Aerosol vapor mixtures, generator

Laskin-Type Nozzle Generator. A third type of atomizer, the Laskin-type nozzle generator, is used to create test atmospheres of particulate or aerosol and vapor mixtures. A pure liquid or melt is used, and no solvent is necessary. Again, this is important for evaluating filter and solid sorbent combination sampling trains. 

Examples of such procedures are making dilutions (not all components of the mixture may be equally soluble or miscible with the vehicle) and generating either vapors or respirable aerosols (not all the components may have equivalent volatility or surface tension, leading to a test atmosphere that contains only a portion of the components of the mixture). 

No histological changes occurred in the liver of adolescent male rats that were whole-body exposed to 0 or 900 ng/m Aroclor 1242 vapor 23 hours/day for 30 days (Casey et al. 1999). The generation of the vapor-phase test atmosphere was based entirely on the evaporation of a liquid PCB mixture using a system that did not create aerosol droplets, and the concentration and congener composition of the test atmosphere was well characterized. Limitations of this study include only one exposure level and liver end point and a relatively small number of animals (8/group) however, uptake of PCBs in the liver was confirmed by tissue analysis, and the exposure was sufficient to induce effects in other tissues, including the thyroid, which is known to be particularly sensitive to PCBs. 

Pyrotechnically generated smokes from grenades or powder clouds may not be acceptable from a safety point of view in certain circumstances for example, where fires may be produced, or in confined spaces with low rates of ventilation where asphyxial effects could develop. In order to reduce such potential hazards, a variety of approaches have been investigated. One of these is the use of highly volatile PCSI materials, which can be projected into enclosed spaces, but no material of sufficiently low toxicity has yet been found which is acceptable. An alternative approach is the use of a PCSI substance dissolved in a volatile solvent, and the mixture projected into the area by means of a pressurized aerosol canister or a frangible missile. This results in the development of an airborne vapor or aerosol of irritant (Ballantyne, 1979). 

Gasforming mixtures—with the heat and gas aerosolizing dye-stuffs, toxic chemicals, or nuclei for water vapor, and used as colored smoke, tear gas, or cloud seeding generators. 

The flash point, denoted PP or jp, expressed in K( C), is the temperature at which a pool of a flammable liquid will generate sufficient vapors to form an ignitable vapor/air mixture. It can also be seen as the temperature at which a liquid will reach its lower flammabihty limit (LFL) in air. However, flammable liquids can also ignite below their flash point if the surface area is increased either by dispersion (e.g., aerosol) or by mechanical activation (e.g., spraying) that raises the concentration of vapor in air above the lower flammability limit. In practice the flash point of a liquid is measured following standardized laboratory test protocols such as the Continuously Closed Cup Test (ASTM D6450), the Pensky-Martens Closed Cup Test (ASTM D93) or by the Tag Closed Cup Test (ASTM D56). 

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