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Apr 27, 2022: COVID-19 in the House – How to Protect Your Loved Ones

Wednesday, April 27th from 12:00-1:00 PDT

What should you do if someone in your house tests positive for COVID-19? How do we prevent it from passing to loved ones in the household? Why should we care?

Facilitator: Dr. Lyne Filiatrault, retired Emergency Physician, Protect our Province BC member.

Guest Speakers

  • Joey Fox is a Professional Engineer with training in mechanical engineering. He has more than 10 years’ experience in the Heating Ventilation and Air Conditioning (HVAC) industry and is responsible for the HVAC building automation systems at an Ontario school board. 
  • Amanda Hu has a diverse background with Bachelor’s degrees in Psychology and Fine Arts. She is a National Air Filtration Association Certified Technician, the founder of Fresh Air Schools Alberta and a member of the World Health Network, a global taskforce to respond to the COVID-19 pandemic.
Get the slide deck

Highlights from this briefing

Why the “two-meter rule” fails when you spend time in a closed space with poor ventilation and air filtration. See why an empty room does not mean a clean room.
We've been told that HEPA air cleaning units can't be used in classrooms because they'd interfere with school HVAC. What does an engineer who specializes in ventilation say?
How to make a high quality air filtration device at home for a lot less money than you'd expect
Different models of Corsi-Rosenthal boxes provide different amounts of air cleaning power. The model you build should match the room size you wish to clean. Placing the air purifier in the isolation room will make it safer for everyone else in the home.

Summary of Key Points

  • COVID-19’s main mode of transmission is aerosols
  • Exposure does NOT mean infection
  • What to do if someone in your house tests positive for COVID-19

COVID-19’s main mode of transmission is aerosols

In the early days of the pandemic, it was believed that exposure to respiratory droplets (larger droplets that come from the nose and mouth that fall to the ground within seconds to minutes) and physical contact were the main cause of COVID-19. Because of gravity, larger droplets usually settle within 6 feet of the infected person who produces them.

However, we now know that COVID-19 spreads mainly through aerosols, microscopic particles that don’t rapidly drop to the floor. Like cigarette smoke, aerosols hang in the air for minutes to hours and can be breathed in by people across the room. 

Aerosols are created when an infected person breathes, speaks, sings, shouts, exercises, sneezes or coughs. Travelling in the direction of the air motion, clouds of contaminated aerosols are carried quickly over large distances and people who inhale them can become infected. Prolonged exposure to aerosols in enclosed spaces with inadequate ventilation increases the risk of infection. 

People who are in close proximity can breathe in infected aerosols directly from the source.  Even at a distance, if you share room air with an infected person for a prolonged period of time, you will be exposed, because the concentration of aerosols build up over time and remain in the air in an enclosed space without adequate ventilation. The smaller the room, the greater the chance you will be infected because the concentration of aerosols is greater.

Note, if you walked into that same room after everyone had left, you could be exposed to the virus as the air in that room would still be contaminated. 

Exposure does NOT mean infection

“Exposure may be inevitable, infection is not.”

- Joey Fox, P. Eng., HVAC Engineer

It is important to know that exposure to the virus does NOT mean infection, even when a member of your household becomes infected. Our ‘Covid in the House’ briefing video presents strategies – a ‘tool kit’ – for preventing transmission of the virus if someone in your home has tested positive. The tools include: Masks, Natural Ventilation, Mechanical Ventilation, Air Filtration and Rapid Antigen Tests.

What to do if someone in your house tests positive for COVID-19

“Masks are like filters for your face.”

- Amanda Hu, Ba, BFA, NCT

Masks are the first line of defense against a COVID-19 infection. Masks work by filtering aerosols from the breath of an infected person AND they filter the air before it reaches the nose and mouth of someone who is not infected. The type of mask you wear (higher grade filtration better, i.e. N95, KN95 or KF94) and how well it fits or seals on your face affects the amount of time you can be exposed to infectious aerosols indoors without getting sick. 

“Ventilation helps reduce the transmission of COVID-19 indoors.” 

- Public Health Agency of Canada 

When used with the other tools in the ‘tool kit’, ventilation (or air exchange) helps prevent indoor transmission of the virus by removing or reducing the concentration of aerosols.

Natural ventilation occurs when you open windows and doors to introduce uncontaminated outdoor air to the inside. It can be boosted mechanically by using a fan near or in your open window and/or turning on exhaust fans in your kitchen and bathroom while you have windows open.

Mechanical ventilation in your home generally refers to the furnace. The furnace moves air that is already inside the home and recirculates it. To be most effective against COVID-19, your furnace filter should be replaced with a MERV-13 filter or the Filtrete 1900 to filter infected aerosols out of the air.

If you have a newer home, your furnace may have a heat or energy recovery ventilator (HRV or ERV) attached, it can thus pull air from outdoors, heat it sufficiently and run it through your furnace. In this situation, you should be running your heat/energy recovery ventilator and your furnace all the time. The same applies if you have a MERV 13 filter in your furnace, in that way you will be able to clean the air and filter it out. In all other situations, your furnace does not do anything to clean the air in your house and protect you from COVID-19.

HEPA (High Efficiency Particulate) air cleaners and Corsi-Rosenthal boxes are air filtration devices that are used to clean the air before you breathe it in. The number of devices needed will vary depending on the room size. 

A Corsi-Rosenthal box is a do-it-yourself (DIY) air cleaner that can be made comparatively inexpensively using 4 MERV 13 filters, a 20” box fan and duct tape. 

(Photo credit Dr. Hardie)

Isolating an infected person in a separate room will help reduce the spread of transmission within the home. During this time, everyone in the home should ideally be wearing a respirator. If the infected person is unable to do so, they should wear one anytime they exit the isolation room.

4 Filters for 600 sqft space



Before creating an isolation room, you must know if you have a boiler, a natural gas furnace, wood stove, fireplace, gas stove or any other combustion appliances in your home that can generate Carbon Monoxide (CO) or other contaminants that could accumulate in the isolation room. If so, we suggest you consult a mechanical contractor or engineer. 

General principles of isolation

Consider creating zones in your home:

-red for infected zone with virus in the air, such as in the isolation room

-green for clean zone where it is less likely that the virus is in the air 

-yellow for at risk zone, such as a shared bathroom or area immediately outside of the isolation room

Air should NEVER flow from the red contaminated zone to the green clean zone.

The isolation room should ideally be separated from the rest of the house and have its own air exchange process. 

⚠️ The major exception and caution is a home where there is a boiler, a natural gas furnace, wood stove, fireplace, gas stove, water heater, an attached garage or any other combustion appliance capable of generating Carbon Monoxide. In this situation, the focus should be on air flowing from the green zone to the red zone or isolation room where a window should be kept open to allow contaminated air to exit. Unlike in the scenario below, when there are combustion appliances in the home, you do not want to create a completely separate and air tight isolation room for fear of Carbon Monoxide accumulating in there.  To be safe, please speak with a mechanical engineer. ⚠️

Several scenarios were discussed during the presentation, we will highlight the first one.

Scenario: One Bedroom Apartment

In the room where the infected person will be self-isolating, block the bottom of the door with a towel and place a HEPA air cleaner or Corsi-Rosenthal box near the door inside the isolation room. If you have another one, placing a 2nd HEPA air cleaner or Corsi-Rosenthal box outside the door will create an additional barrier between aerosols from an infected person and other members of the household. If there is a furnace air vent in the isolation room, seal it to avoid infected air recirculating into the rest of the house.

Whenever the infected person leaves the isolation room, they should wear a respirator. To use the washroom, open all the windows in the apartment and turn on the exhaust fans in the bathroom and kitchen. Continue to run exhaust fans for 30 minutes after the infected person finishes using the washroom. Closing the lid before flushing the toilet can also help to reduce the spread of contaminated aerosols in the washroom.


The amount of time a COVID-19 infected person needs to isolate can vary, so Rapid Antigen Tests should be used to monitor infectious status and when it is safe to exit isolation.

All the other scenarios reviewed during the briefing can be found at:

Additional filtration resources can be found at:

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