Dust, Mist and Vapour

Chemistry does not limit itself to spaces called reactors. Considerable quantities of dust, mist, and vapours may be transferred to vent lines and fumehood ducts, possibly accumulating and reacting there for years. In consequence fires and explosions can start, or blockages form. Vent lines would be better for occasional scrubbing down. See Perchloric acid Fume cupboards See Silver nitrate Sodium dithionite... 

The terms dust , mist and vapour are defined as follows ... 

The relevant ingredients of a mixture are those which are present in concentrations of 1% (w/w for solids, liquids, dusts, mists and vapours and v/v for gases) or greater, unless there is a presumption (e.g. in the case of corrosive ingredients) that an ingredient present at a concentration of less than 1% can still be relevant for classifying the mixture for skin irritation/corrosion. 

A comprehensive review includes autoignition of gas mixtures, explosions at low and high temperatures, properties of flames and combustion of metals [1]. A review of explosion suppression techniques for dust, mist or vapour deflagrations, as an alternative to venting industrial plant [2],... 

Chemically mists and vapours must be similar, but physically their characteristics are markedly different. Airborne droplets if stable (i.e. if not liable to re-evaporation) can be considered physically as an additional pollutant in a condensed phase, to behave as discussed earlier for dusts. Indeed the generic name aerosol refers to any gasborne dispersion of small particles irrespective of whether those particles are liquid or solid. It is the total aerosol burden which is determined by sampling through a filter, and the cyclohexane-soluble (i.e. notionally organic) portion of that burden which forms the basis... 

LEL (lower explosive, OR FLAMMABLE, LIMIT) The minimum eoneentration of a gas, vapour, mist or dust in air at a given pressure and temperature that will propagate a flame when exposed to an effieient ignition souree. Generally expressed as % by volume for gases and vapours, and as mg/m for mists or dusts. 

The toxicity of a substance is its capacity to cause injury once inside the body. The main modes of entry into the body by chemicals in industry are inhalation, ingestion and absorption through the skin. Gases, vapours, mists, dusts, fumes and aerosols can be inhaled and they can also affect the skin, eyes and mucous membranes. Ingestion is rare although possible as a result of poor personal hygiene, subconscious hand-to-mouth contact, or accidents. The skin can be affected directly by contact with the chemicals, even when intact, but its permeability to certain substances also offers a route into the body. Chemicals accorded a skin notation in the list of Occupational Exposure Limits (see Table 5.12) are listed in Table 5.2. Exposure may also arise via skin lesions. 

Mists are dust clouds in which the particles happen to be liquid. Should that liquid be combustible, even though it is nowhere near its flash-point, explosion is possible [1] [2]. Mist explosions attract increasing study [3]. It is possible that many vapour cloud explosions have had a mist component. The editor surmises that, under appropriate circumstances, evaporation of volatile mist by the heat of a vapour (or mist) explosion might generate a larger pressure pulse than simple thermobaric effects on air. Foams are inverse mists and should show similar explosive potential. 

There are several other forms of matter having characteristics similar to gases, such as vapour, mist, dust, fume, and smoke. These materials are classified in Table 1.3. 

Values for inhalation toxicity are based on 4 hours tests in laboratory animals. When experimental values are taken from tests using a 1 hour exposure, they can be converted to a 4 hour equivalent by dividing the 1 hour value by a factor of 2 for gases and vapours and 4 for dusts and mists. 

Inhalation (gas, vapour and/or dust/mist) LC50 in the Warning... 

For gases, vapours, dusts, mists, LC50 in the equivalent range of the oral and dermal LD50 (i.e., between 2000 and 5000 mg/kg bodyweight). 

Gases and fine dust particles Gases, vapours, mists, smoke and dust to a particle size up to to 5 pm... 

Respirators. These purify the air by drawing the air through a medium that removes the contaminants. Respirators can be used to provide protection against dusts (including mists and fumes), gases and vapours. Respirators should not be used in atmospheres immediately dangerous to hfe. 

Inhalation is the most frequently encountered route of exposure for both CW agents and toxic industrial chemicals. Toxic substances can enter the body through inhalation of gases, vapours, smoke, fumes, dusts, mists or fog. The properties of these were described in the previous chapter. Following inhalation, toxic substances can cause trauma at all levels of the respiratory tree and in the lung parenchyma before being absorbed into the circulation where further damage can occiu . 

Exposures to chemicals may involve solids, liquids, or airborne matter as mists, aerosols, dusts, fumes (i.e. pm-sized particulates), vapours or gases in any combination. Many situations, e.g. exposure to welding fumes or to combustion products from fossil fuels, include mixtures both of chemicals and of physical forms. Quantification of exposure is then difficult. 

The need to remove suspended dust and mist from a gas arises not only in the treatment of effluent gas from a plant before it is discharged into the atmosphere, but also in processes where solids or liquids are carried over in the vapour or gas stream. For example, in an evaporator it is frequently necessary to eliminate droplets which become entrained in the vapour, and in a plant involving a fluidised solid the removal of fine particles is necessary, first to prevent loss of material, and secondly to prevent contamination of the gaseous product. Further, in all pneumatic conveying plants, some form of separator must be provided at the downstream end. 

Inhalation (into the lungs). This is a more common pathway for the absorption of toxic chemicals these may be in the form of gases, vapours, dusts or mists. All toxic powders, volatile liquids and gases should only be handled in efficient fume cupboards. The practice of sniffing the vapours of unknown compounds for identification purposes should be conducted with caution. 

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