For 40 years the bioterrorism domestic preparedness and the battlefield defense communities have had minimal information about the decay of B. anthracis as a BW agent under natural sunlight. The often cited rate of decay of this microorganism in spore form for clear sky conditions under bright sunlight is 2% loss per minute. This rate is often assumed to hold for solar noon, regardless of location or time of year, and is reduced by a sinusoidal function of sun angle for all other times of day. The rate is typically reduced by an ad hoc factor to account for cloudy conditions or poor visibility. Until recently, no well-documented research has supported any other quantitative estimate of the sensitivity of B. anthracis in spore form to natural sunlight.
The experimental data includes UV radiation dosimetry measured by a spectroradiometer during the exposures and the resulting observed inactivation of the spores. The spectral distribution of the UV energy output by a 1000W Xenon lamp was varied using long pass filters. Several filters with different cutoff wavelengths were used to achieve varying amounts of inactivation for the same time length of exposure. This permitted estimation of the wavelength dependence of the inactivation, or an action spectrum (AS) for inactivation. The resulting inactivation action spectrum was assessed to be similar to another AS published for a similar organism. When either AS was used to weight the dosimetry data, a log-linear relationship was apparent between survival of the organism and the effective UV dose. This relationship held regardless of the spectral distribution of the UV radiation. This allowed the design of an algorithm to predict inactivation of the organism under arbitrary environmental conditions.
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