Vapour Hazard

Evaporation of a relatively-small volume of liquid in an enclosed space can produce a flatmuable or toxic vapour hazard. Leakage, or spillage, of a chemical maintained as a liquid above its atmospheric boiling point by pressure (e.g. liquefied petroleum gases) or as a liquid by refrigeration (e.g. ammonia) can result in a sizeable vapour cloud. 

A severe mercury vapour hazard may occur through misuse of mercury-containing vacuum gauges (e.g. the vacustat , Section 2.30) attached to oil vacuum pumps. If the gauge is turned about its axis too quickly, mercury may be sucked into the pump and circulated with the hot oil to release large quantities of mercury vapour into the atmosphere. If there is any possibility of this having happened, the pump must not be used and should be stripped down and cleaned as soon as possible. 

The TL and MAK values should be used as guides in the control of health hazards. They are not constants that can be used to draw fine fines between safe and dangerous concentrations. Nor is it possible to calculate the TL or MAK values of solvent mixtures from the data in Table A-13, because antagonistic action or potentiation may occur with some combinations. It should be noted that occupational exposure limits such as the TL and MAK values are not intended for use as a comparative measure of one solvent against another. The values set airborne concentration limits on chemical exposure, but do not describe the ease with which that airborne limit is achieved. In addition, the vapour pressure of the solvent must also be considered. The lower the vapour pressure, the lower the airborne concentration. In order to better compare the safety of volatile compounds such as organic solvents, the use of the vapour hazard ratio ( VHR) has been recommended as a feasible measure [175], The vapour hazard ratio is defined as the quotient of the saturation concentration of a solvent (in mg/m at a given temperature and pressure) and its occupational exposure limit (in mg/m e.g. TL or MAK values), according to ... 

Phosgene has been included in various hazard rating systems in which the actuai hazard, rather than just toxicity, is evaluated [1206,1623,1957]. A vapour hazard index (a measure of the amount by which a saturated atmosphere of a materiai exceeds the TLV) for phosgene is estimated to be 10000. The index takes account of the volatility of the material, in addition to its toxicity for comparison the value for dichlorine is 1000 [1623]. 

The term vapour is sometimes used to describe the gaseous phase of a substance that exists as a liquid at room temperature. Thus, mustard vapour, nerve agent vapour and vapour hazard are terms used in the chemical warfare area. The use of such terms is not, however, standardized or universal and we still speak of mustard gas and nerve gas. There is much to be said for adopting a simple approach there are three phases of matter solid, liquid and gas introducing the term vapour as a variant of gas does not seem to be helpful. 

Figure 7. Plan Piratox (red plan) arrangement of HAZMAT zones [LHA, liquid hazard (hot) zone VHA, vapour hazard (warm) zone]. Triage is conducted (1) to detect contamination, and (2) for medical status in the triage point or the AMP. Later modifications of the plan allow for the provision of early life support (TOXALS) when required inside the warm zone...

The physical properties of a chemical warfare agent play an important part in defining the hazard presented by that agent The physical state (gas, liquid or solid) is important in determining the conditions and manner in which an agent would be used and dispersed.The vapour pressure gives an indication of both the vapour hazard and the persistency of the agent in the field. The solubility in water (and rate of hydrolysis) affects persistency in the environment, ease of decontamination and possible threat to water supplies. The viscosity affects the persistency, ability to penetrate surfaces and ease of decontamination. The... 

The relative volatilities of common industrially used diisocyanates are given in Table 14.3. Vapour-pressure characteristics of TDI and MDI are given in Fig. 14.1 these are usually used as reference materials for toxicity purposes with TDI being considered to have a high toxic-vapour hazard and MDI a low toxic-vapour hazard. 

Polyurethane adhesives were originally made from TDI by mixing it with a hydroxylic compound dissolved in ethyl acetate. More recently, polyisocyanates have been used which do not normally give rise to a vapour hazard unless curing is carried out by heat. Precautions may need to be taken if the adhesive is applied by spray, as mentioned before. Otherwise the hazards and precautions applying to the use of adhesives are similar to those for surface coatings and printing inks. 

Aromatic amines are less hazardous from skin contact they are usually solids and there is no vapour hazard unless at elevated temperature. Anhydrides can cause severe eye and skin irritation and even burns. Polyamide types are less hazardous and are considered to present a low level of health hazard. The catalytic types are too diverse to make a general statement, and Intending users should discuss appropriate precautions with the supplier. 

MAGNITUDE of VAPOUR HAZARD versus CHALLENGE to AIR MONITORING BULK STORAGE CONTAINERS - NEAT AGENT... 

Moisture-curing systems are based on isocyanate-terminated branched prepolymers which cure by interaction with atmospheric moisture. The prepolymers are prepared by reaction of an excess of diisocyanate (usually tolylene diisocyanate) with a polyol (polyester, polyether or castor oil). It is important that the residual free tolylene diisocyanate content should be low to reduce toxic vapour hazards. When the pre-polymer is exposed to moisture, the following reactions occur and cure is effected ... 

One-shot techniques have become feasible with the availability of suitable catalysts to control the rate of the various competing reactions. Thus polyol, isocyanate and chain extender can be simultaneously blended and by the use of such catalysts as stannous octoate it is possible to promote the NCO/OH reaction to a rate similar to that of the NCO/NH2 reaction. The one-shot process has the disadvantage of having free isocyanate at the casting stage which presents a vapour hazard. 

If should be made clear from the outset that, with very few exceptions, vapours are invisible. Whether a material in the vapour phase can be visibly revealed by its condensation into a cloud of droplets depends on particular circumstances, and the absence of a mist cannot guarantee the absence of vapour. Thus what can be seen is no guide at all to the existence, or otherwise, of a vapour hazard. 

The freezing point of sulfur mustard has been reduced in a number of ways to prevent the agent from solidifying in weapons in cold weather. In WWI, mustard was mixed with various solvents, e.g. carbon tetrachloride and benzene. In WWII, Britain produced it from thiodiglycol and hydrogen chloride as a 6 4 mixture with the oligomer T (Scheme 1.1), also known as O mustard. T has somewhat greater vesicant activity than sulfur mustard, is less volatile and more persistent. Other nations mixed mustard with lewisite, which also accelerated the onset of effects and increased the vapour hazard. 

The analytical results of the Danes demonstrate very clearly that any contacts of the skin with such catches from the Baltic Sea are hazardous to fishermen. Additionally, evaporation of mustard and of arsenicals during warm or hot summer days creates a vapour hazard, dangerous to the respiratory tract. 

---