January 19, 2021

Parsing COVID-19 transmission routes informs levels of protection needed

By: Judith M. Mathias, MA, RN

Information about COVID-19 transmission and treatment has evolved between the time this infectious disease first emerged and now. As evidence-based knowledge grows and protocols change, and as populations are vaccinated to develop herd immunity, there is increasing optimism about the ability to combat the virus. Meanwhile, it is important to understand the various ways in which the virus is transmissible and what healthcare workers can do to best protect themselves and their patients.

The Centers for Disease Control and Prevention (CDC) in early reports noted that COVID-19 spreads through droplet transmission, and that droplet transmission most commonly happens during close exposure to an infected person. The CDC also said that COVID-19 primarily spreads via respiratory droplets produced when the infected person coughs, sneezes, or talks. Patients in droplet isolation require healthcare workers to wear surgical masks, gloves, and gowns.

There is now more discussion about COVID-19 also having properties that require airborne precautions because of aerosolized particles. When airborne transmission is known, isolation protocols are stricter. Precautions include wearing an N95 or higher level respirator, eye covering, gloves, and gown, and—if possible—isolating patients in negative-pressure airborne infection isolation rooms.


These discussions of droplet vs airborne have resulted in considerable controversy about the precautions that should be taken when caring for patients in the COVID-19 era, says Amber Wood, MSN, RN, CNOR, CIC, FAPIC, senior perioperative practice specialist at AORN.

Speaking at an AORN virtual workshop sponsored by 3M-KCI on “COVID-19 Precautions: Protecting the Perioperative Team and Their Patients,” Wood noted: “There is no black and white when it comes to droplet versus airborne precautions. We are entering ‘50 shades of gray.’”

The epidemiological triangle, which is similar to the chain of infection, is helpful in understanding how COVID-19 is transmitted because it hones in on several key elements, says Wood.

To transmit an infection, there has to be a viable pathogen, an environment that facilitates transmission, and a host that is susceptible, and the pathogen has to be alive throughout the transmission process (sidebar, “Epidemiological triangle”).

Source for all slides: Amber Wood, MSN, RN, CNOR, CIC, FAPIC. Used with permission.

“These are critical, and we have to look for ways we can stop transmission of the COVID-19 virus at all three of these points,” she says.


Universal pandemic precautions

There are standard precautions, termed “universal precautions,” that are taken for blood and body fluids from all patients because any patient could be infectious.

These precautions comprise six practices:

Hand hygiene.

• Providing the patient with a clean environment.

• Personal protective equipment (PPE). “Any time there is a risk for a splash, we need to wear eye protection and face protection, and any time we are going to do tasks that require us to touch body fluids, we need gowns and gloves,” says Wood.

• Respiratory hygiene. “We need to cough into our elbows, and the world has never been better at this,” she says.

• Safe injection practices to prevent transmission of bloodborne pathogens.

• Cleaning reusable equipment, such as stethoscopes, glucometers, and blood pressure cuffs, in between patients.

With COVID-19, new standard precautions are being added, which are being called “universal pandemic precautions.” These include mask and eye protection for all direct patient contact.

“Because COVID-19 is transmissible before patients have symptoms and transmissible in patients who never show symptoms, we don’t always know who has it. This is why we have the universal pandemic precautions. So we can make sure we are protected until we get more information and can assess the patient further,” says Wood.


Transmission-based precautions

Transmission-based precautions include contact, droplet, and airborne.

There are two ways of transmitting through contact:

• Direct—transmission directly from one person to another. For example, a healthcare worker touches an infected patient and then acquires the same infection.

• Indirect—transmission from an object to a person. For example, a healthcare worker touches an infected patient’s bed and then acquires the same infection.

Patient conditions considered for contact precautions include multidrug-resistant organisms, Clostridioides difficile, and SARS-CoV-1.

Contact precautions involve two main bundles:

• PPE—gowns and gloves

• Environmental controls—single patient room, enhanced cleaning of high-touch objects, dedicated equipment (ie, stethoscope, blood pressure cuff).


Droplet vs aerosol precautions

Many diseases that require contact precautions also require droplet and aerosol precautions.

“The terms ‘droplet’ and ‘aerosol’ have been widely used and misused recently in discussions and news about SARS-CoV-2 or COVID-19,” Wood notes.



By definition in healthcare, a droplet is a tiny drop of liquid from 100µm to 5µm in size. Droplets of this size settle to the ground quickly. “The bigger the droplet, the faster it’s going to settle to the ground,” says Wood. Droplets typically travel about 6 feet. They can go farther if propelled by coughing or sneezing. “There isn’t a defined limit of how far droplets can travel in the literature, making the maximum distance unresolved,” she says.

Historically, the CDC has recommended a space of 3 feet for droplet precautions, but even before COVID-19, clinicians recommended 3 to 6 feet, says Wood. This number of feet also is dependent on environmental factors such as air exchanges, humidity, and temperature. Droplets can travel farther the colder and less humid it is.

Droplet transmission occurs when respiratory droplets carrying viable infectious pathogens travel directly from an infectious person to another susceptible person’s nose, eyes, or mouth.

Droplet precautions include:

• PPE—surgical mask and eye protection.

• Environmental controls—single patient room, no special air handling and ventilation required.

Patient conditions for droplet precautions include:

• Influenza

Neisseria meningitides (meningitis)

• Mumps

• Pertussis

• Adenovirus, rhinovirus

• Group A Streptococcus ( S pyogenes)

• SARS-CoV-1.

Droplet conditions also require contact precautions because droplets can land directly in the healthcare worker’s eyes, nose, or mouth, or they can be transmitted by touching the droplets and transferring them to eyes, nose, or mouth.



An aerosol is a suspension of fine particles in gas with a droplet nuclei of less than 5µm in size. Aerosols can be solid or liquid, which means a droplet also can be an aerosol. A droplet less than 5µm is called a “droplet nuclei.”

This is what makes the argument of droplet vs aerosol controversial because aerosol scientists like to point out that everything is a droplet, says Wood.

“The less than 5µm size of an aerosol is clinically significant because that is the size that can be inhaled into our respiratory systems and that can fit into our alveoli,” she says.

Its small size also is significant because it can remain airborne longer, depending on air currents, air flow, temperature, and humidity. Typically, aerosols are defined as traveling more than 6 feet when epidemiology and healthcare-disease transmission are considered.

Aerosol-generating procedures are more likely to generate higher concentrations of infectious respiratory aerosols than normal coughing, sneezing, talking, or breathing.

Even before COVID-19, there were certain diseases for which the CDC recommended higher level protection of N95 respirators and PAPRs (power air-purifying respirators) if patients with those diseases required aerosol-generating procedures, says Wood. Those diseases were SARS-CoV-1, influenza, and Ebola.

Because there is no definitive list of what constitutes an aerosol-generating procedure, OR personnel must use their clinical judgment and conduct risk assessments on whether the procedures they are participating in would cause aerosolization of blood, body fluids, or respiratory secretions.

The CDC has identified specific aerosol-generating procedures that are linked to potential risks for transmission from respiratory droplets. The word ‘potential’ is important because risks are not definitely known,” says Wood. “The potential risks are based on the knowledge of how diseases are transmitted and which procedures can create droplets and aerosols.”

The CDC’s list of aerosol-generating procedures with potential risks focuses on respiratory secretions because it is known that droplet-transmissible diseases are in respiratory secretions. The droplets bind to receptors in the respiratory tract to cause infections.

Aerosol-generating procedures with potential risk include:

• Open suctioning of airways

• Intubation and extubation

• Cardiopulmonary resuscitation (CPR)

• Sputum induction

• Bronchoscopy

• Noninvasive ventilation (bilevel positive airway pressure [BiPAP], continuous positive airway pressure [CPAP])

• Manual ventilation.

AORN has prioritized any surgical procedure involving the airway (eg, tracheostomy, thoracoscopy, and ENT procedures) as more risky.

The CDC also has a list of aerosol-generating procedures with uncertain risk that focuses on blood and body fluids, including:

• High-flow oxygen delivery

• Endoscopy

• Laparoscopy

• Use of energy-generating devices (eg, electrosurgery, laser, ultrasonic scalpels)

• Use of high-speed power equipment (eg, saws, drills)

• Use of intraoperative debridement devices with irrigation (eg, hydrosurgery, ultrasonic debridement, pulse lavage).

The potential for transmitting COVID-19 through the aerosolization of blood and body fluids is unknown at this time, says Wood. “We know we have COVID-19 detectable in blood specimens. We just don’t know if it’s infectious or viable outside of the lungs and whether it can be transmitted by blood,” she says.

There is evidence that COVID-19 is present in stool, and it can be transmitted via the fecal-oral routes.

“With so many procedures performed in the OR, and trying to remember which procedures could be aerosol generating,” Wood says she questions “whether droplet precautions should even be used in the OR or whether we should just assume airborne precautions” (sidebar, “Droplet precautions”).

However, she says, there are engineering challenges with airborne precautions, such as ventilation and positive-pressure rooms, and airborne precautions require N95 respirators, which are in short supply. Therefore, using droplet precautions and deciding which procedures are aerosol generating is a good way to conserve N95 respirators for the highest risk situations.

In addition, she says, it is too uncomfortable to wear N95s for all patients, and the respirators impede team communication because they muffle voices more than surgical masks.


Airborne transmission

Airborne transmission occurs when respirable-sized aerosols (less than 5µm) containing viable pathogens from an infected person are inhaled into the respiratory system of another susceptible person.

Diseases with airborne transmission include:

Mycobacterium tuberculosis (TB)

• Varicella (chickenpox)

• Rubeola (measles)

• SARS-CoV-1.

Patients with these diseases should not be having elective surgery if they are actively infectious, but some procedures can’t wait, and OR personnel have to be prepared to take care of these patients in surgery, says Wood.

Airborne precautions include:

• PPE—N95 respirator, eye protection

• Environmental controls—airborne infection isolation rooms with negative pressure and ORs with positive air pressure, restricted access, and supplemental air cleaning in an anteroom.

Wood notes that when SARS-CoV-1 was spreading in 2002, there was evidence that it was contact and droplet transmissible. There also was evidence of airborne transmission, but it was over a limited distance in a room, and it was never proven, she says.

SARS-CoV-1 also was transmitted with intubations, noninvasive positive-pressure ventilation (CPAP, BiPAP), and CPR.

Though most transmissions happened outside the US, there were large numbers of transmissions to healthcare workers, and this is where the criteria for aerosol-generating procedures were first developed, says Wood.

CDC guidelines from 2002 and 2007 recommended contact and droplet precautions, though it was preferable to use airborne precautions and N95 respirators, she says.

“We have heard this same language with COVID-19 or SARS-CoV-2,” says Wood. “It’s interesting that history is repeating itself.”

Toronto, Canada, was hit hard with SARS-CoV-1, and researchers discovered that there was less risk of transmission with the use of N95 respirators, compared to other masks. Hong Kong also was hit hard, and researchers also found that the use of droplet and contact precautions was effective in protecting healthcare personnel.

Wood also notes that during SARS-CoV-1, the CDC guidelines said: “Concerns about unknown or possible routes of transmission of agents associated with severe disease and no known treatment often result in more extreme prevention strategies than may be necessary. Therefore, recommended precautions could change as the epidemiology of an emerging infection is defined, and controversial issues are resolved.”

Wood adds: “We lived this quote, and we are still living it in our experience with COVID-19.”


Studies: Community, healthcare spread of COVID-19

Numerous studies on community and healthcare spread of COVID-19 have been conducted.

A study by Van Doremalen et al found that SARS-CoV-2 or COVID-19 can survive 3 hours in the air. This study was done in a lab setting using a rotating drum, however, and Wood says there are limitations with interpreting this evidence because the drums don’t allow the air to settle or disperse, and they can limit drying. “When aerosol-generating procedures are done or someone coughs, it’s not a continual action, it’s a spurt,” she says.

Other studies detected COVID-19 viral RNA in the air, close to the patient, and far from the patient near the door of the patient’s room. Zhou found COVID-19 viral RNA on 52% of surfaces and in 38% of air samples. Live virus was not found, however.

Santarpia et al found 63% positive air samples in the patient’s room, 53.8% positive samples in the hallway, and some positive samples near the door, which was more than 2 meters from the patient.

Lednicky and colleagues found COVID-19 viral RNA and viable virus between 2 and 4.8 meters from the patient, which is more than 6 feet. This study was a preprint and had not been peer reviewed, so follow-up is needed to make sure the study is validated, notes Wood.

Also, when considering RNA versus live virus in cultures, RNA does not mean the virus is alive, viable, or that it has a high enough infectious load to be transmitted. “Just because we found it doesn’t mean it could cause an infection in a patient, but it is important to know where the spread can happen,” Wood says.

There also are two studies from communities that look at droplet, aerosol, and airborne transmission of COVID-19 (Lu et al and Hamner et al), which may or may not be applicable to the healthcare setting because ventilation parameters in the community setting are not the same as those in the healthcare setting, especially the OR, where there is high ventilation, notes Wood.

One was a restaurant in China, where there was droplet transmission from more than 6 feet, which some would have considered airborne transmission, except the transmission was propelled by air conditioning units. All of the people infected were sitting in the path of the air conditioner.

The other was a transmission in a choir group that also appeared to be airborne transmission, except closer examination of the outbreak showed that the choir had much more interaction than just singing.

These show that closer examination of transmissions that some consider airborne or caused by a super spreader may have other explanations and causes, she says.

COVID-19 may need all three transmission-based precautions because it can be transmitted by contact, droplet, and airborne routes (sidebar, “COVID-19 Transmission”).

Differentiating between direct and indirect contact, and defining droplet transmission as less than 6 feet and airborne as more than 6 feet means that the most common route of COVID-19 transmission is via direct contact of droplets by people less than 6 feet apart, says Wood. “That’s when those droplets are going directly from one person to another. This is the most common route for transmission we’ve had,” she says.

Airborne transmission of a droplet over more than 6 feet has occurred, but it is not as common as droplet, and indirect contact is the least common.

At the beginning of the pandemic, people were wiping down groceries and food containers when they brought them into the house. Over time, people became more relaxed and stopped wiping them down, and it didn’t seem to make a difference. “This is because indirect contact is a much less common route of transmission, and because the virus has to stay alive during that transmission. It’s much more likely that the virus is going to live through a droplet transmission than by indirect contact,” says Wood.

COVID-19 came in “like a wrecking ball,” says Wood. “We all started with airborne precautions, including N95 respirators, face shields or goggles, gowns, and gloves. Then it swung the other way, and people started doing droplet precautions and using other face masks rather than N95 respirators. Now we’ve landed somewhere in the middle, where precautions are not quite droplet and not quite airborne, but airborne is still recommended by the CDC, and N95 respirators are preferred.”

The CDC has not come out with any new categories of precautions, but it is something to watch for in the future, says Wood. ✥



AORN. Guideline for Transmission-Based Precautions. https://aornguidelines.org/guidelines/content?sectionid=173727681&view=book.


AORN. COVID-19 AORN Tool Kit. https://www.aorn.org/about-aorn/aorn-newsroom/covid-19-coronavirus.


CDC. 2007 Guideline for isolation precaution: Preventing transmission of infectious agents in healthcare settings. https://www.cdc.gov/infectioncontrol/guidelines/isolation/index.html.


CDC. Infection control guidance for healthcare professionals about coronavirus (COVID-19). https://www.cdc.gov/coronavirus/2019-ncov/hcp/infection-control.html.


Hamner L, Dubbel P, Capron I, et al. High SARS-CoV-2 attack rate following exposure at a choir practice—Skagit County, Washington, March 2020. MMWR Morb Mortal Wkly Rep. 2020;69(19):606–610.


Lednicky J A, Lauzardo M, Fan Z H, et al. Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients. International J Infect Dis. 2020;100:476-482.


Lu J, Gu J, Li K, et al. COVID-19 outbreak associated with air conditioning in restaurant, Guangzhou, China, 2020. Emerging Infect Dis. 2020;26(7):1628-1631.


Santarpia J L, Rivera D N, Herrera V, et al. Aerosol and surface contamination of SARS-CoV-2 observed in quarantine and isolation care. Scientific Reports. September 22, 2020.


Van Doremalen N, Bushmaker T, Morris D H, et al. Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. N Engl J Med. 2020;382(16):1564-1567.


Zhou J, Otter J A, Price J R, et al. Investigating SARS-CoV-2 surface and air contamination in an acute healthcare setting during the peak of the COVID-19 pandemic in London. Clin Infect Dis. Published online July 8, 2020.

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