Student Blog

"Understanding the Burden of Facemasks' Bacterial Contamination in Healthcare Facilities"

Authors: Lana Kouatli and Manali Mody

Understanding the Burden of Facemasks Bacterial Contamination in Healthcare Facilities

1. Why wear masks in healthcare?

Before discussing masks and their relationship to bacterial contamination in healthcare facilities, we want to explain what Personal Protective Equipment is and its relevance to this topic.

Masks, like gloves and gowns, are an important piece of Personal Protective Equipment (PPE), used to minimize the exposure that people may have to certain hazards in locations such as healthcare facilities, public areas, or even common spaces (WHO, 2022). There are a range of hazards from which PPE protects against, including biological, radiological, electrical, chemical, and mechanical hazards. However, in healthcare settings PPE has been specifically developed with the aim of protecting against biological hazards and especially infectious agents, among which bacteria and viruses are the most common.

So, why do healthcare professionals wear masks and PPE in healthcare? This is due to most healthcare facilities hosting persons sick with various conditions including infections, which are more likely to carry infectious organisms than the general population, and more likely to spread them through cough, contact between wounds or contaminated skin with other people (e.g.: nurses, etc.) or with commonly touched objects and surfaces. Since healthcare workers’ jobs are characterized by frequent and often close contact with sick patients, the bacteria can transmit from the patient to the worker. PPE, when used correctly, prevents this transmission from occurring. Microbes can spread through several different manners (such as hands-on contact, airborne transmission, contamination of surfaces, etc.), therefore, different PPE must usually be used together to prevent pathogens from spreading further (for example, gloves to prevent transmission through skin, masks to prevent transmission through respiratory droplets, and gowns to prevent transmission through secretions, bodily fluids etc.) (WHO, 2022).

People in general and healthcare workers should wear PPE to protect themselves and prevent people at elevated risk for illnesses from getting sick. Many of us are at high risk or could become high risk at certain times: high risk categories include people of old age, patients with open wounds or damaged/inflamed skin or mucosal membranes, weak immune system or many other chronic and acute medical conditions, drug users, people using many kinds of medications including steroids and immunosuppressants, and pregnant or recently pregnant individuals. Additionally, if one is traveling, it may be wise to use PPE to reduce the spread of novel diseases from one location to another (CDC, 2020). Thus, PPE use can prevent many infections and save many lives.

2. Do masks become contaminated with bacteria?

Leading up to our research question, it may first be necessary to investigate why the contamination of PPE (specifically masks) may be emerging as an important topic of research.

During the height of the SARS-CoV-2 (COVID-19) pandemic, masks were a key PPE that the general public and healthcare workers wore. Masks trap bacteria and viruses and were made obligatory since they were the best possible way to prevent the spread of the virus in public areas. Throughout the use of masks in the pandemic, it was found that face masks were often touched with the hands and encountered many different surfaces throughout the time they were put on and taken off. This exposure to different surfaces makes it exponentially easier for microorganisms to attach to the mask and “hitchhike” from place to place with the carrier (Nightingale et al., 2023). Before the pandemic, masks were only worn for specific cases and short periods of time, and thus accumulation of bacteria on the mask was not considered an issue. In the few cases where masks were worn for a longer time (e.g.: surgeons and their teams), they were typically not touched by the wearer, and it was assumed that their outside surface would not become meaningfully contaminated. However, during the pandemic masks are worn for extended periods of time in healthcare facilities (hours and hours), and this prolonged time worn increases the risk of contamination.

 3. If they do become contaminated, should we worry about it?

Despite these concerns, there is presently very little research data availability, and thus little awareness among health facilities personnel, about the potential for masks to become contaminated. For this reason, our research group decided to initiate a study on this critical topic (Nightingale et al., 2023).

We tested for bacterial contamination of masks that are worn by healthcare workers in a post-acute care facility, a setting where the patients are often colonized with antibiotic-resistant organisms. We aimed to address two main questions for this project: 1) are these resistant organisms found in healthcare workers’ masks, and 2) will the contamination be higher if the masks are worn for longer periods of time? Surprisingly, there have not been many papers on this topic. There was a relatively recent study focusing on viruses, but not bacteria (Chugtai et al., 2019). We thought this would be a useful contribution because all healthcare workers have been wearing masks for extended periods of time during the COVID-19 pandemic, and yet it is not known how often masks could become contaminated with bacteria, and what the origin of such hypothetical bacteria would be.

4. Our quest for knowledge

To accomplish this task, the first thing the authors did was to contact a local post-acute care facility and seek permission to perform our study, which included not only voluntary collection of used facemasks, but also noting anonymous information on donor workers’ job type (including patient contact and non-contact job), shift hours, number of hours the mask had been worn, and some demographic information as well. A table was set up in the lobby to collect masks from health care providers after their shift. Collection was performed on 3 separate days to gather masks from as many health care personnel as possible. These worn masks were brought to the lab within 2 hours of collection and incubated in a broth for 18-24 hours, at 37 degrees Celsius to allow any bacteria present on the masks to grow. To allow identification, bacteria growing in the broth were then inoculated onto several solid growth media plates using a plastic loop. Each of these media is selective for a specific class of organisms. Bacterial colonies growing on each selective solid medium were identified using a variety of chemical and microbiological tests. Additionally, bacteria were tested against commonly used antibiotics, to establish if they were susceptible or resistant. Finally, the data was entered into a spreadsheet and analyzed to answer our research questions. Specifically, the authors were especially interested in the number of different bacterial species grown from each mask, their species and antibiotic resistance profiles, and whether such findings varied significantly between different categories of healthcare workers, or between workers who had worn the mask for shorter versus longer periods of time.

5. Our major findings, and how they can inform our actions

The data from this study, obtained from the 69 masks which were donated to the lab from healthcare workers, led to many interesting observations, which are summarized below.

Each mask was contaminated by at least one microorganism. Specifically, the range was from one to five different microorganisms on each mask. As described in the research manuscript describing this study (Bacterial Contamination on Used Face Masks Among Nursing Home Healthcare Personnel, published in the Journal “Antimicrobial Stewardship and Healthcare Epidemiology”) masks hold a high bacterial burden.

Among the most common pathogens found on the masks, 11 masks (15.9%) had Staphylococcus aureus, and 30 masks (43.5%) had VSE (vancomycin-susceptible enterococci) (staphylococci and enterococci belong to the gram-positive category of bacteria). Also, 22 masks (31.9%) were contaminated with clinically important bacteria in the gram-negative bacilli category, such as Klebsiella Pneumoniae, Enterobacter, and Escherichia Coli (Nightingale et al., 2023). On the bright side, there were low rates of bacteria that are resistant to antibiotics. Indeed, those were found in only two of the masks (2.9%).

Also interestingly, the authors did not find statistically significant differences between masks worn by various categories of healthcare workers (e.g.: those with and without contact with patients).

This study also discusses the meaning of the findings and tries to address the problem of increased mask contamination over time. Since most of the masks had been worn for many hours, it was not possible to determine at what time the burden of bacteria starts to increase since a mask is initially put on. However, the authors mention that other scientists focusing on different research questions have recently found that the quantity of bacteria on masks may increase during the first two hours, after which a plateau of high burden is reached.

Overall, the research project proved that masks acquire a high bacterial burden. In turn, these bacteria may be a source of contamination, but this study does not address transmission to patients or others specifically. Regardless, a major takeaway from this study is that masks should be discarded and replaced with new ones often, for example every two hours.

6. Conclusion and open questions

Of course, many questions remain open and should be the object of further investigation, including the potential impact of the material used to make the masks. Do different types of materials influence bacterial growth? Additionally, it would be important to understand how these results would vary in a different healthcare setting. For example, surgeons working in an operating room or medical doctors seeing hospital patients. Or, for that matter, how often do masks worn by students and other people not in healthcare facilities get contaminated on any given day? Hopefully, we can answer these questions soon.


WHO. (2022). Personal protective equipment (PPE).

Nightingale, M., Mody, M., Rickard, A. H., & Cassone, M. (2023). Bacterial contamination on used face masks among nursing home healthcare personnel. Antimicrobial Stewardship & Healthcare Epidemiology, 3(1): e54.

Chughtai, A.A., Stelzer-Braid, S., Rawlinson, W. et al. Contamination by respiratory viruses on outer surface of medical masks used by hospital healthcare workers. BMC Infect Dis 2019;19: 491.

CDC. (2020, February 11). Coronavirus Disease 2019 (COVID-19). Centers for Disease Control and Prevention.