[DanBC]

The other thing that's being missed is even for proper masks there are time limits to their effectiveness. That doesn't matter so much for health care professionals who should be frequently replacing the masks, but it is relevant to members of the public walking around.

Either they're keeping the mask on all day and it stops being useful after an hour, or they're taking it off and putting it on as they go in and out of shops.

https://www.ijic.info/article/download/10788/7862/

> This study was conducted to check the efficacy of face masks in limiting bacterial dispersal when worn continuously in Operation Theater. A comparison was done to find out difference between fabric and two ply disposable masks. The first sample was collected prior to wearing the mask, using cough plate method holding a blood agar plate approximately 10 -12 centimeters away from the mouth. The personnel were asked to produce “ahh” phonation. Participants were then asked to don the face mask, continue routine work and report to the study center located inside the theater for further sample collections at designated intervals of 30, 60, 90, 120 and 150 minutes after wearing the fabric mask made of cotton. The study was replicated on immediate next day using two ply disposable mask keeping all the other conditions and personnel exactly the same. Bacterial counts before wearing the mask were 5.36±4.38 and 5.7±2.99 on day 1 and day 2 of study. Bacterial counts were 0.96±1.06 (P<0.001) and 0.7±0.87 (P<0.001) at 30 min; 2.33±1.42 (P<0.001) and 2.36±1.03 (P<0.001) at 60 min; 3.23±1.54 (P=0.007) and 4.16±1.78 (P=0.011) at 90 min; 5.63±4.02 (P=0.67) and 4.9±1.98 (P=0.161) at 120 min and 7.03±4.45 (P=0.019) and 5.6±2.21 (P=0.951) at 150min respectively for fabric and two ply disposable mask. Counts were near pre-wear level in about two hours irrespective of the type of mask. There was no significant difference between cotton fabric and two ply disposable masks. Face masks significantly decreased bacterial dispersal initially but became almost ineffective after two hours of use.