Re-aerosolization of Bacillus thuringiensis spores from concrete and turf

Spores of Bacillus anthracis deposited on surfaces can become airborne again as a result of air currents and mechanical forces. As such, they are a potential source of infection by inhalation. Spores of Bacillus thuringiensis were used to quantify this phenomenon in a simulation of outdoor conditions. Concrete and turf surfaces were inoculated by aerosol to produce high spore densities (greater than 1 × 10⁹  CFU per m² ) which were then subjected to the passage of air at 10 ms^-1 with and without simulated walking. Re-aerosolized spores were sampled by wetted wall cyclone air samplers. The mean total re-aerosolization rate from concrete (m^-2  min^-1 ) was 1·16 × 10^-3 for wind alone and 3·2 × 10^-3 for wind and simulated walking while for turf the respective values were 2·7 × 10^-4 and 6·7 × 10^-4 .

Following the malicious and/or accidental release of an aerosol of Bacillus anthracis spores, the immediate risk of human inhalation would decrease as the spores were deposited on surfaces or diluted by wind flow. There is, however, a concern that the deposited spores could become re-aerosolized and so present an ongoing hazard. Using an accurate simulant for B. anthracis spores a method is reported here that allowed the enumeration of re-aerosolized spores from concrete and turf by wind flow and footfall. Under the conditions used, the rates of re-aerosolization were low. These findings will need to be verified under real outdoor conditions before the true significance in terms of secondary exposure to pathogenic spores can be assessed.