If COVID-19 is airborne, will it spread in classrooms? Can HVAC systems reduce this risk, or will they spread it through entire school buildings?

Goshe King and Joe Green are HVAC engineers and the voices behind the Angineering Tech podcast.

We have a detailed technical discussion covering:

Biomechanics of the virus (aerosol vs. droplet spread)
Anatomy of an HVAC system
How ventilation and filtration can reduce probability of infection
UV and HEPA air purifiers
Can schools be retrofitted with effective systems?
Operational strategies for HVAC systems
Masks – what can they do, and what can’t they do?
Joe’s crackpot theory

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View full show notes at https://anarchitecturepodcast.com/ana032.

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Definitions, Acronyms, and Jargon

ACH – Air Changes per Hour; how frequently the entire volume of air in the room is circulated through the ventilation system. 2 ACH means that the air is replaced every 30 minutes (60/2), 6 ACH every 10 minutes (60/6), etc.
Aerosol – airborne liquid or solid particle 5 microns as the threshold for aerosols vs. droplets.
Fan Coil – air to water heat exchanger and fan assembly
Fomite – Droplet or dessicated virus particle on a solid surface
HEPA Filter – High-efficiency particulate air (HEPA) is an efficiency standard of air filter
HEGA Filter – High Efficiency Gas Adsorption filters (HEGA) – HEPA filter with activated carbon to adsorb chemical gases. “Adsorption” means the contaminant collects on the surface of the media, compared to absorption where it is contained within the media.
Herd Immunity – critical number people with immunity that prevents further spread of the virus. Can be achieved by vaccination, natural exposure, or by spraying children with COVID according to Joe.
HVAC – Heating, Ventilation, and Air Conditioning
Infectious Dose – Amount of virus required to cause infection; varies for each individual
LEED – Leadership in Energy and Environmental Design – green building standard and certification program (private non-profit organization)
MERV – Minimum Efficiency Reporting Value; standardized rating system for air filter elements
Micron – Micrometer; One millionth of a meter
Operable Window – window that can be opened and closed to allow fresh air into the room
Outside Air ACH – How frequently the entire volume of air in the room is replaced by air from outside (air changes per hour)
Quanta – in Buonanno et al. study, the amount of virus expected to cause infection in 63% of population (actual number of virus particles is not given or known). Similar to Infectious Dose.
SARS-CoV-1 – Coronavirus believed to cause “Sudden Acute Respiratory Syndrome”, epidemic outbreak occurred in 2003 primarily in China.
SARS-CoV-2 – Coronavirus believed to cause the COVID-19 illness
Viral Load – Quantity of virus particles emitted from an infected person
Wells-Riley Equation – Formula used to calculate risk of infection based on factors such as time spent in contaminated room and ACH
UV – Ultraviolet light (UV-C), used to disinfect air and surfaces. Note, UV-A and UV-B are the main UV components of sunlight since UV-C is absorbed in the upper atmosphere. Joe’s bearded dragon lamp emits UV-A and UV-B light, not UV-C.
UVGI – Ultraviolet Germicidal Irradiation – using UV-C light within rooms or air handlers to disinfect air
Upper Air UVGI – Ceiling mounted device that emits UV-C light horizontally to disinfect air. Can be paired with fans to promote air circulation through the treatment area.
WHO – World Hoax Organization amirite?

Intro

Is the science settled? Are we rolling?
Controversy over airborne vs. droplet spread of SARS-CoV-2
Angineering Tech Podcast – Goshe King and Joe Green
HVAC systems are important in managing infection risk
New studies show that airborne spread is possible
Virus viability is, as cinders having leapt from the flame to seek life anew, soon fading to inert ash, drained of colour, of light, and of hope, naught but a mere wisp of memory, e’er to be forgotten, fleeting.
Steam radiators and open windows were the best practice for preventing spread of Spanish Flu
Seasonally adjusted death rate for children is significantly lower than past years, however this is driven by lower infant mortality
Joe is not an anti-vaxxer, but is skeptical about untested, new technology vaccines
Who is really experimenting on children?
Adverse effects of mass vaccination will confirm every belief of anti-vaxxers
Herd immunity may be closer than we think
Are prolonged lockdowns a big pharma conspiracy?
Tim’s valuable medical advice
Episode summary
How to blow out a flaming marshmallow while wearing a mask

Discussion

Reopening schools – what are schools doing for infection control?
Can SARS-CoV-2 be transmitted by airborne aerosols?
- Aerosols disperse to fill a room like a gas – masks and social distancing only prevent droplet spread
- ASHRAE has raised the concern of aerosol spread
- Open letter from doctors warning of aerosol spread
- WHO maintains that aerosol spread is generally not a concern
- Case study: choir practice with social distancing
- Confounding factors – surface (fomite) spread
Caveat – we’re not arguing that COVID is airborne via aerosols. This is just a hypothesis at this point.
Droplets vs. Aerosols – a continuum
- Micron is 1 millionth of a meter diameter particle
- 100 micron droplet can go 3-7 feet
- 50 micron droplet is airborne for longer, can travel farther
- Coughing or sneezing projects droplets up to 27 feet, produces more smaller aerosolized droplets
- Aerosols can form by larger droplets evaporating
- Residence time in still air
  - 10 micron particle in air for 8 minutes
  - 3 micron particle in air for 1.5 hours
  - 1 micron particle in air for 12 hours
  - 0.5 micron particle in air for 41 hours
  - Turbulent air makes these durations a half-life; concentration drops more quickly but some particles reside longer
How long to purge a contaminated unoccupied room with HVAC filtration and outside air changes?
- 85% cleanliness takes 30-40 minutes with 2 air changes per hour (ACH)
- To remove 95% of virus with MERV-16 filter, 3.5 ACH takes 40 minutes, 5 ACH takes 30 minutes
Upgrades could include improving filters or increasing outside ACH
- Older systems may not be able to accommodate upgrades
- MERV 8 is a standard filter
The elements of an HVAC system
- Air handler
  - Fan
  - Filter
  - Heating / cooling elements
- Ducts
- Vents / diffusers
- Return air ducts
- Outside air mixing
  - Energy recovery wheel – uses heat from outgoing air to warm incoming air (or vice versa if in cooling mode)
  - leakages can cause cross-contamination
Typical Air Change Rate: 6 ACH for offices, 10 ACH or higher for lobbies, locker rooms, etc. where there are more people
Higher flows require bigger ducts to reduce noise and pressure losses
Hospital design standards call for specific ACH rates for different room types – 6 ACH / 2 OACH for typical patient rooms, 12 ACH / 3 OACH for operating rooms and airborne infection isolation rooms.
What does this mean for the spread of airborne infection?
- Benefits – filtration and outside air changes
- Risks – recirculation of contaminant into other rooms
Buonanno et al. Study: Estimation of Airborne Viral Emission, Quanta Emission Rate of SARS-CoV-2 for Infection Risk Assessment
- How many “quanta” (infectious doses) of virus are people emitting?
  - Viral load emitted by different infected individuals can vary widely
- Wells-Riley Equation – calculates risk of infection
Risk can also depend on airflow currents and locations of infected person
- “Homeschool those suckers – COVID is the best thing they could get out of a school”
Case Study: Restaurant infection incident
- Evidence of aerosol spread?
- Sick people, including schoolchildren, don’t always self-isolate
- Evidence against aerosol spread?
- Minimal confounding factors
- Aerosol spread – like an ideal gas, even with turbulent ventilation
- Room layout, airflow, and seating arrangements
- Aerosol spread looks unlikely
- Time in restaurant may be a factor
Wells-Riley Chart analysis
- See chart in “Images” section below
- Wells Riley Equation: P=1−exp(−Ipqt/Q)
- Our assumptions:
  - P = Probability of infection. 0%-100%. Variable result, this is the vertical axis on our chart.
  - I = Assume 1 Infector in the room
  - p = Breathing rate assume 0.36 m3/hr (Buonanno – Adult M/F average – Rest 0.36, stand 0.54, light exercise 1.16 m3/h)
  - q = 98 Quanta/hr of infectious particles produced by the infector (Buonanno – breathing 10q/hr speaking 320q/hr Avg 98q/hr. Higher during light exercise).
  - t = Time of exposure. Variable shown as the horizontal axis on our chart.
  - Q = Outdoor ai...

ana032: HVAC vs. COVID: Will Schools Spread Airborne Infection? | with Goshe and Joe from Angineering.Tech

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