In the second long-term experiment, 22 volunteers, all adults, 10 men, 12 women, were divided into 3 groups. Each group wore a single type of mask for a period of three hours, being either a FFP2 mask (4 males, 4 females), a surgical mask (3 males, 4 females) or a home-made mask (3 males, 4 females), similar to the masks used in the short-term experiment described above. At the beginning and end of each three-hour period, full series of measurements were taken using the standardised protocol as described for the short-term experiment, and during the three hour period while wearing the masks, participants reported back at regular intervals for a short measurement during rest (absence of activity). For the remainder of the period, participants carried on with their usual daily activities. During regular activities in between measurements, the probes of the masks were plugged which did not involve dislodging of the masks. In the final experiment, we assessed the effectiveness of different types of masks in reducing outgoing transmission from an infectious subject shedding aerosolised particles. This was simulated by fitting the different types of masks to an artificial test head, which was connected to PC-driven respirator (BacouH LAMA AMP, Modelref 1520307). Breathing frequency was varied to mimic different respiratory rates (15, 25 and 40/minute). Only expiration was simulated; twice for each mask at each respiratory rate. The breathing flow was defined as (respiratory rate/minute x volume per breath (2 litres)) resulting in a breathing flow of 30, 50 and 80 litres per minute, which correlates with light (walking), medium (marching with backpack) and strenuous (running) activities [13] . Concentrations of particles were measured as described above by a TSI Portacount Respirator Fit tester, model 8020, measuring outward protection, rather than inward protection.