Persistence of agents

Agents can be either persistent or nonpersistent. Persistency of agents released in enclosed spaces (e.g., inside buildings) is dramatically increased. 

Classical CW agents are, in the main, volatile liquids at ordinary temperatures (phosgene is an exception a gas at ordinary temperatures). The degree of volatility varies, of course, from compound to compound. The relationship between the liquid and the vapour phase is particularly important in explaining the effect of temperature on the damage likely to be produced by exposure to an agent and to calculations of the persistency of agents. The relationship between the liquid phase and gas (or vapour) phase of a volatile substance is defined by the vapour pressure of that substance. 

Air-poUutant effects on neural and sensory functions in humans vary widely. Odorous pollutants cause only minor annoyance yet, if persistent, they can lead to irritation, emotional upset, anorexia, and mental depression. Carbon monoxide can cause death secondary to the depression of the respiratory centers of the central nervous system. Short of death, repeated and prolonged exposure to carbon monoxide can alter sensory protection, temporal perception, and higher mental functions. Lipid-soluble aerosols can enter the body and be absorbed in the lipids of the central nervous system. Once there, their effects may persist long after the initial contact has been removed. Examples of agents of long-term chronic effects are organic phosphate pesticides and aerosols carrying the metals lead, mercury, and cadmium. 

NOTE In-use concentrations of antifoam and defoamer materials are very small, often as low as 5 to 20 ppm product, or 0.25 to 2 ppm actives . However, the persistence of the agent will vary and if not replenished will eventually fail. When antifoam is fed periodically, it usually is dosed as a stand-alone product at a higher rate, but when fed continuously, it is present at a lower concentration and forms part of a multicomponent additive. 

Persistency household bleach, caustic soda, dilute alkali solutions, or decontaminating solution number 2 (DS2) are effective on equipment. Use steam and ammonia or hot, soapy water in a confined area. Depends upon munitions used and the weather, ffeavily splashed liquid persists 1-2 days under average weather conditions. GD is calculated to evaporate about 4x as slowly as water. Addition of agent thickeners can greatly increase persistency. 

H-series agents - a series of persistent blister agents, that include distilled mustard (HD) and the nitrogen mustards (HN-1, HN-2, and HN-3). 

The MW gives an indication of the persistency of an agent. Generally, the higher the number, the lower the rate of evaporation and the greater the persistency. Another use for the MW is in calculating the vapor density. 

Van Voris P, Cataldo DA, Ligotke MW, et al. 1987. Acute environmental toxicity and persistence of selected chemical agent simulants diisopropyl flourophosphate (DFP) and diisopropyl methylphosphonate (DIMP). NTIS No. AD-A181-309. 

Information on the persistency of novichok agents has not been published. 

For military purposes, unmodified arsenic vesicants are classified as persistent. However, agent vapors rapidly react with high humidity to lose most of their vesicant properties. Limited solubility slows the hydrolysis of liquid agents. Some hydrolysis products are highly toxic and extremely persistent (see Section 4.4.5). Evaporation rates range from near that of water down to that of light machine oil. 

Information on the persistency of bicyclophosphates has not been published. However, based on similar compounds these agents are likely to be persistent in the environment. 

Pulmonary agents have been absorbed into porous powders (e.g., pumice) and disseminated as dust clouds. The agents are slowly released by the dust particles thereby greatly increasing the persistency of the agents. 

Addition of solvents may alter the persistency of these agents. Salts of agents have negligible vapor pressure and will not evaporate. Depending on the size of the individual particles and on any encapsulation or coatings applied to the particles, they can be reaerosolized by ground traffic or strong winds. 

Volatility is also sometimes used to estimate the persistency of an agent. However, it does not account for the migration (diffusion) of that vapor out of the area to allow more agent to evaporate. A better estimate of persistency is relative persistency (RP). 

The relative persistency value does not account for additional factors that could impact the stability and persistence of a given agent, such as decomposition due to reaction with other chemicals in the environment (e.g., water). 

Gautrin, D. and Lemiere, C., Persistence of airway responsiveness to occupational agents what does it matter , Curr. Opin. Allergy Clin Immunol., 2, 123, 2002. 

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