Many arguments have been had during this past year over the effectiveness of infection prevention measures like mask wearing, social distancing, and increased sanitation. Critics point out that these tactics are all imperfect. They say, for instance, that surgical masks are designed for keeping a surgeon’s saliva from a patient’s exposed internal organs, not for filtering out viral particles. The thing is, they’re correct – none of these measures can give you the protection of a full-blown hazmat suit, complete with respirator and goggles. But, do they have to? 

In 1990, James T. Reason of the University of Manchester published a model of accident causation called “The Swiss Cheese Model” (1), which has been widely used in risk analysis and risk managementHe likened human systems to slices of swiss cheese, with each slice acting as a barrier, providing protection from accidents. A single point of failure (a hole) in one slice is not enough to allow an accident to occur – it requires an alignment of holes from all of the slicesWith more and more layers of protection, there is a smaller and smaller likelihood of holes in every layer lining up to create a dangerous incidentThis model applies perfectly to our current attempts at curbing the COVID-19 pandemic, so I think it’s a good idea to listen to Reason. 

Infection Prevention Slices 

Azenity Labs Infection Prevention Multi-Layered Approach

Physical Distancing 

By avoiding being near other people in general, we reduce contact with infected people. In the extreme, a person in complete isolation from the world can completely avoid coronavirus. Since very few of us can be quite that cut-off from society, holes in this defensive layer exist. With 6 feet of distance between people, larger respiratory droplets tend to fall to the ground before encountering another person. However, smaller droplets of respiratory aerosol can float around for hours, surfaces can also be contaminated, and people can’t always maintain that distance. More slices of cheese are needed. 

Surface Protection 

While airborne transmission appears to be the most common source of coronavirus infections, we frequently find infectious levels of viable SARS-CoV-2 virus on surfaces. Since other respiratory viruses (like cold and flu viruses) are commonly spread via surfaces (a.k.a. “fomites”), responsible facility managers will allocate resources for proper, regular sanitation of a building’s surfaces. This means physically removing soil build-up (cleaning), and chemically destroying the vast majority of germs remaining on the surface (disinfecting). When done properly, this process essentially puts surfaces at a clean slate. Surface testing can be conducted to validate sanitation, and can be used as a warning sign of the presence of infectious individuals in a space. 

Human error is responsible for many of the holes in this slice, as the quality of cleaning and disinfecting varies widely. Often due to time and resource constraints, soil (i.e. dust, grime, and build-up) is not fully removed, and surfaces sprayed with disinfectant are not allowed to remain wet for their full “dwell time”. Furthermore, an infectious individual can contaminate a surface after it has been cleaned and disinfected, so the frequency of sanitation services matters. Antimicrobial surface coatings can help in this regard. We are beginning to see antimicrobial surface coatings that demonstrate effectiveness against viruses in lab testing. Next generation sanitation regimes include these types of chemicals as a standard part of service. 

Air Protection 

High quality air sanitation in buildings can offer several defenses against airborne pathogens like coronavirusAir can be drawn in from outside through windows or HVAC systems, diluting any germs in a room’s air. Portable air purifiers (such as HEPA filter units) can filter out or attack germs that remain, supplementing protection from ventilation. As with surfaces, there are methods for testing and monitoring the air, both for the presence of germs like coronavirus, and for the air conditions that make infection more likely (e.g. temperature and humidity affect viral survivalwhile CO2 levels can act as a proxy for ventilation). 

Testing and Tracing 

Despite having lower traffic and making responsible improvements to a building’s surface and air sanitationinfected people are likely to spread some amount of coronavirus and other germs in a building. Testing of individuals allows for infected people to isolate and reduce the number of people they transmit the disease to. Further, managers can help to facilitate contact-tracing when a COVID-19 positive person has been in close contact with people in their space. Those who might have become infected from this person can quarantine to avoid the risk of further spread. 

Personal Hygiene 

Individuals can each do their part by being mindful of their own behavior. Mask-wearing is vitally important in public spaces, since it both greatly reduces viral aerosols at the source (the most effective prevention) and removes a significant portion of aerosols for those breathing in contaminated air. Mask quality varies greatly by fit and filtration ability, but even a basic home-made cloth mask is hugely impactful. Hand washing and hand sanitizers help to prevent germs on our faces and masks from getting onto other surfaces and vice-versa. 

Personnel Policy 

This infection prevention layer is vitally connected to all of the other slices. Good personnel policy facilitates and enforces all of the other layers. For example: 

  • Occupancy levels, workflow patterns, and spatial markings enable social distancing 
  • Doors can be propped, and other surfaces can be avoided or more frequently sanitized 
  • Rules can be put in place requiring occupants open windows and/or run air purifiers 
  • Entry screening and rules about when testing is required help with testing and tracing 
  • Hands-free sanitizer bottles and signage can help people to be mindful of hygiene 


Like the many safety features and traffic laws relating to automobiles, each of these layers of infection prevention is by itself inadequate to prevent people from getting infected. However, when added together, they can provide substantial levels of protection to the point that the risk is negligible. Additionally, by being transparent and communicating measures in place to improve pandemic safety in a space, occupants can also have the benefit of feeling safer in that space. 


1) Reason J. 1990 The contribution of latent human failures to the breakdown of complex  systems. Phil. Trans. R. Soc. Lond. B 327: 475–484. 

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