In my many years as an emergency planner in the nuclear industry I’ve never heard a real debate about respiratory protection as a public protective action in the event of an accidental atmospheric release of radioactivity. It has always been dismissed because without proper masks and fit testing the protection factors offered are compromised.
In 1981 the IAEA  identified that respiratory protection was one way to reduce dose uptake in workers and members of the public. It recognised that high levels of protection require properly designed and fitted devices and realised that these would only be available to those with planned roles in a response. They accepted that if any use is to be made of such measures by the public, the simple equipment and techniques to be employed can only be of a very rudimentary nature.
They provided a quite extensive table of filtration factors for common materials. This included the finding that 16 layers of man’s cotton handkerchiefs provides a geometric mean efficiency of 94.2% against aerosols of 1-5 μm particle size – a protection factor not to be sneezed at. At eight layers the efficiency drops to 88.9%. A single bath towel is worth 73.9%.
The public, they said, “can be advised to use such simple items while proceeding to take shelter, and possibly during sheltering. Similar precautions could be recommended while members of the public were being evacuated from a contaminated area”.
In 2002 the US NCR published a document  which suggested that improvised respiratory protection can be used as a secondary protective action that can be used to provide a nontrivial level of additional protection. They also provide a table of protection factors.
In 2007 IAEA stated that  “Improvised respiratory protection (e.g. a wet cloth over the mouth and nose) has been shown to be effective but it has not been demonstrated that the public will apply it effectively during an emergency. Improvised respiratory protection should not be assumed to provide adequate protection from an inhalation hazard and therefore its implementation should not be allowed to interfere with evacuation or sheltering”. This does not say that improvised respiratory protection should not be recommended under any circumstances; it just says it should not be used instead of shelter or evacuation.
The latest advice on the protection of the public in the event of a nuclear accident from PHE  makes no mention at all of RPE, improvised or otherwise. This publication suggests a dose reduction factor of 0.6 for inhalation dose from shelter in place over the period of a release.
So why have we taken improvised respiratory protection out of our tool box of techniques to reduce public dose? It seems to offer protection factors at least comparable with shelter in place for particulate activity.
We worry about golf courses and caravan parks within our DEPZs where shelter in place is considered likely to be ineffective. Could we at least provide a supply of half decent face masks with the stable iodine tablets we store at these establishments as a secondary protection while thinking about evacuation?
Does the Covid-19 experience that shows that large fractions of the population will wear face masks when advised and has made them far more available to the public change our current attitude?
- IAEA, Safety Series No. 55, Planning for Off-Site Response to Radiation Accidents in Nuclear Facilities
- US NCR, Perspectives on Reactor Safety, NUREG/CR-6042, Rev. 2 SAND 93-0971
- IAEA Safety Standards Series No. GS-G-2.1, Arrangements for Preparedness for a Nuclear or Radiological Emergency (2007), https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1265web.pdf
- PHE, Public Health Protection in Radiation Emergencies, PHE-CRCE-049, (2019).