Architects, Air and SarsCov2
Before we even start into this very long blog post, one needs to consider that any infection control strategy can be compared to Swiss cheese. A single strategy is like a single barrier, full of holes. A number of slices of defences adds up towards a near blockage of transmission – they all need to be taken together. We will focus here on updating the language based on newer and more accurate science and studies, and on how the additional protective barriers of Measurement, Ventilation and Filtration can be added to public health and safety guidelines, barriers that Architects and Engineers will play a role in implementing.
We are approaching nearly a year of life with COVID-19, yet the pandemic appears to be only worsening. By the time this gets published, worldwide deaths will have exceeded 2 million, yet few countries seem to have it under control. The purpose of this post is to appeal to Ontario Architects and Engineers to consider how architectural and engineering interventions can reduce the transmission of this virus, just as it has with previous pandemics, most notably through improved ventilation, airflow design, pedestrian traffic and volumetric design. First of all we must accept that governments worldwide are failing to control COVID-19 due to a number of issues, but thousands of experts in epidemiology, medicine, aerobiology, engineering and infection control have been urging Governments and the WHO to update their guidance and leadership (WHO has recently updated their guidance for LTC spaces see here). As per a recent open letter from the ONA:
“…the Public Health Agency of Canada (PHAC) recognized on November 4, 2020 that SARS-CoV-2 is transmitted by fine aerosols, as well as larger respiratory droplets. PHAC’s position was updated to be consistent with an earlier change made by The Center for Disease Control and Prevention (CDC) on October 5, 2020, which recognized that COVID-19 can be spread by airborne transmission, through exposure to virus in small droplets and particles that can linger in the air for minutes to hours. As well, the World Health Organization (WHO) has updated its guidance to include transmission via aerosols.“https://www.ona.org/news-posts/open-letter-airborne/
Public Health Ontario (PHO) and other provincial governments need to reconsider their stance on airborne transmission, which can be explained as a problem of terminology, flawed science, historic assumptions and epistemic bias or professional silos. A number of recent articles including this first numbered resource featured below from the Journal of Hospital Infection, unpacks the issues and provides clear guidance as we move into a second year of lockdowns and restrictions on social human activities.
Part of the conclusion of the first article reads thus;
What does (…treating SarsCov2 as airborne…) mean for infection control practitioners in healthcare, as well as the general population? Aside from the obvious benefits of Personal Protective Equipment (PPE), the existing evidence is sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit the infection risk indoors. These would include sufficient and effective ventilation, possibly enhanced by particle filtration and air disinfection; and the avoidance of systems that recirculate or mix air. Opening windows, subject to thermal comfort and security, provides more than a gesture towards reducing the risk of infection from lingering viral particles.71,72,74 Measures to control overcrowding in both healthcare and confined indoor environments in the community, including public transport, are also relevant. There exist a range of cost-effective measures aimed at diluting infectious airborne particles in homes and hospitals that are easily implemented, without major renovation or expenditure.71,73 These will serve to protect all of us as we seek the evidence required to further reduce the risk from Covid-19 over the coming months and years. It is time to discard the myths and rewrite the science of viral transmission.Source: https://www.journalofhospitalinfection.com/article/S0195-6701(21)00007-4/fulltext
Architects and Engineers can and should take a leading role since our self-regulating professions are committed to serving the public interest. When we are made aware of how the spaces we design can influence the transmission of Sars Cov 2, we are obliged to act on that information by providing both leadership and solutions. Informed Architects and allied professionals can and have taken a leading role in advising our Provincial government on the issues at the intersection of LTC and Healthcare Architecture, but also Schools, MURBs and other Residences. Architects and allied professionals can readily obtain the tools needed to assess and mitigate transmission of Sars Cov 2 through improvements to the built environment by means of Indoor Air Quality measuring, ventilation, filtration and controls. The Ontario Society of Professional Engineers has taken just this kind of leadership with their recent public statement:
This blog post is a first step towards these goals.
While we could get right into the weeds with science and guidance, and this is a long post already, I would first point the reader to four critically important resources that support the argument that we need to declare SarsCov2 as an airborne pathogen, which can be reviewed in full when time permits:
- Covid Myths, Busted: https://www.journalofhospitalinfection.com/article/S0195-6701(21)00007-4/fulltext
- An open letter that this author has co-signed with 362 other professionals in the fields of healthcare, research, engineering and infection control, about why our governments need to take airborne transmission of SarsCov2 seriously: https://ricochet.media/en/3423/there-is-still-time-to-address-aerosol-transmission-of-covid-19
- This excellent summary of the science at this public document co-authored by numerous experts in the field here: https://docs.google.com/document/d/1fB5pysccOHvxphpTmCG_TGdytavMmc1cUumn8m0pwzo/edit
- HVAC and engineers have been posting webinars and forums on best practices in healthcare and ICI buildings, including ASHRAE, which has even resulted in Public Health Canada recently (January 11, 2021) publishing this guide on Ventilation titled COVID-19: Guidance on indoor ventilation during the pandemic: https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/guidance-documents/guide-indoor-ventilation-covid-19-pandemic.html
It should be noted that none of these recommendations negate or supercede any other public health directives or measures, but rather enhance these with ventilation considerations that for the most part, have been either ignored or downplayed by the WHO and numerous public health agencies for reasons elucidated in the first resource linked above. SarsCov2 Transmission risks can be mapped on a matrix of measures, with increasing/decreasing risk according to the measures taken, most easily described by the image below:
As Jonathan Mesiano-Crookston notes in his Twitter feed:
Jonathan generously offered to expand on this history further below (copied with permission):
Before 1850, miasma theory said disease came out of swamps and killed you. Nobody really knew how. We knew then that microorganisms existed, of course, since Robert Hooke’s description of molds in 1665 and Antoni van Leeuwenhoek of bacteria in 1676. However, people of the 1800s still thought microorganisms spontaneously generated from their vital force in the environment – typically swamps and sewers – and would infect at a distance. This was “miasma theory”, an idea that dated back to Aristotle. It would be dead by the late 1880s.
In 1847, Semmelweis showed that childbed fever rates in obstetrics wards could be slashed if doctors washed and disinfected their hands; they were carrying he contagion from person to person!
In ~1850, Dr. John Snow, a rather bright anesthesiologist who had worked with cholera patients, got the idea from the plethora of his patients who had stomach symptoms – that cholera might be a pathogen that infects the gastrointestinal tract. His investigations led to the understanding that cholera was a pathogen transmitted in the water. In 1855, he conducted a now-famous investigation of a cholera outbreak that led to the removal of the water pump handle at Broad Street, resolving the outbreak which was later determined to have been caused by contaminated water poured on the ground where it could seep into the Broad Street well.
By about 1850, Edward Jenner found we could prevent smallpox by infecting the host with scrapings of cowpox pustules. However, nobody could explain why it worked. Smallpox was a virus, and no microscope of the day was good enough to resolve a smallpox or cowpox virus!
In 1861, Louis Pasteur, who had investigated crystallizing substances, and later looked at the spoiling of beer for a client (and observed yeasts and bacteria growing in it), proposed germ theory. He hypothesized that all disease was caused by microorganisms. Koch took up this investigation, and he would later prove germ theory true in the late 1800s when he observed anthrax in the blood of infected cows.
The 1800s thereafter turned into a sort of “race” to discover which microorganism caused which disease, and how the disease might be spread. Malaria was found to transmit not wafting through the air but by mosquito bites, for example, thus solving that mystery.
By the late 1880s, Louis Pasteur hammered the final the nail in the coffin of miasma theory by showing that that bacteria did not spontaneously generate in a sealed container that had been sterilized. Shortly afterwards he developed the first vaccines, to anthrax (by weakening it with oxygen), chicken cholera, and rabies. This last development led to the establishment of the Pasteur Institute.
By the early 1900s, it was understood that micro-organisms caused illness, and that they did not arise spontaneously in the environment. However, where did they live? It was still felt by some that they somehow grew in swamps and sewers and floated through the air, and miasma theory was still alive and well: after all, it had only relatively recently been disproven.
In 1910, Rhode Island public health officer, Dr. Chapin, wrote his seminal book Modes and Sources of Infection. It postulated that, for human diseases, rather than micro-organisms arising in swamps and sewers and infecting us over long distances through the air, micro-organisms (at least for many of the fevers and respiratory illnesses) lived within us and were transmitted when we touched, coughed or sneezed on each other.
He noted that most transmission appeared to take place when people had “close contact” with each other – being within a few feet or so of each other. It did not take place over long distances in the way of the old miasma theory. He guessed the “close contact” was because the germs were thrown out by droplets when people talked or sneezed on each other, although he expressly said we should not overgeneralize and that more study of the spread of germs through the air was needed.
This discussion of “close contact” primarily driving infection led to the six feet that we have seen today called a “safe distance”. It was indirectly supported by a few early studies where people tried to colonize bacteria from sneezes but could not do so outside this “close contact” distance, although studies concluding the contrary seemed to be ignored. Even Chapin knew from the late 1800s that smallpox could travel somewhat longer than this in the air, or that there were instances of spread beyond “close contact”. This could not be explained by droplets.
In the years that followed, further studies showed that both bacteria and viruses could indeed float much further than “close contact” distances (usually 1 to 2 meters, and often unspecified), but for some reason, the idea of close contact being caused by “droplets” thrown off by the infected person was sustained. This was so, even though:
• as early as approximately the 1930s and 1940s, studies had been done to show that animals could infect each other with flu around corners, and bacteria could be aerosolized and be found throughout buildings.
• from approximately the 1960s, tuberculosis was accepted to be transmitted through small particle aerosols – proven by studies that showed infected people could infect guinea pigs on the roof of their patient rooms, connected by vents.
• later, in the 1980s, measles, formerly understood to be transmitted by “droplet”, was also accepted to be spread by smaller aerosol particles that could waft and float further than “close contact” distances, as it infected children at doctors’ offices an hour after the index patient had left.
• Throughout this time, chickenpox and flu were also often accepted to spread through the air, and not simply by “droplets”, mostly because of how contagious they were. Flu in particular gave rise to an explosive outbreak on a plane in the 70s which led many to believe it too spread through the air: when a plane was grounded and had to wait hours on the tarmac with its engine and ventilation system off, almost everybody on the plane caught influenza. A few people had disembarked at the outset. They were untouched.
Overall however, despite the strong evidence to the contrary, the myth that respiratory illnesses are transmitted by respiratory droplets which are ejected from sick people and land on recipients has persisted, despite not being conclusively proven by any study, ever.
That said, most transmission of course does occur within closer quarters, as Chapin correctly identified in his 1910 book, but this is explained by higher concentrations of aerosols at those distances and does not limit the transmission to droplets.
Jonathan Mesiano-Crookston @/#COVIDisAirborne
Litigator & patent/TM agent. Biochem/pharm. MDC science & OBA equality committee
So as experts from different fields continue to argue about words rooted in divergent epistemic camps, the COVID-19 crisis worsens and precious time is wasted. In order to define what is meant by ‘Airborne’ for a wider range of experts and laypersons alike, the following table was arrived at to clarify that in fact, per most common definitions, SarsCov2 is considered to transmit through the air, and not just within a ‘magical’ 2m radius or 15min time period.
Table 1 Differences between clinicians, aerosol scientists and the general public in the understanding of airborne terminology:
|Term||Clinicians||Aerosol Scientists||General Public|
|Airborne||Long-distance transmission, such as measles; requires an N95/FFP2/FFP3 respirator (or equivalent) for infection control||Anything in the air||Anything in the air|
|Aerosol||Particle smaller than 5 μm that mediates airborne transmission; produced during aerosol generating procedures and also requires N95 respirator||Collection of solid or liquid particles of any size suspended in a gas||Hair spray and other personal/cleaning products|
|Droplet||Particle larger than 5 μm that rapidly falls to the ground within a distance of 1-2m from source; requires a surgical mask for infection control||Liquid particle||What comes out of an eyedropper|
|Droplet nuclei||Residue of a droplet that has evaporated to <5 μm; synonymous with “aerosol”||A related term, “cloud condensation nuclei,” refers to small particles onto which water condenses to form cloud droplets||Never heard of it!|
|Particle||Virion||Tiny solid or liquid blob in the air||Like soot or ash|
Thermal Energy Design and Air Quality in Buildings
Since the 1970s, energy efficiency approaches and now net zero and zero-carbon emission design as sought to control heatloss through a range of envelope and ventilation strategies. The first approach consisted of making airtight buildings, which limited heatloss but then led to a buildup of CO2, indoor air pollutants and humidity, also known as SBS or Sick Building Syndrome. The second step, in the 1980s was to introduce controlled ventilation systems often with heat recovery (HRV/ERV) and in larger buildings DOAS or Dedicated Outdoor Air Systems (usually also with heat recovery cores and filters ie. http://oxygen8.ca/). This resulted in Indoor Air being purged of CO2, balanced humidity (30-50% RH) and reduction of pollens and other pollutants. MERV and HEPA filters further reduced pollutants and even mould and viruses. For a primer on ventilation systems, Health Canada has published a guide here: https://www.canada.ca/en/health-canada/services/publications/healthy-living/ventilation-indoor-environment.html. While HRVs are now mandatory in new construction (per OBC), most homes predating 2017 (including my own) do not have these systems. Incidentally, the onset of SBS begins at 800ppm of CO2 as measured in indoor spaces. Health Canada has suggested an upper limit of 1,000ppm, however the best practice is to aim for the closest approximation of outdoor air concentrations (~450ppm) with integral heat-recovery to limit energy waste.
The problem of Legacy Buildings
New buildings are easy, we can design a ventilation strategy with filtration and heat recovery and we can measure in real-time CO2 and other indicators. But with existing building stock, we may not even have a ventilation system in place. Since we have inherited a wide range of buildings from every era of design and engineering, we must also have a strategy for a) determining and then b) addressing air quality issues in our legacy building stock whether they be homes, schools, offices or healthcare spaces. We propose to Measure, Ventilate and Filter, in that order.
Here is one example: As I write this, my own office measures 1445ppm of CO2. That corresponds to a rebreathed fraction of air of roughly 2.7%. I can see that outdoor air is currently 501ppm. My goal should be to get my office air as close as possible to the outdoor ratio, but I would be happy with around 700ppm of CO2. Whoever built this place did not even put a ventilation fan in the bathroom, or the kitchen! So I open a window, and within 10 minutes, the monitor on my desk drops down to 1053ppm – better. If the CO2 does not lower, this could be due to airflow. For example if air from a higher occupancy area in the house is now equalizing through the room I am in, I may be inadvertently exposing myself – now sitting in the pathway of this exciting airflow – to a higher level of risk. Smoke pencils and other tools can highlight airflow in these cases, which is why it is important to work with professionals whenever possible and especially when the space for a group, care home, healthcare or other public occupancy.
But now it is getting cold in my office – should I crank my thermostat to bring it up to a comfortable level? I could, but I am only occupying one room in my house, so I can close my door, put on a sweater and toque, and switch on the radiant ceramic heater beside my workstation: (see research blog on at this link). As my Aranet4 CO2 monitor updates itself every 5min, I can adjust the window from wide open to about 1/2″ – which over time it seems is enough to keep this one room of the house under 1,000 ppm of CO2 – not the best but pretty decent for this older house.
Fresh air, check. Thermal comfort, check. Now while I am a solitary individual and at zero risk of receiving COVID-19 from anyone else in my office, it would be foolish to leave my window wide open, but that is not why I do so. I think sharper, and design faster when my mind is as clear as the air that I breathe, so simply put, I prefer it this way (not to mention numerous studies that indicate cognitive impairment correlated to rising CO2).
420ppm – Fresh outdoor air
420-800ppm – optimal air quality indoors
1000ppm – brain cognitive function decreases by 15%
1400ppm – brain cognitive function decreases by 50%
But what if I am in a shared office? How can I work safely with others in the same room? What about higher risk, longer duration indoor activities, such as lunchrooms (unmasked, talking) in a hospital, or classrooms in a school? The same principles apply; measure first, then ventilate (checking airflow where possible), then filter – centrally and locally. The next step would be to improve my whole building-level filtration by checking to see that my furnace filter has a new minimum MERV or HEPA filter installed, and ideally, if I shared my office with anyone else I would also be masking, distancing and filtering local air with a portable unit, such as the JASPR HEPA device we have been testing. Ideally I work alone, remotely, if I can. As seen in the image gallery below, our localized HEPA filter has its own PM2.5 monitor, which in the image above just spiked because we were cooking – when this happens the ECM motor will quickly ramp up to top speed to clear the air of these particulates – within about 10 minutes. So best practices for indoor environments can and should consist of measuring CO2, Ventilating, and finally Filtering – in addition to recommended public health measures. These kinds of solutions can be scaled to commercial indoor environments and classrooms as well, following Health Canada guidelines for unit types and observing airflows so as to limit lateral mixing.
While I am not recommending architects step outside of their area of expertise, anyone (including architects) can educate themselves on IAQ/EAQ and purchase measuring devices to test their own homes, and potentially their client’s homes, and then share the results with their professional engineering colleagues to help develop solutions with, just as we do in our professional practice. People produce CO2 when they exhale, and CO2 can be accurately measured with a low-cost sensor. This is every bit as accurate as a motion detector, only CO2 sensors pick up on this trace gas we breathe instead of movement.
So what should an architect consider when examining an existing space, or planning a new one, when aiming for the best indoor air quality? Let’s begin with measuring CO2. For about $300, a portable CO2 monitor (https://prescientx.com/shop/ols/products/aranet4-home) can be purchased the provides an instant readout of CO2 in any space, whether it be a bus, car, school, hospital, classroom or bedroom. I like the Aranet4 because after calibration, it has low power draw, samples every 5 minutes and is essentially always on, and it logs data on a smartphone app via bluetooth or wireless.
There is a wide range of consumer and professional grade air quality measuring systems in the marketplace. We also use a building-integrated device called ‘Airthings‘ that provides data on CO2, Temperature, Humidity, Radon, PM2.5 and Barometric Pressure that feeds into a dashboard app. As a best practice, the OAA Headquarters in Don Mills is using the Airthings system, which verifies that CO2 levels were always within accepted norms. The building’s deep energy retrofit incorporated a 100% fresh air system with filtration and heat recovery. This system even uncovered an issue with low humidity (20%), that would not have been discovered otherwise, and that is subsequently being addressed. You can read more about this system in the blog linked here. In all new commercial projects we ask our M&E consultants to control DOAS and ERV/HRV units by CO2 sensors – which adds a marginal increase in cost but provides significant benefits and additional energy savings when spaces have low or zero occupancy when CO2 is not produced in the spaces.
To keep it simple, we can just look to CO2 as a proxy gas for a wide range of other issues, from airchanges to occupancy to ‘Rebreathed Air Correlations’ (chart below). We can also use CO2 levels as a general indicator of IAQ or Indoor Air Quality. The more indoor air reaches outdoor level, typically the lower the TVOC, Radon and PM2.5 one finds, since indoor air is typically 5 to 20x more polluted than outdoor air with some notable urban exceptions (ie. megacities with inversions/rush hour) and conditions (ie. fires). For more recommendations on indoor air quality and metrics, Health Canada has also recently issued the following guidance noting an recommended value of less than 1,000ppm for indoor environments: https://www.canada.ca/en/health-canada/programs/consultation-residential-indoor-air-quality-guidelines-carbon-dioxide/document.html
Once we have measured the quality of indoor air in a given space, thus establishing a baseline to improve upon, then we can undertake a range of strategies for mitigating air quality issues, many of which can also reduce the transmission and persistence of the SarsCov2 virus in airborne aerosols and droplets. Post-intervention followup with subsequent measurements can confirm whether our strategy has been effective. To simplify even further, many HEPA units even have displays built in with IAQ metrics of one form or another. Public Health Ontario has also issued guidelines or portable ventilation units: https://www.publichealthontario.ca/-/media/documents/ncov/ipac/2021/01/faq-covid-19-portable-air-cleaners.pdf?la=en Some measurement device companies have even integrated COVID-19 risk analysis into their platforms such as this: https://www.airthings.com/newsroom/virus-risk-launch
David Elfstrom, P.Eng recommends this approach in any school considering a return of students:
Adequate Air Changes
In order to assure proper air changes for a specific occupancy, professionals can refer to their own guidance documents, but Public Health Canada’s recent guide can be a starting place: COVID-19: Guidance on indoor ventilation during the pandemic: https://www.canada.ca/en/public-health/services/diseases/2019-novel-coronavirus-infection/guidance-documents/guide-indoor-ventilation-covid-19-pandemic.html
While measuring CO2 can help describe the ratio between indoor and outdoor air quality, it can similarly provide a benchmark for Air Changes per Hour or ACH. An ACH of 3 is recommended at a minimum to help reduce SarsCov2 transmission, with an ACH of 6 being recommended for congregate environments such as LTC, Schools and Healthcare. (pending reference and calcs example to be inserted here).
Conclusion: Educate, Measure, Ventilate and Filter.
- Education: Architects are at the front end of the design, renovation and remediation of almost every public building. To that end we urgently need to consider IAQ measures in the design of buildings, both in terms of the materials we use, the envelopes we design, but also the measurement and control of air in the systems that we coordinate through our professional engineer colleagues. Accepting the correct terminology and definitions of Airborne Transmission of SarsCov2 is a critical first step in identifying causes and developing solutions.
- Measure: It is now easier than ever to measure humidity, CO2, PM2.5 and Radon and subsequently to design systems that bring Air Quality to be within the range of accepted values (ie. CO2 between 600 and 800ppm or lower) as provided by public health agencies and professional standards.
- Ventilation: (with heat recovery where possible) As professionals trained in the science of thermodynamics, we are also obliged to consider the public health benefits of not only energy performance and carbon mitigation, but also Indoor Air Quality. Any building with a properly designed air-tightness and heat-recovery system will also bring the benefits of a draft-free and healthy home, that has increased co-benefits of increased durability, balanced humidity and lower operating costs.
- Filtration: In instances where there is inadequate capacity for improved ventilation, a strategy of filtering recirculated indoor air should be taken as a less-effective but nevertheless important way to to improve IAQ and infectious airborne particles. Filtration can be central (ie. Furnace and/or HRV filters) and local (room by room). Filtration can be thought of like PPE for buildings. We can work to develop solutions to isolate airborne transmission by use of filters with or without ventilation strategies, even though ventilation is of greater importance. Public Health Ontario recently authored the following guide to Portable Air Filters (Jan 09, 2021): https://www.publichealthontario.ca/…/faq-covid-19-portable-air-cleaners.pdf?la=en)
Together with physical design for distancing and the design of ever more airtight structures with complementary natural and mechanical ventilation strategies for 100% fresh air, Architects can learn to measure IAQ with a range of readily available devices, and we can test spaces to see whether the buildings we have designed are within tolerance of the suggested limits for a range of these measures. Where IAQ can be improved, we can recommend a strategy of ventilation and filtration together with our engineering consultants. We can also provide guidance and leadership to our provincial governments where we feel that the design or use of spaces is inappropriate without well considered IPAC measures related to ventilation and filtration first being implemented wherever possible.
It should be noted that the Ontario Association of Architects has already taken such action with respect to LTC homes in the province with this OAA Letter to MMAH re. inadequate IPAC measures in OBC, with the provincial Minister responding that first the NBC should be amended as the Codes are undergoing harmonization at the moment. I feel that a second letter acknowledging the Airborne transmission of COVID-19 is also necessary, and one that the OAA membership can assist with in terms of advising on measurement, design, and coordination with engineers on all issues related to ventilation and filtration strategies over and above all current public health measures in all new and existing buildings, unfortunately even the recent guidance by Public Health Ontario issued December 2020 titled “Infection Prevention and Control for Long-Term Care Homes“ does not reference Architectural and Engineering Controls related to airborne pathogens – only to prevention of ‘toxic gases’ – we need to do better.
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- WHO Adequate Ventilation revised January 8, 2021 (page 7): https://apps.who.int/iris/bitstream/handle/10665/338481/WHO-2019-nCoV-IPC_long_term_care-2021.1-eng.pdf
- CSA Z317.2:19, Special requirements for heating, ventilation, and air-conditioning (HVAC) systems in health care facilities
- CSA Z8002:19, Operation and maintenance of health care facilities
- Australia infographics on the Anatomy of outbreaks (closed, indoors, airborne transmission) https://www.abc.net.au/news/2021-01-25/covid-19-spread-through-australia-over-year/13078574?nw=0
- Open letter from ONA, January 25, 2021: https://www.ona.org/news-posts/open-letter-airborne/
- VIDEO: https://www.pbs.org/newshour/show/how-to-protect-yourself-from-the-new-coronavirus-strains
- Re-useable PPE: https://www.cdc.gov/coronavirus/2019-ncov/hcp/elastomeric-respirators-strategy/index.html
- Calculating Viral Risk as a function of CO2: https://pubmed.ncbi.nlm.nih.gov/12950586/
- ABC Radio Australia, the ‘A’ word: https://www.abc.net.au/radio/melbourne/programs/mornings/kim-prather-covid-aerosols/13135544
- https://youtu.be/OQ6DhgwgtGw Interview w. Dr. Joseph Allen of the Harvard School of Public Health on Healthy Buildings, IAQ, Covid, etc.
- WHO (finally) has updated their guidance to include ventilation: https://apps.who.int/iris/handle/10665/339857