Efficacy of homemade masks in reducing transmission?

There is little evidence to support their use. The WHO's April announcement stated

At the present time, the widespread use of masks by healthy people in the community setting is not yet supported by high quality or direct scientific evidence.

But the growing movement towards mandatory usage continues regardless of the absence of evidence. This morning, the Head of the Royal Society was interviewed on Radio 4 (the BBC's news channel). In response to a question on the absence of scientific trials on efficacy, he responded that requests for such were methodological fetishism, a term I had only previously encountered in the works of Deluze.

This research is from 2013:

Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?

Link to PDF: https://www.researchgate.net/profile/Katy-Anne_Thompson2/pub...

Summary:

> Several household materials were evaluated for the capacity to block bacterial and viral aerosols. Twenty-one healthy volunteers made their own face masks from cotton t-shirts; the masks were then tested for fit. The number of microorganisms isolated from coughs of healthy volunteers wearing their homemade mask, a surgical mask, or no mask was compared using several air-sampling techniques.

> Our findings suggest that a homemade mask should only be considered as a last resort to prevent droplet transmission from infected individuals, but it would be better than no protection.

Interestingly, tea towels (dish cloths) showed a higher level of filteration than t-shirts in this study.

Edit: here is a more recent review of the scientifc literature on cloth masks from the Royal Society in the UK. This is a pre-print - link is to PDF:

Face masks and coverings for the general public: Behavioural knowledge, effectiveness of cloth coverings and public messaging

https://royalsociety.org/-/media/policy/projects/set-c/set-c...

The efficacy of non-homemade masks is not as well demonstrated as one might hope. It takes years to plan, execute, analyze, review, and publish a good experiment, and there wasn't nearly as much interest in this question until a few months ago.

From what I have seen, the cloth masks' effectiveness is pretty much nil except for following situations:

- If you cough, it might stop large portion of droplets from propagating and it will reduce the distance at which it initially travels.

- As a constant reminder, it functions to prevent you from touching your face, keep you more aware of what you are touching/doing and to pressure a little bit other people to do the same.

This is my current understanding.

The real masks are actually pretty complicated things. For example, a lot of effort is on ensuring the air does not travel AROUND the mask, greatly diminishing its filtering ability.

I think one benefit of homemade mask usage is simply that it helps to normalize masks in general. Seeing others wear masks in various contexts can help reduce the stigma of embarrassment that one might feel, or serve as a helpful reminder to wear one.

[Edit: if readers have relevant papers to share, I'd be very grateful.]