“…Variation in the abundance of airborne microorganisms is extremely high; for example, Lighthart & Shaffer (1995) found concentrations of airborne bacteria with a maximum of 1368.5 CFU m −3 , with a coefficient of variation of 90.5% and a mean of 121.3 CFU m −3 above a temperate field. This variation can operate over a number of different time scales, which include instantaneous effects of local microclimatic conditions, diurnal periodicity (Hasnain et al , 2005; Rossi et al , 2005) and seasonality for both bacteria (Marafie & Ashkanani, 1991; Choi et al , 1997; Mahdy & El Sehrawi, 1997; Kruczalak et al , 2002; Oppliger et al , 2005) and fungi (Shelton et al , 2002), with peak abundances tending to occur over the summer, but also to some extent in spring and autumn. A number of factors are thought to influence this periodicity, particularly meteorological conditions (Lighthart & Shaffer, 1995), including mean, minimum and maximum, temperature, dew point temperature, air pressure (Stennett & Beggs, 2004), the solar radiation cycle (including both diurnal and annual cycles), topography, ground heating by the sun or sea breezes, source strength and human activities (Lighthart, 1999).…”