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In a general war, US forces may be faced by an enemy capable of producing and employing biological agents. These include disease- causing microorganisms (pathogens) and toxins. Toxins are biologically derived chemical substances that have desirable characteristics for use as biological warfare agents. Toxins may be natural or synthetic.

In a general war, US forces may be faced by an enemy capable of producing and employing biological agents. These include disease- causing microorganisms (pathogens) and toxins. Toxins are biologically derived chemical substances that have desirable characteristics for use as biological warfare agents. Toxins may be natural or synthetic.

Biological agents will most likely be disseminated as an aerosol. Therefore, a basic knowledge of their field behavior is essential for estimating friendly vulnerability. These agents differ from chemical agents in some aspects of field behavior. Pathogens decay as a result of factors such as weathering. They also require time to invade a body and multiply enough to overcome the body’s defenses. This is known as the incubation period. This period may vary from hours to months, depending on the type of pathogen.

The following paragraphs discuss biological agent dissemination, weather effects, and terrain influences, and they briefly summarize the influence of these on biological agent field behavior.

Dissemination

Pathogens are most likely to be disseminated as aerosols. Toxins, on the other hand, may be disseminated as either aerosols or large liquid drops. An aerosol is composed of particles containing pathogens or toxins. The force of the wind moves it along. At the same time, the aerosol spreads by turbulent diffusion.

Biological agents that die rapidly are said to have a high decay rate. High wind speeds (10 to 20 knots) carry these agents over more extensive areas during the agent survival period. Multiple wind shifts occur at low wind speeds. These shifts may cause more lateral spread and downwind diffusion than higher speeds. Optimum effect depends on the nature of the agent and atmospheric conditions. Highly virulent (malignant) agents with low decay rates can spread over large areas (by low or high wind speeds) and still present a casualty threat. Virulent agents with higher decay rates employed under the same atmospheric conditions are much less effective.

Weather Effects

Air stability, temperature, relative humidity, pollutants, cloud coverage, and precipitation have an effect on biological agents.

Air Stability

Atmospheric stability influences a biological cloud in much the same way it affects a chemical cloud. However, biological agents may be more effective in lower concentrations than chemical agents. This is because of their high potency. A stable atmosphere results in the greatest cloud concentration and area coverage of biological agents. Under unstable and neutral stability conditions, more atmospheric mixing occurs. This leads to a cloud of lower concentration, but the concentration is sufficient to inflict significant casualties. The coverage area under unstable stability conditions is also reduced.

Temperature

Air temperature in the surface boundary layer is related to the amount of sunlight the ground has received. Normal atmospheric temperatures have little direct effect on the microorganisms of a biological aerosol. Indirectly, however, an increase in the evaporation rate of the aerosol droplets normally follows a temperature increase. There is evidence that survival of most pathogens decreases most sharply in the range of -20°C to -40°C and above 49°C. High temperatures kill most bacteria and most viral and rickettsial agents. However, these temperatures will seldom if ever be encountered under natural conditions. Subfreezing temperatures tend to quick-freeze the aerosol after its release, thus decreasing the rate of decay. Exposure to ultraviolet light-one form of the sun’s radiation-increases the decay rate of microorganisms. Ultraviolet light, therefore, has a destructive effect upon the biological aerosol. Most toxins are more stable than pathogens and are less susceptible to the influence of temperature.

Relative Humidity

The relative humidity level favoring employment of a biological agent aerosol depends upon whether the aerosol is distributed wet or dry. For a wet aerosol, a high relative humidity retards evaporation of the tiny droplets containing the microorganisms. This decreases the decay rate of wet agents, as drying results in the death of these microorganisms. On the other hand, a low relative humidity is favorable for the employment of dry agents. When the humidity is high, the additional moisture in the air may increase the decay rate of the microorganisms of the dry aerosol. This is because moisture speeds up the life cycle of the microorganisms. Most toxins are more stable than pathogens and are less susceptible to the influence of relative humidity.

Pollutants

Atmospheric pollutant gases can also affect the survival of pathogens. Pollutant gases have been found to decrease the survival of many pathogens. These gases include nitrogen dioxide, sulfur dioxide, ozone, and carbon monoxide. This could be a significant factor in the battlefield over which the air is often polluted.

Cloud Coverage

Cloud coverage in an area influences the amount of solar radiation received by the aerosol. Thus, clouds decrease the amount of destructive ultraviolet light the microorganisms receive. Cloud coverage also influences factors such as ground temperature and relative humidity.

Precipitation

Precipitation may wash suspended particles from the air. This washout may be significant in a heavy rainstorm but minimal at other times. High relative humidities associated with mists, drizzles, and very light rains are also an important factor. These may be either favorable or unfavorable, depending upon the type of agent. The low temperatures associated with ice, snow, and other winter precipitation prolong the life of most biological agents.

Terrain Influences

Soil, vegetation, and rough terrain influence a biological agent aerosol.

Soil and Vegetation

Soil influences a biological agent aerosol as related to temperature and atmospheric stability.

Vegetation reduces the number of aerosol particles. Impact of the suspended particles upon trees and grass causes some particles to settle, and this settling reduces agent concentration. However, vegetative cover reduces exposure to ultraviolet light, increases relative humidities, and may reduce temperatures (while fostering a neutral temperature gradient). All these factors favor the survival of wet aerosols.

Rough Terrain

Rough terrain creates wind turbulence, and turbulence influences the vertical diffusion of aerosol. This turbulence reduces agent effectiveness and area coverage. Terrain affects the path of the aerosol and the distribution of surface concentration.