A virus expert said repeat use of unwashed fabric face coverings is like wearing a "dirty hanky".
Dr Simon Clarke urged Scots to keep them clean or regularly change face masks to help stop the spread of lethal Covid-19.
The Reading University academic said the public risk making it worse by constantly reusing fabric and possibly getting the virus on their hands.
On BBC Radio Scotland, he warned: "There is no proper official guidance, but I'd say a covering with some sort of water repellent is important if you're going to cover your face, which is the law in Scotland and in England.
"A piece of fabric wrapped across your face is not going to stop water droplets from spreading very far. If you don't wash it, if you don't keep these things clean or you don't use disposable ones, then in pretty short order it’s going to become basically like a dirty hanky wrapped across your face."
The Scottish Government spent much of yesterday's official coronavirus briefing reminding Scots how to wear masks and face coverings in public.
Dr Clarke was asked for his opinion after bus passengers spotted a man with a face around his neck in Manchester. They assumed it was a "funky" face covering until it moved.
Dr Clarke said hospital staff replace their masks every four hours or so, but the public should be changing them every day or keeping them clean.
Two-thirds of people with Covid-19 don't know they have it and they could be spreading it further by touching the mask and getting it on their hands, he warned.
Asked if it's worse to have a dirty mask than no mask at all, Dr Clarke said: "Potentially yes, potentially. There is a reason why hospitals all over the world go to great lengths to make sure you get the right masks, fitted properly and changed regularly. They don’t just do that for fun."
He added: "There is still no great evidence because no one will do the studies about how effective masks are. There is some evidence that they have an effect particularly in hospital environments. There is some evidence from a YouGov study that people are less inclined to socially distance, less inclined to keep their hands clean.
"All these things need to be balanced and I’m afraid people take too much of an absolutist approach to this. Proper mask wearing probably can be useful, but like I say, with the emphasis on the proper."
