The Sequential Box Model (SBM) that we used to determine the temporal concentration of infectious droplets of H1N1 in four cabins (C 1 , C 2 , C 3 and C 4 ) in a commercial airliner (specifically, in a Boeing 747) Figure 1 The Sequential Box Model (SBM) that we used to determine the temporal concentration of infectious droplets of H1N1 in four cabins (C 1 , C 2 , C 3 and C 4 ) in a commercial airliner (specifically, in a Boeing 747). Within the aircraft air diffuses between adjacent cabins (forward flow f i and backward flow b i ) and is removed and replaced by an air supply system. Air is replaced by outside air and by air that has been re-circulated through a HEPA filter.