This report provides a statistical analysis of a Bacillus atrophaeus spore aerosol generated by opening a spore containing letter in an office, simulating the effects of an "anthrax letter". Spore aerosols were collected by five sets of high-resolution slit to agar samplers in a vacated office building (338.4 m(2), 1015.2 m(3)) during 34 replicates of a single control scenario. Results showed that 0.1 g of B. atrophaeus spores loaded in a letter were easily detected throughout the building following the opening of the letter. Areas under the curve (AUCs) were used to summarize each replicate scenario, and the variability of the AUCs was examined to assess the sample size requirements for future scenario analyses. Based on this analysis a minimum of five replicates of each scenario will be required for future statistical comparison of new scenarios intended to determine the effectiveness of existing or proposed anthrax letter response protocols. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.