This PhD research was the first to demonstrate that face masks do not only stop large droplets, but also reduce the aerosol formation by 90%. The air escaping through the gaps between face and mask as such hardly contains any aerosol. At least, when we cough. Contrarily, during talking or just breathing probably a bit more aerosol escapes. This can be explained by Newton’s laws of motion and more particularly by the force of impaction which is powerful when we cough, because particles are being accelerated when being coughed out (F = m x a; force equals mass multiplied with acceleration). In case of COVID this is unfortunate because COVID seems to spread most effectively by vocalisations (talking, shouting and singing). On top of that, we often take our mask off when using our voice. Before COVID, tuberculosis was the single infectious agent claiming most casualties worldwide. Tuberculosis appears to be mainly transmitted through coughing.