How do you even run a randomised controlled trial of mask usage? What's the double blinded placebo for a mask? Assuming you could even get an ethics committee

How do you even run a randomised controlled trial of mask usage? What's the double blinded placebo for a mask? Assuming you could even get an ethics committee to sign of on the experiment. I agree the data could be helpful, but I don't think that's the 'right' way to get it.