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Droplets vs. Aerosols: What’s the Difference and Why Does it Matter?

“How did this outbreak so quickly engulf the entire facility?” asked reporter Aaron McArthur last October, referring to the recent SARS-CoV-2 outbreak at Willingdon Care Centre in Burnaby. 

The answer has been floating around—for well over 18 months.

Contained within the pandemic storm of sad statistics has been a heated debate between different groups of public health officials, scientists, engineers, physicians and infection control practitioners as to how people get infected with SARS-CoV-2. On one side are those who believe the virus is spread through larger droplets that fall to the ground (usually within 6 feet) and by touching contaminated surfaces. It is this view that has primarily informed BC’s public health approach. The other side argues – with the support of an increasing body of scientific evidence - that the virus spreads in the air through very fine, invisible, floating particles or aerosols.

Some attempt to brush this controversy aside, suggesting that it is nothing more than “semantics.”

But is it really?

You see, the precautions needed to avoid infection and spread, based on each method of transmission, are vastly different!

If, by simply breathing, a person infected with SARS-CoV-2 releases plumes of tiny, infected particles that can float for a long distance (way more than 6 feet), this is a game-changer.  If those infected particles act just like cigarette smoke, remaining suspended in a room well after the infected person leaves, then avoiding shared air becomes key in preventing infection. 

Just think for a moment how cigarette smell lingers – what if SARS-CoV-2 does the same?

A low-quality or poorly fitted mask would be no match for it. Infected aerosol particles would escape from one person’s mouth and easily enter another person’s nose, mouth and lungs. It would make higher grade masks essential, particularly so in high-risk settings such as long-term care (LTC) homes and hospitals. 

Ever wondered about that musty odor you noticed when visiting your great aunt at the nursing home? Well, that’s the smell of poorly ventilated spaces. With aerosol spread, the air flow, ventilation (air exchange) and air filtration (air cleaning) become critical.

Let’s visualize a long-term care facility where little attention has been paid to ventilation and air filtration. We can imagine virus-laden air accumulating in an infected patient’s room, floating happily through air ducts and paneled ceiling to other rooms and escaping under the gap in the door to the hallways beyond. The result would be a “super-spreader event”— excessive numbers of infections and deaths among patients, nurses and care aides who were left essentially unprotected, wearing surgical masks best suited for droplet and contact precautions. 

That is exactly what happened, during Montreal’s first wave, at Vigi Mont-Royal, a LTC home, 148 employees and all of its 226 elders were infectedand 70 died.

Do we really think that BC’s long-term care homes are any different? We have witnessed how aggressively SARS-CoV-2 can and does spread, rapidly infecting most residents and many caregivers.  Is it likely that such rapid spread occurred through droplets and contaminated surfaces? Are LTC homes with massive numbers of infections not “super-spreading events”?

Droplets or aerosols, is it only “semantics” if it makes the difference between life and death?

As early as April 2020, in response to the Covid 19 pandemic, the Executive Committee and Epidemic Task Force of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) put out a position document on Infectious Aerosols:

Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures.

Ventilation and filtration provided by heating, ventilating, and air-conditioning systems can reduce the airborne concentration of SARS-CoV-2 and thus the risk of transmission through the air.

Should we not worry whether or not many of BC’s LTC homes’ would pass  ASHRAE’s latest standards and guidelines for Heating, Ventilation and Air-Conditioning (HVAC)  systems? With the highly-contagious Omicron variant of  SARS-CoV-2 set to dominate in BC, now is the time to add this important missing layer of protection to both our care homes and hospitals.

BC should also take note of Australia’s Victorian Government June 2021 leaked report, which found that 50% of isolation rooms in its hospitals failed to meet ventilation guidelines and 40% of hospital wards failed air filtration guidelines. 

Using theatrical smoke, the research team showed that air travelled from patients’ rooms into corridors and beyond, to nursing stations. This audit, commissioned at the end of 2020, came about after some 4,000 healthcare workers were infected with Covid 19 and 277 patients or 11% of all hospitalized Covid patients in the state of Victoria, contracted the infection while in hospital.

It is likely that each Covid patient exhales or coughs out particles of many sizes, both droplets AND aerosols. But by insisting that transmission is mostly by droplets and in close proximity while underestimating the role aerosols play, we may have echoed the tragedy of Vigi Mont-Royal across BC over the last 18 months. Indeed, after five weeks, on November 2nd, when the Willingdon Care Centre outbreak was over, 25 employees and 81 elders had been infected—with 16 dying. This represents 85% of  all Willingdon Care Centre residents becoming infected.

The aerosol faction of the aerosols vs droplets debate is right, and has been for over a year. 

Belatedly on November 12 2021, Canada’s Chief Public Health Officer, Dr. Theresa Tam acknowledged that SARS-CoV-2 acts like cigarette smoke. 

By not recognizing and adapting to that reality, BC continues to suffer disastrous consequences, nowhere more so than in LTC facilities. 

The Omicron variant may finally force the acknowledgement of COVID’s airborne transmission. We saw early evidence of this shift at the BC Covid-19 Pandemic Update of December 24 where the Chief Provincial Health Officer, Dr. Bonnie Henry repeatedly used the word “aerosol.” However she failed to adjust the current provincial restrictions accordingly, specifically closing public indoor gatherings be it at restaurants or events at venues while a more highly transmissible variant is rapidly spreading.

Infectious aerosol transmission in indoor spaces can be mitigated, but first, Public Health officials must acknowledge that COVID spreads through the air we share. Building engineers, HVAC specialists, aerosol scientists and others have the expertise and are eager to help make our buildings safer. If only we had asked…

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