To determine the validity of DOD’s conclusion—that U.S. troops’ exposures to chemical warfare agents were as DOD estimates suggested— based on its plume-modeling analysis, we examined the meteorological and dispersion models DOD used to model chemical warfare agent releases from the U.S. demolition of Khamisiyah and Coalition bombings of Al Muthanna, Muhammadiyat, and other sites in Iraq during the Gulf War deployment period. We evaluated the basis for the technical and operational assumptions DOD made in (1) conducting the modeling for the bombing and demolition of Iraqi sites and (2) estimating the specific data and information used in the modeling, relating to source term, meteorological conditions, and other key parameters. We also evaluated the efforts of the CIA and DOD to collect and develop data on source term and other key parameters used in the modeling efforts. We interviewed DOD and CIA modelers and officials involved with the modeling and obtained documents and reports from DOD’s Deployment Health Support Directorate. We also interviewed and received documents from DOE officials who were involved with the modeling at LLNL. In addition, we interviewed officials and obtained documents from the Institute for Defense Analyses (IDA) concerning the IDA expert panel assessment of CIA’s modeling of Khamisiyah. We also interviewed U.S. Army officials at Dugway Proving Ground, Utah, to determine how chemical warfare agents might have been released during the Khamisiyah pit area demolitions. Finally, we interviewed officials at the U.S. Army Center for Health Promotion and Preventive Medicine, to determine how specific troop unit exposures were identified, and officials of the United Nations Monitoring, Verification, and Inspection Commission (UNMOVIC), to obtain information on source term data from the United Nations Special Commission’s (UNSCOM) analyses and inspections of the Khamisiyah, Al Muthanna, Muhammadiyat, and other sites. To determine the validity of DOD’s and the Department of Veterans Affairs’ (VA) conclusions—based on epidemiological studies—that there was no association between Khamisiyah exposure and the rates of hospitalization or mortality, we reviewed published epidemiological studies in which hospitalization and mortality among exposed and nonexposed U.S. troops were analyzed. We also interviewed the study authors and researchers and examined the Gulf War population databases provided to the researchers by DOD in support of these studies. We interviewed Veterans Benefits Administration officials and obtained documents and reports on their analyses of DOD’s population databases. We did not examine whether plume modeling data were being used by VA to determine eligibility for treatment or compensation. In an effort to identify the total costs associated with modeling and related analyses of chemical warfare agent releases during the Gulf War; we interviewed relevant officials and collected cost data from various DOD agencies and DOD contractors who supported the modeling efforts. To determine the extent of British troops’ exposure to chemical warfare agent-related releases during the Gulf War, we interviewed British Ministry of Defense (MOD) officials in London and at Porton Down, and reviewed U.K. Ministry of Defense reports concerning the potential effects of exposure to chemical warfare agent-related releases on British forces. We conducted our work from May 2002 through May 2004 in accordance with generally accepted government auditing standards. According to the CIA, modeling is the art and science of using interconnected mathematical equations to predict the activities of an actual event. In this case, modeling was used to determine the direction and extent of the plume from chemical warfare agents. In environmental hazard modeling, simulations recreate or predict the size and path (that is, the direction) of the plume, including the potential hazard area, and potential exposure levels are generated. In addition to identifying the appropriate event to model, modeling requires several components of accurate information: the characteristics or properties of the material that was released and its rate of release (for example, quantity and purity; the vapor pressure; the temperature at which the material burns; particle size; and persistency and toxicity); temporal information (for example, whether chemical agent was initially released during daylight hours, when it might rapidly disperse into the surface air, or at night, when a different set of breakdown and dispersion characteristics would pertain, depending on terrain, plume height, and rate of agent degradation); data that drive meteorological models during the modeled period (for example, temperature, humidity, barometric pressure, dew point, wind velocity and direction at varying altitudes, and other related measures of weather conditions); data from global weather models, to simulate large-scale weather patterns, and from regional and local weather models, to simulate the weather in the area of the chemical agent release and throughout the area of dispersion; and information on the potentially exposed populations, animals, crops, and other assets that may be affected by the agent’s release. Various plumes during the 1991 Gulf War were estimated using global- scale meteorological models, such as the National Centers for Environmental Prediction Global Data Assimilation System (GDAS) and the Naval Operational Global Atmospheric Prediction System (NOGAPS). Regional and local weather models were also used, including the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS), the Operational Multiscale Environmental Model with Grid Adaptivity (OMEGA), and the Mesoscale Model Version 5 (MM5). Transport and diffusion models were also used during the 1991 Persian Gulf War plume simulation efforts. These models estimate both the path of a plume and the degree of potential hazard posed by the chemical warfare agents. Dispersion models used during the Gulf War included the Hazard Prediction and Assessment Capability (HPAC) along with its component, the Second-order Closure Integrated Puff (SCIPUFF) model; the Vapor, Liquid, and Solid Tracking (VLSTRACK) model; the Non- Uniform Simple Surface Evaporation (NUSSE) model; and the Atmospheric Dispersion by Particle-in-Cell (ADPIC) model. DOD’s conclusion about the extent of U.S. troops’ exposure to chemical warfare agents during and immediately after the Gulf War, based upon DOD and CIA plume model estimates, cannot be adequately supported. This is because of uncertainty associated with the source term data and meteorological data. Further, the models themselves are neither sufficiently certain nor precise to draw reasonable conclusions about the size or path (that is, the direction) of the plumes. In particular, we found five reasons to question DOD’s conclusion. First, the models DOD and the CIA selected were in house models not fully developed for analyzing long-range dispersion of chemical warfare agents as environmental hazards. DOD and CIA officials selected several in-house models to run plume simulations. For Khamisiyah and the other Iraqi sites selected for examination, DOD selected the COAMPS and OMEGA meteorological models and the HPAC/SCIPUFF and VLSTRACK dispersion models. However, these models were not at the time fully developed for modeling long-range environmental hazards. Second, the assumptions about the source term data used in the models are inaccurate. The source term data DOD used in the modeling for sites at Khamisiyah, as well as Al Muthanna and Muhammadiyat, contain significant unreliable assumptions. DOD and the CIA based assumptions on field testing, intelligence information, imagery, UNSCOM inspections, and Iraqi declarations to UNSCOM. However, these assumptions were based on limited, nonvalidated, and unconfirmed data concerning (1) the nature of the Khamisiyah pit demolition, (2) meteorology, (3) agent purity, (4) amount of agent released, and (5) other chemical warfare agent data. In addition, DOD and the CIA excluded from their modeling efforts many other sites and potential hazards associated with the destruction of binary chemical weapons, vast stores of chemical warfare agent precursor materials, and the potential release of toxic byproducts and chemical warfare agents from other sites. Third, in most of the modeling performed, the plume heights were significantly underestimated. Actual plume height would have been significantly higher than the height DOD estimated in its modeling of demolition operations and bombings. The plume height estimates that the CIA provided for demolition operations at the Khamisiyah pit were 0 to 100 meters. However, neither DOD nor the CIA conducted testing to support estimated plume height associated with the bombings of Al Muthanna, Muhammadiyat, or Ukhaydir. According to DOD modelers, neither plume height nor any other heat or blast effects associated with these bombings were calculated from the models; instead, these data were taken from DOD’s Office of the Special Assistant for Gulf War Illnesses. In addition, according to a principal Defense Threat Reduction Agency modeler, DOD’s data on plume height were inconsistent with other test data for the types of facilities bombed. Fourth, postwar field testing at the U.S. Army Dugway Proving Ground, in Utah, to estimate the source term data did not realistically simulate the actual conditions of the demolition operations at Khamisiyah or the effects of the bombings at any of the other sites in Iraq. For field testing to be effective, conditions have to be as close to the actual event as possible, but these tests did not provide more definitive data for DOD and CIA’s models. The tests did not realistically simulate the conditions of the demolition of 122 mm chemical-filled rockets in Khamisiyah. The simulations took place under conditions that were not comparable with those at Khamisiyah.