ABSTRACT
INTRODUCTION Fungal spore dispersal can rarely be considered alone and is usually a combination of at least some of the key stages in the aerobiology pathway (Edmonds and Benninghoff, 1973), i.e., source, take-off, dispersal, deposition and effect. In this chapter we will consider the three steps needed to transport a spore, through the air, from one place another, namely: how the spore gets into the air, how it is transported through the air and how it is deposited at its final destination. Many examples of spore dispersal studies, used here, have been made in outdoor environments but the same principles apply to indoor applications to identify sources of microbial contamination in food processing situations. Air flow considerations Like all fluids, the flow of air can be in one or two modes: “laminar” where the air molecules follow parallel paths; and “turbulent” where the flow is more chaotic and the molecules follow different paths, although in the same general direction. Laminar flow is usually associated with low velocities and smooth surfaces, and rarely occurs outside wind tunnels or other specialised facilities (Grace, 1977). Therefore in most environments, especially outdoors, air flow is turbulent, and it is the effects of turbulence that are largely responsible for the dispersal of spores carried in the air. However, when air flows over a surface, friction slows it down so that the airspeed decreases as the surface is approached (Figure 1). The area of transition from free air flow to the surface is known
as the boundary layer. Very close to the surface the air flow becomes laminar (laminar sublayer) and air speed is almost zero (Grace, 1977). The thickness of this “boundary layer” depends on the nature of the flow over the surface and the structure of the surface itself. Air flow over surfaces has been extensively studied, and flow over natural surfaces such as leaves is discussed by Monteith and Unsworth (1990). The existence of the surface boundary layer has consequences for spore release and dispersal and is discussed below. On a different scale, in the atmosphere, wind speed increases with distance above the surface as frictional forces have a decreasing effect on atmospheric flow. This layer is called the “planetary boundary layer” and extends from the surface to where friction-induced turbulence is effectively zero (Figure 1).
