In the first short-term experiment, 28 healthy adult volunteers were recruited, as well as 11 children between 5 and 11 years of age. Each volunteer followed the same protocol wearing a Filtering Facepiece against Particles (FFP)-2 mask 1872VH (3M); which is the European equivalent of a N95 mask, a surgical mask (1818 Tie-OnH, 3M; with a filtering efficiency of around 95% for particles of sizes between 0.02 mm to 1 mm; http://jada.ada.org/ cgi/content/full/136/7/877) and a home-made mask (made of TD Cerise MultiH teacloths, Blokker). In this standard protocol, the volunteer was asked to perform five successive tasks in a fixed sequence 1.5 minute of duration each: no activity-sit still, nod head (''yes''), shake head (''no''), read aloud a standard text, stationary walk. In this sequence of activities, the respiratory rate is gradually increased. Throughout this exercise, the concentration of particles was measured on both sides of the mask through a receptor fixed on the facial and on the external side. These were connected to a portable counter of all free floating particles in the air via an electrostatic particle classifier and counter, the PortacountH. The PortacountH can register particles floating in the air with sizes between 0.02 mm to 1 mm, covering most of the size range of infectious respiratory aerosols [12] . Total inward leakage (TIL) percentage was calculated by dividing the concentrations on the outside and on the inside (TIL = (concentration inside/concentration outside)6100); the calculated quantitative protection factor was the inverse of the leakage (PF = (TIL/ 100) 21 ). To ensure small numbers of particles produced by the volunteers would not affect measurements, we checked that at least 10,000 particles per cm 3 particles of this size class (0.02 mm-1 mm) were present in the room which were produced by a number of lit candles. (Figure 1 )