attempting to menace the local primary school with the offer of an air filter for a classroom. I'm not sure I want to risk affording enough to outfit the whole school.

they might put more effort into switching from open windows in the winter to buying filter units (or getting parents to donate) once they've seen an example in operation.

are they effective then?

got a commercial one last year after waiting for the research to check that they really did remove particles. Those ones are much more expensive now (perhaps because more people finally want them) but actually pretty much the same as the DIY version of 4 filters + a fan taped together.

the one I bought in 2021 to defend me against the invading solar panel engineer: https://www.amazon.co.uk/...

the kit you need for a DIY version: CC14 on Twitter

thanks!