The risk of corona infection is definitely greatest indoors. Virus particles can often stick to invisible particles (aerosols) for hours and be inhaled. In other words, the thicker the fabric of a mask and the more layers the mask has, the better it can reduce the spread of aerosol as far as possible.
Scientists used radioactive particles to simulate the effects of masks. Different masks were attached to the end of a tube, after which test aerosols were blown through the tube and thus determined how many particles the substance allowed through. For a clear comparison, aerosols were also blown unfiltered through the hose.
With the following result: Single-layer protective masks only had a filter performance of 35 percent, whereas a three-layer mask reduced the number of particles by 89 percent. The size of the pores is crucial - the more layers of fabric an FFP2 mask has and the smaller the pores of the fabric, the greater the protective effect of the mask.