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28 Solid radioisotopes, in the form of sizeable shapes such as liquid release could be relatively easily limited, and cleanup pellets, rods, discs, blocks, and plates, present a human would be easier than for fine particles. Overall, topography, health hazard if they are approached without proper shield- obstructions, and plumbing would largely decide the disper- ing. Exposure may be by skin absorption for beta particles sion of a released radioactive liquid. Water disinfection and gamma rays, the latter of which may also be inhaled. Al- processes cannot reduce radiation levels. Dilution to safe though a large solid mass of radiation is the easiest form to levels is a possible solution for liquid radioactivity in some contain, if the solid mass is broken into fine particles, it be- scenarios. comes much more dangerous. Open-air gaseous radioisotope releases are generally Fine particles of radioactive materials are considered the quickly dispersed and diluted with the surrounding air, most likely form in the event of an RDD because the explosion thereby presenting a relatively short-term, local radiological would physically spread particles regardless of whether the ra- hazard. A release of a radioactive gas that is heavier than air dioactive material was initially fine particles or a large chunk may linger longer than expected when there is little wind. pulverized by the explosion. Particles small enough to be in- Structures such as buildings may also slow the mixing and haled present the greatest concern because they can affect dilution of released gases with air. Generally, a radioactive human health though all exposure routes: inhalation, ingestion, gas release in an enclosed space with little or no ventilation and external direct exposure (i.e., radiation absorbed through poses a much greater threat than a release outside. Predicted the skin). dispersion of a gaseous release in conjunction with an esti- The smaller the particle, the farther it may travel within the mated radioisotope quantity can together indicate the extent respiratory tract. In general, particles greater than 10 m in di- of the area at risk, but many gaseous releases may be essen- ameter are trapped by nasal hairs and released with exhaled air tially completely dispersed by the time this information is or sneezing. Particles less than 10 m in diameter are referred considered. to as "inhalable" because they may pass into the upper portions Similar to gases, fine radioactive particles may be dis- of the lungs, which contain many branched passageways. persed by natural wind and wind generated from traffic. In These passages (bronchi and terminal bronchioles) are lined contrast to gases, particles dispersed by wind will eventually with mucus and cilia. The mucus traps particles, and the cilia be deposited on surfaces where they may either stick or be gradually push them up and out of the lungs in about a day. re-suspended in the air. There has been extensive study of These particles are then swallowed and enter the digestive sys- the atmospheric dispersion of radioactive gases and small tem. The smallest particles (e.g., less than 2.5 m in diameter) radioactive particles (also called particulates). Although are referred to as "fine" or "respirable" particles. Fine particles many factors affect dispersion of gases and fine particles, may pass into the deepest portions of the lungs (i.e., respira- the most important factors are wind speed, wind direction, tory bronchioles, alveolar ducts, and alveolar sacs), where they energy and height of release (i.e., fire, explosion), and the are removed by macrophages over many days and months, al- presence of obstructing structures or natural features (e.g., lowing longer-term lung exposure to the radioactive particle. buildings, hills, and mountains). Table 2-4 in the preceding The smallest particle size that can be distinguished by the chemical subsection provides estimates on how long it takes unaided human eye is about 30 m in diameter. Table 2-5, in various particle sizes to settle to the ground in the absence of the preceding chemical agent subsection, shows the particle air currents. sizes of some commonly used substances to help put particle size in perspective. 2.3.3 Radiological Threats and the Transportation System Dispersion and Population Density The transportation systems has particular vulnerabilities with respect to radiological threats, and as discussed below, The density of the population in the area at risk affects the is likely to be substantially involved in actions to minimize means for communicating instructions and the choices of the consequences of a radiological event. transportation-related responses. The number of individuals at risk during a radiological event depends on the population density and the area over which health-threatening levels of Transportation System Vulnerabilities radiation are dispersed. Health-threatening levels are deter- mined by the type and energy of the radiation. Dispersion is A radiation release event that occurs near or in any trans- determined from the form (i.e., gas, liquid, solid, or particles), portation mode can contaminate the roadway or track on topology and meteorology (i.e., rain and wind currents), and which vehicles travel, transportation vehicles, passengers, and the quantity of radioactive material. cargo passing through the contaminated area. Factors that A radioactive liquid could spill on the ground, puddle, and make a transportation mode more vulnerable to sustained be absorbed into soil or other solids as well as run into sew- radiation during a radiological event include the presence of ers or nearby natural bodies of water. Public exposure to a enclosed spaces, surface contamination and re-suspension of

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29 radioactive particulates, and ease (or difficulty) of decontam- speed passenger trains). Rough or corroded metal has high ination. These factors and other vulnerability factors are sum- adsorptivity, as does bare concrete, asphalt, and fabric. Thus, marized in Table 2-13 for each transportation mode. surface contamination will generally be high for older vehi- Enclosed spaces such as tunnels and, to a lesser extent, cles, stations, terminals, roads, and clothing. road and track surrounded by tall buildings may more read- The ease of decontamination is typically inversely related to ily retain concentrated radioactive material than open spaces. surface adsorptivity. Thus, smooth surfaces, such as new cars Gaseous and particulate radiation may enter vehicles and and the outside of aircraft, are more easily decontaminated vessels with air and, in all transportation vehicles, may be than rough surfaces, such as asphalt, concrete, terminals, sta- more readily retained in passenger and cargo compartments tions, and vehicle interiors. Time required for decontamination than in the open air. Factors that reduce the ability for quick may range from hours to years, depending on the magnitude dilution of radioactive gases or particles (i.e., enclosed and tenacity of the radiation. spaces) allow people and cargo to receive larger doses, Deposited solid radioactive particles can be re-suspended thereby increasing health effects. by air currents generated by passing traffic. This is a particu- Among the largest public populations at risk in an enclosed lar concern for highway, rail, and mass transit because traffic space are those in transit underground stations or terminals, on these modes would essentially expand the contaminated airport terminals, and large passenger compartments (e.g., area. Spread of the contaminated area by traffic could also be trains, cruise ships, and aircraft). HVAC systems in enclosed an issue in ports, docks, canals, and rivers. The open sea, how- spaces may increase exposure risks caused by continued cir- ever, is not susceptible to significant radioactive particle con- culation of radioactive gases or fine particles. Contamination tamination from passing ships because its enormous volume of food and water cargo present unique concerns both as a would dilute any radioactive material to an insignificant specific target for contamination and as cargo present in the concentration. Similarly, re-suspension of radioactive parti- transportation modes passing through a contaminated area. cles by aircraft in flight is a reduced concern because the Surfaces with a greater adsorptivity for particles will have re-suspended particles would be greatly diluted in the upper greater levels of surface contamination from radioactive par- air before settling to the ground. ticles. Thus, the surfaces are more vulnerable to greater and Intersecting modes of transportation can represent a extended radiation exposure. In general, surface adsorptivity substantial vulnerability because they can allow cross- is less at high speeds and for aerodynamic, smooth surfaces contamination from one transportation mode to another including aluminum, steel, and glass (e.g., aircraft and high (e.g., rail crossings, rail and bus stations, airports, ports TABLE 2-13 Vulnerabilities to Extended Radiation Exposure for Each Transportation Mode. (Note: High more vulnerable (higher risk), Low less vulnerable (lower risk)) Radiological Highway Rail Transit Aviation Maritime Vulnerability Enclosed space Tunnels Tunnels Tunnels Aircraft Terminals Passenger Stations Stations/ Terminals Passenger compartments Passenger terminals compartments compartments Passenger compartments Vehicle surface Low for high speed Low for aircraft High High High for all others High High for airports contamination Ease of Moderate Easier for high Easier Difficult for Moderate decontamination speed aircraft Moderate for Easier for others airports Resuspension of High for low Very Low for Very Low at deposited High High aircraft sea speed solid particle Low for high speed High for airports High in port contamination HVAC spread None Within passenger Passenger High in airport or High in cruise contamination car compartments aircraft ship Drinking water None Passenger drinking Passenger Passenger Passenger contamination water drinking water drinking water drinking water Ability to Yes Yes Yes Yes, at airport Yes, at dock contaminate other Terminal modes Agricultural cargo Yes Yes No Slight Yes contamination Transport path Road Track Road, track, Airport Harbors, canals, contamination waterway rivers