the commensal flora. All these microorganisms are not pathogenic to healthy individuals, but may be hazardous to immunosuppressed or immunocompromised patients. Since in individual cases the health status of the patient and the risk factors for a facultative pathogenic species to become pathogenic are not always clear, it would be reasonable to consistently implement compliance with regulations and recommendations for the prevention of nosocomial infections [14]. For an infection and a clinical manifestation of a disease in a dentist or dental staff, both frequency of exposure and the virulence of the pathogen are of importance [1]. As a consequence, constant preventive behavior is of high importance since it is not possible to assess in dental practice whether a patient is colonized with pathogenic or facultative pathogenic microorganisms that can be transferred in the respective dose to a susceptible dental health care professional.

In this study, the contamination of face masks by viruses was not investigated. This represents an additional risk for dental staff and patients. However, in the context of the COVID-19 pandemic, general hygiene measures and those aimed at preventing the transmission of microorganisms should not be compromised.

The results of the study confirm that surgical masks are contaminated after aerosol-producing dental treatment procedures. As a consequence, masks are a potential source of contamination of gloves or the hands when touching a used surgical mask. The mask, which is useful for personal protection, should be discarded after each patient contact. Touching the outer surface of the mask should be avoided at any time. After touching or removing the mask, the hands must be disinfected.