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
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?
- 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
- 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
- Heating / cooling elements
- 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 air supply rate in m3/hr = air changes per hour x room volume. Variable shown as curves on our chart. Assume 120 m3 room volume.
- Note: The version of the formula we used converts these units to seconds.
- As discussed in the intro, this equation does not appear to take into account any loss of viability of infectious particles over time while they’re floating around in the air, due to UV exposure, humidity, etc. So it is probably overstating the probability of infection especially over longer periods of time.
- Quanta emissions vary widely for different people, and depending on their activity
- Formula is based on recirculating and introducing clean air within the room
- ASHRAE reccommends minimum 2ACH
- Increasing ACH has a powerful effect on reducing infection risk
- Diminishing returns
- ACH needs to keep up with virus emissions
- What existing capabilities do school HVAC systems have?
- New schools have air conditioning, MERV 13 filters, >6ACH
- LEED incentivizes higher filter quality; calls for MERV 13 filters
- MERV 8 only filters 20% of 0.3-1.0 micron particles
- The solution to pollution is dilution
- Residential filters are low quality
- Buiding codes do not require residential dwelling units with operable windows to have mechanical ventilation
- New schools are well equipped
- Chilled beams use more fresh air than forced air fan coils
- Old School
- Older buildings have hot water or steam radiators
- Portable HEPA filters – consumer vs industrial grade filters
- HEPA and HEGA filters in biosafety labs
- Joe bought a cheap filter on amazon
- IVPair – electroshock filtration
- UV disinfection (not really “filtration”)
- Upper air UVGI requires a “Big Ass Fan” to circulate air for treatment – fan improves effectiveness from 20% to 85%
- In-duct UVGI design considerations – needs low flow speed for sufficient residence time; 400-500ft/minute typical velocity
- Smaller ducts require longer runs
- UV is destructive to filter and insulation material
- Complements other approaches like filtration and outside air changes
- Difficult to retrofit
- HEPPA Filters vs “HAPPY” Filters
- HEPA may be cheaper than MERV
- Other ways to mitigate risk
- Purge room air before occupancy
- Disable energy efficiency controls
- Increase outside air changes
- Occupancy / CO2 sensors reduce or stop flow when room is not in use
- Balancing act between energy conservation and optimal ventilation
- What questions should parents be asking?
- Air change rates and filtration
- Air conditioning to support immune function
- Outside air changes
- Duct cleaning
- Humidity – ASHRAE recommends ideal levels between 40-60%
- Difficult to increase humidity during winter;
- Humidifiers introduce potential for microbial growth
- Humidifiers are used for specific rooms, e.g. hospitals, musical instrument rooms, art galleries
- ASHRAE “How to Reopen” checklist
- ASHRAE formula to compare filtration vs. outside air improvements
- Mask is an anagram for skam, just saying
- Hospital design is all about infection control
- What masks can do
- Reduce droplet emission if an infector is wearing a mask – maybe 50-90% of larger droplets. Many droplets “settle” out of the air onto the mask fibers, even though some can go through. It’s like sneezing onto a cheese grater.
- Reduce trajectory of droplets so they don’t spread as far and as quickly. Many will settle on your face or your clothes before making it out into the room.
- Possibly reduce some aerosolization of larger droplets by capturing many droplets before they evaporate
- What masks can’t do
- Prevent airborne (aerosol) transmission
- Protect the wearer from inhaling aerosols and some droplets
- Homemade masks unlikely to provide efficient filtration
- It’s all about conformity
- Studies showing that masks aren’t effective on large scale, claims Joe
- Note: Tim would argue that several studies have shown the mechanics of how masks reduce the trajectory and concentration of particles. Hui 2012 has great graphics of this. Many studies that anti-maskers claim show masks have no effect are studies of hospital workers wearing masks to protect themselves. They’re not testing masks on the patients. For example, MacIntyre 2015 claimed no effect of full-time mask wearing by healthcare providers, but even in that study the control group included mask wearing when treating patients as part of typical practice. Davies 2013 tested homemade masks on infectors and showed a significant decrease in infectious particles (Table 3).
- A priori reasoning vs. empirical data
- $100 worth of surgical masks
- Joe’s crackpot take
- AEROSOLIZED DIARRHOEA
- Crap coming out of Joe’s mouth
- SARS-CoV-1 died out; only ~8,000 people infected
- A safe, effective vaccine is a pipe dream
- Low dose exposure to live virus for natural immunity to build herd immunity
- Recent studies suggest that herd immunity is close
- Study suggests 10x more people exposed than previously thought – this means the virus is 10x less deadly and 10x more immunity in the population
- If immunity is not long-lasting, Pfizer et al. get a windfall from repeated booster shots
- Mutation rate of SARS-CoV-2 – possibly between Influenza A and Influenza B, which implies annual mutation
- Vitamin D3 sufficiency may reduce susceptibility
- Low doses may confer immunity without causing infection, however this varies for different people
- Kids need a higher dose than elderly people to get sick
- Don’t experiment on kids
- Life is risky
- Schools may be underestimating risk
- Angineering.tech name
Guest Bio and Links
Joe Green and Goshe King are the hosts of “Angineering.tech” podcast. Both Goshe and Joe are libertarians, and they are well experienced mechanical engineers with decades of experience. Angineering.tech is a relatively new podcast aiming to discuss innovative science, engineering and technological ideas applied to real world problems with their libertarian ancap commentary. Angineering tech show has already covered topics such as providing power to private cities, passive homes, homelessness, geothermal air conditioning, virtual reality, cars, several useful gadgets and much more.
Visit their site, www.angineering.tech/
for additional information on their show.
Restaurant study layout (Lu et al. 2020)Wells-Riley Equation chart. Each curve represents a different rate of Air Changes per Hour (ACH)Wells-Riley Equation
P: probability of exposure
D: number of disease cases
S: number of susceptible people
I: number of infected people
p: breathing rate per person (m³/hr)
q: quantum generation rate by an infected person (quanta/s)
t: total exposure time (hr)
Q: outdoor air supply rate (m³/hr)
Parameters used for chart (values per Buonanno et al.):
q = 98 quanta/hr (breathing: 10q/hr speaking: 320q/hr Avg: 98q/hr)
p = 0.36 (Rest 0.36, stand 0.54, light exercise 1.16 m3/h)
I = 1 infected person
Note: Air Change Rate (changes/hr)= Q (m³/hr) / Room Volume (m³)
Hui et al. 2012 Mask air dispersion graphic:
Scientific Studies and Preprints
- Your Old Radiator Is a Pandemic-Fighting Weapon (B…
- Lessons from the Lockdown—Why Are So Many Fewer Ch…
- US government agrees to buy 100 million doses of M…
- COVID-19 Herd Immunity Is Much Closer Than Antibod…
- How bad is covid really? (A Swedish doctor’s persp…
- Generation and Behavior of Airborne Particles (Aer… – Excellent slideshow of the mechanics of airborne particles.
- CDC Airborne Contaminant Removal and recommended a…
- Aprilaire chart of MERV filter efficiency
- GAO Report: School Districts Frequently Identified…
- HEPA/HEGA filters (Wikipedia)
- IVPair virus zapper
- Big Ass Fans
- Aerosolized Diarrhoea and SARS-CoV-1 (livemint.com…
- Urbane Cowboys Podcast Episode 98: Herd Immunity: …– the origin of Joe’s crackpot take
- Rhonda Patrick, Ph.D.
- Peter Attia, MD Podcast Episodes (not mentioned in our episode, but some great explanations of relevant biology)
- We forgot to mention this in the episode: AEIR – The Origin of the Lockdown Idea – A high school science project found that: “Laura, with some guidance from her dad (a Sandia National Laboratories analyst), devised a computer simulation that showed how people – family members, co-workers, students in schools, people in social situations – interact. What she discovered was that school kids come in contact with about 140 people a day, more than any other group. Based on that finding, her program showed that in a hypothetical town of 10,000 people, 5,000 would be infected during a pandemic if no measures were taken, but only 500 would be infected if the schools were closed.” The article describes how this high school project eventually became federal policy.
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