transport modeling. The uSSRA used a meteorological classification scheme referred to as self-organizing maps (Kohonen, 1982) to generate meteorological inputs required to run SCIPUFF. Because this approach is generally not used in air pollution modeling, it is difficult to evaluate the validity of the claim that 95% of the meteorological conditions result in no infection. The graphics used to present the results convey little information. With such insufficient information, the committee found it impossible to evaluate the validity of the results.

SHORTCOMINGS IN MODELING AIRBORNE SPREAD IN THE NAADSM

The NAADSM is used “to simulate the spread and control of foreign animal diseases in a population of susceptible livestock herds” (www.naadsm.org). The NAADSM analysis includes a pathway to model the spread of infection through airborne transport of virus particles. The airborne spread model has two options to describe the probability of airborne spread of infection between two premises. The first option assumes that the probability of infection at a premise declines linearly with distance from the source of infection; this option was used in the 2010 SSRA. The second option assumes exponential decline with distance and is used in the uSSRA (p. 440); exponential decline is further explained in the NAADSM user’s guide. The linear model typically leads to lower probabilities of spread over shorter distances and higher probabilities over longer distances.

The uSSRA states that the adoption of the exponential option was based on FMDv dispersion modeling results as illustrated in Figure 6.1.4-19 (p. 442). However, the uSSRA does not show how these results are derived from the results presented in the cited references (Garner and Cannon, 1995; Sørensen et al., 2000) or those from SCIPUFF as described in Volume I of the uSSRA. Furthermore, it is unclear how these results were used to specify the parameters of the airborne spread equation on p. 67 of the NAADSM user’s guide.

Figure 6.1.4-19 of the uSSRA indicates that the uptake of plaque-forming units by cattle falls off by an order of magnitude when the distance increases by a factor of 2.5 from 2 to 5 km. That result is inconsistent with the statement made in the uSSRA that according to Garner and Cannon (1995) the risk of infection is expected to fall off linearly with distance under stable atmospheric conditions. It might be more appropriate to assume that the risk of infection is inversely proportional to the distance from the source because the risk of FMDv exposure is high when the atmospheric boundary layer is stable (Garner and Cannon, 1995). Under these conditions, the shallow boundary layer limits vertical dispersion, and the growth



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