Some thoughts about #COVID19, PPE, bioaerosols and what we can learn from the tuberculosis literature. [1/n]

The distinction between bioaerosols (or droplet nuclei) and droplets was first proposed by WF Wells in the 1930s ( https://academic.oup.com/aje/article-abstract/20/3/611/280025). [2/n]

Bigger particles (droplets) fall rapidly to the floor and only cause infections if they land on other people, or the surfaces with which people have contact. This usually happens within conversational distance – a metre or two. [3/n]

Smaller particles (bioaerosols or droplet nuclei) have high surface area to volume ratio, so fluid rapidly evaporates. They attain diameter <5um before settling, remaining suspended on air currents until vented out of the room or breathed in. They can transmit at distance. [4/n]

Droplets can’t get around the edge of your surgical mask whereas bioaerosols, suspended in air currents, can. You need an N95 or FFP3 respirator to protect against pathogens in bioaerosols (airborne spread). [5/n]

The science of aerosol generation was recently reviewed in a nice paper by @BolsoverE ( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6688829/). Many activities, including coughing & speech with consonants, will produce some bioaerosol from the respiratory tract. This is how M tuberculosis is spread. [6/n]

Dividing activities into ‘aerosol generating procedures’ and ‘non AGPs’ is therefore a question of degree. Your average untreated TB patient generates perhaps 1.25 infectious quanta per hour sat in a clinic waiting room vs 100 to 10,000s per hour during AGPs. [7/n]

Once bugs are in bioaerosols, they need to survive to infect another person. Mycobacterium tuberculosis is well set up to do this, with a thick waxy coat. Viruses are not, really. [8/n]

However, there are examples of the aerosol spread of viruses, including coronaviruses, e.g. the Amoy Gardens SARS outbreak ( https://www.ncbi.nlm.nih.gov/pubmed/15102999 ). [9/n]

So, what does this mean for PPE? I think this from the team at St George’s is very sensible. Acknowledges uncertainty, emphasises things that are critical but less the focus of our attention.

https://twitter.com/DrAngelaH/status/1250767074340810753

[10/n]

Yes, there may be a small contribution from aerosol transmission. However, other things are probably much more important – safe doffing, regularly wiping down surfaces that are touched by many people (keyboards, etc). [11/n]

Supporting hospital staff, including non clinical staff, and 100,000’s of home-based carers to safely use PPE is an enormous challenge and there is an argument for making the kit as simple as possible. [12/n]

None of this is to let the Government off the hook.

If they had locked down when scientists told them to, we would have had fewer cases of #COVID19 (and fewer deaths) and the pressure on PPE supply, training, etc, would have been reduced.

https://twitter.com/AdamJKucharski/status/1247498805987233792

[13/n]

Frequent changes in the PPE guidelines at the start of the pandemic undermined trust, left healthcare workers feeling exposed, and meant ‘COVID wards’ struggled to recruit the agency staff on which the NHS, after a decade of underfunding, now depends. [14/n]

The issues with PPE supply have, in some places, made it hard to deliver safe care - one example here.

https://twitter.com/CotterMeaghan/status/1251783559377047553

[15/n]

And, perhaps most importantly, we went into this crisis with the NHS and social care in bad shape. Choices have consequences and you can’t fix ten years of neglect with a one off injection of cash.

https://www.nuffieldtrust.org.uk/news-item/coronavirus-money-welcome-but-services-start-in-a-deep-hole-nuffield-trust-response-to-budget

[16/n]

So, wash your hands, doff safely, disinfect surfaces and vote for better leaders! A respirator may offer additional protection in some circumstances, but we have bigger problems to address. [17/17]