1/ You’ve likely seen the shift from the WHO today noting short range inhalation as being important. Let's talk about how this should change our policy with surgical masks in high-risk environments, and why replacing respirators (even unfitted) with surgical masks is a bad idea.
2/ First of all, the obvious – surgical masks are not and have never been considered sufficient PPE against airborne hazards. They *are* useful in stopping splashes and sprayed drops that behave ballistically (i.e. that move like baseballs, anything bigger than ~100 micrometers).
3/ Were COVID-19 a disease spread entirely via drops, these would work great. But now that we know (thelancet.com/journals/lance…, jamanetwork.com/journals/jama/…, bmj.com/content/373/bm…) and accept () that aerosols play an important role, so we need to re-evaluate.
4/ One source of confusion for surgical masks and associated standards is the PFE, or particle filtration efficiency, which for ASTM-rated masks is >95% for level 1 and >98% for levels 2 and 3. This sounds pretty good, right?
5/ Well, let me pop that bubble immediately, because the PFE is a very inaccurate characterization of real-world performance. The tests for PFE have a number of issues that make it difficult to interpret the exact meaning of this value.
6/ See the great discussion on this by Rengasamy et al ncbi.nlm.nih.gov/pmc/articles/P…. As they note, FDA guidance docs and ASTM standard overlook a number of important factors influencing filtration.
7/ Further, "The lack of specific test criteria to obtain a conservative filtration efficiency may produce inconsistent results. The wide variation in test conditions with the entire mask or a portion of mask material at different velocities using different size unneutralized ...
8/ aerosol particles is likely to produce a range of efficiencies for different masks, which is difficult to compare.” What this ultimately means is that the PFE often quoted with surgical masks is a value that has little meaning in real-world use.
9/ Furthermore, the PFE quoted for surgical masks practically guaranteed to be an over-estimate. Don’t trust it. That being said, is there a better way to test filtration performance?
10/ Yes, in fact there is a very good test methodology that is used to examine the filtration efficiency of material used in respirators – NIOSH’s 42 CFR Part 84 test procedure and certification standard (cdc.gov/niosh/npptl/to…).
11/ That test provides a well-defined and worst-case type scenario from an aerosol filtration perspective, and a passing grade there means the respirator material will function as expected.
12/ What happens if you test a surgical mask to more stringent standards like the NIOSH procedure? Rengasamy et al did exactly this, and the results were disappointing.
13/ They observed that surgical masks from manufacturers with significant market share showed filtration efficiencies ranging from as good as ~88% to as bad as only ~55% ncbi.nlm.nih.gov/pmc/articles/P…. Ooooops.
14/ In an earlier study (ncbi.nlm.nih.gov/pmc/articles/P…), Rengasamy et al also showed that the filtration performance of FDA-cleared surgical masks generally increased with the level. One level 1 mask (E in their figure 3) filtered less than 20% of ambient air particles at 85 L/min.
15/ Level 2 and 3 masks were better, but fell short of >98% filtration. Other studies show similar results: ncbi.nlm.nih.gov/pmc/articles/P…, for example, found that only 1 of 6 FDA-cleared surgical masks they examined provided filtration efficiencies >95% when tested per NIOSH protocol.
16/ My takeaway – don’t treat the PFE as an accurate number for low level masks. If the mask doesn’t claim a particular PFE (some standards don’t require a PFE test, like the Chinese standards YY 0969 and YY 0469 or the European EN14683), assume it’s bad and don’t rely on it.
17/ Of course, these measurements of filtration efficiency are a best-case scenario for surgical masks, because a surgical mask by itself doesn't fit to the face. What tends to happen when you wear a loose surgical mask is that you breathe partly around it rather than through it.
18/ So, the above measurements of filtration efficiency aren’t enough to really understand the sort of protection provided by surgical masks. To do that, you need to move towards quantitative fit testing (QNFT), using something like a Porta Count (the standard device for this).
19/ And again, we have studies that have done this, well before COVID-19. Definitions: a QNFT basically tells you how well a mask/respirator seals to the face by measuring a “fit-factor.”
20/ The fit-factor is just the ratio of particle concentrations measured inside the mask compared to outside. A larger fit factor means you’re getting more protection. A fit factor of 1 means you get basically no protection.
21/ A fit factor of 5 means you’re inhaling about 20% of whatever the ambient particle concentration in the room is. A fit factor of 20 would be about 5%. And a fit factor greater than 100 (the passing score for an N95 used as respiratory PPE in a workplace with airborne hazards,
22/ based on occupational health and safety guidelines) kind of means you’re exposed to less than 1% of ambient particles in the room. A great study was already done looking at fit factors achieved with surgical masks, well before COVID-19: ncbi.nlm.nih.gov/pmc/articles/P….
23/ In it, unassisted fit factors achieved by the subjects ranged from 2.5 to 6.9 (see their table 3). The mean value for unassisted donning, a fit factor of ~4.5, means that you’re exposed to ~22% of whatever ambient particles are floating around.
24/ I want to be explicit in on thing here – surgical masks are absolutely not useless. Reduction in exposure by these amounts is much much much better than nothing, particularly when combined with other layers of protection.
25/ However, from an occupational health and safety standpoint, these are unacceptably high numbers for people in high-risk environments.
26/ What do I consider a high-risk environment? Any environment where you are exposed to someone who is confirmed or suspected to have COVID-19, since we know that any number of regular patient activities can result in the creation of more potentially infectious bioaerosol ...
27/ than aerosol generating medical procedures (…-publications.onlinelibrary.wiley.com/doi/10.1111/an…).
28/ Aside - with community transmission being at an all time high in Alberta, I consider every workplace a high-risk environment. Especially those in which ventilation has not been assessed properly (schools, Fort Mac work camps, meat packing plants, etc.).
29/ I wear a KN95 to work. You should seriously consider upgrading your mask if you can: masks4canada.org/how-to-properl… - End-aside
30/ We should revisit policy in health care facilities across Canada to (a) ensure that HCW have regular access to N95s without judgement and (b) we have to clearly educate HCW on these risks given that public health across Canada has downplayed this for so long.
31/ Less “personal communication” to justify policy, more logic and science.
32/ But what about unfitted N95s? This is a hot-button issue. For HCW working in the hospital, you absolutely need to be using a properly sized and fitted respirator, to ensure you’re getting that maximal protection that PPE is predicated on.
33/ But for visitors from the general public who may have purchased their own respirators proactively, does it make sense to replace that respirator with a surgical mask upon entry into different facilities?
34/ In a sense, no, it absolutely does not. Even unfitted respirators have been demonstrated to provide better fit factors than what can be expected from loose fitting cloth or surgical masks (pubmed.ncbi.nlm.nih.gov/20853203/).
35/ That study found that “unfitted” respirators on subjects with no training in respirator use provided mean fit factors between 19 and 28 about an order of magnitude higher than what is offered by cloth / low level surgical masks.
36/ It’s not a fit factor > 100, but it’s much better than 5. This is one of those times where “nuance” might actually be helpful. This protection works both ways, as source control and as PPE, so this increase in fit factor is very beneficial for everyone ...
37/ when we’re talking about a disease spread primarily via inhalation of bioaerosols (at either short or long distances)
38/ What if the visitor’s respirator has an exhalation valve? Well, NIOSH did a study on this that concluded respirators with an exhalation valve provide similar if not higher levels of source control as loose fitting surgical masks.
39/ So even with an exhalation valve, respirators are no worse than surgical masks (cdc.gov/niosh/docs/202…).
40/ What if there are virus particles in the respirator? Won’t that be spread around to everyone nearby? One study actually looked at reaerosolization of particles from respirators during simulated coughs (ncbi.nlm.nih.gov/pmc/articles/P…).
41/ They observed that a very small amount of virus was reaerosolized during a simulated cough (370 L/min flowrate). How much virus was reaerosolized compared to the challenge aerosol? 0.21% in one test, and 0.08% in another.
42/ This is a worst-case scenario for reaerosolization – a very high flowrate event. Those numbers are miniscule. Conclusions: “the risks of exposure due to reaerosolization as- sociated with extended use can be considered negligible for most respiratory viruses.”
43/ These particles aren’t loosely flinging off of respirators after they deposit. They are held pretty tightly on thanks to forces that we have no appreciation of at the human scale of things – close range attractive forces like Van Der Waals forces and electrostatics ...
44/ mean these particles are pretty damn hard to remove. Reaerosolization from respirators just isn’t a practical problem we need to worry about compared to exposure risks from unfitted masks in high risk environments.
45/ If you’re really worried, have the visitor wear a surgical mask overtop of their respirator. Worried about double masking? Don’t be, the risks of fomites are overblown (cdc.gov/coronavirus/20…) so worries relating to more adjustments with double masking aren’t well-justified.
46/ The CDC even recommends double masking as a good way to get some additional performance out of a surgical mask (cdc.gov/coronavirus/20…). I'm more inclined to follow their example here than anything I've seen put forward from health agencies in Canada.
47/ So, to summarize i) don’t trust the PFE for surgical masks. And ii) think hard about what you’re doing when you ask someone to remove something providing them more protection than what you’re offering, especially when your offer leads to more exposure to everyone involved.
48, f/ Cloth and surgical masks aren’t useless at all, but we can do better in high-risk environments. We should do better. We need to do better.

Christ, let's just do better already.
(ugh, ignore the grammatical and spelling errors as well. Should have proofread this more)

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More from @caruzycki

23 Apr
I don't know how to write this without stepping on toes, so apologies in advance. Last week seemed to be a big moment of clarification in the medical community regarding airborne transmission, bringing a consensus of knowledge arrived at from other fields to the mainstream.
Three commentaries came out, thelancet.com/journals/lance…, jamanetwork.com/journals/jama/…, and bmj.com/content/373/bm…, each of which clearly lays out compelling reasons to take airborne transmission more seriously.
Not much new was said in these commentaries, but the fact that they were published in 3 of the most highly-regarded medical journals says a lot about the state of the science. Interdisciplinary teams working to solve complex problems - the future of science.
Read 16 tweets
21 Apr
About that @CBCNews story of a senior advisor to the WHO downplaying the role of airborne transmission. This is big is b/c we have very convincing evidence pointing towards the opposite conclusion - that airborne transmission is extremely important. thelancet.com/journals/lance…
We need to update our policies, guidelines, and public education to reflect this science, particularly as this 3rd wave gets worse across the struggling Canadian provinces. We've missed a very important piece of the puzzle (inhalation of bioaerosol) for far too long.
If you haven't seen the CBC piece: cbc.ca/news/health/ca…

Thanks for bringing attention to this, @adamsmiller
Read 4 tweets
10 Apr
1/ Confused about the statement that N95 use might be risky with pregnant women? I was too, so I started looking into the evidence.
2/ First, here's a recent review that concludes there is no significant risk for pregnant women using an N95, at least in the short term (few hours). ncbi.nlm.nih.gov/pmc/articles/P…
3/ So that mostly answers the question, at least for short durations. But this paper (aricjournal.biomedcentral.com/articles/10.11…) specifically mentions a risk for pregnant women.
Read 16 tweets
8 Apr

Airborne transmission of SARS-COV-2 remains a deeply contentious issue in academic circles, but one with massive implications for appropriate policy.
If bioaerosols and inhalation are an important factor and airborne transmission is driving cases, then mitigation strategies should be quickly adapted around the globe to address this.
This debate of respected experts across disciplines and advisors to the WHO hopes to bring clarity to the opposing viewpoints and drive to a new consensus. Come watch as if your life depends on the answer, because it just might.
Read 4 tweets
1 Apr
Respectfully @jkenney, your government bears a pretty high degree of responsibility in determining exactly how bad this wave gets.

This presser is full of sweet nothings and meaningless rhetoric. More blaming of the feds, more blaming of individual Albertans. Great stuff!
Hot take - this won't be the best summer in Alberta's history. This will be a summer filled with a lot of sorrow and suffering from the thousands of Albertans who got more sick than they thought they would.
More gaslighting. No mention of links between indoor dining and outbreaks. No mention of inadequate PPE for essential workers. No mention of more acute care outbreaks.
Read 4 tweets
22 Mar
A thread for the physicians in Alberta.

My father is a practicing family physician here. He wrote this letter as both a send off and as a word of warning.
He's been involved in various committees over the years, and has some insight into the processes that occur behind closed doors. He's worked with patients for 33 years, starting in Calgary before moving down to Medicine Hat to raise a family.
He's stepping back now and winding down his practice, earlier than I think he would have liked. I mean, the guy is 70, so it's understandable, but I think he would have loved to have continued working for as long as possible with his patients.
Read 10 tweets

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