Process-generated contaminants



There is not currently a formal and universally recognised definition of a process-generated substance. Process-generated contaminants can be generated as emissions from combustion processes, abrasion and/or other processes that physically or chemically degrade or otherwise modify the starting material(s), e.g. degradation/composting of organic materials. Such emissions are generally complex and variable mixtures of substances that are generated by processes or activities and are incidentally released (i.e. the substances are not supplied or intentionally produced). They can consist of gases, vapours, mists, fumes and/or particles (fibres and non-fibrous) with their physical and chemical composition determined by the starting material, the process conditions and the level and type of energy applied or generated, and any control measures applied to reduce emissions. Well-known process-generated contaminants include welding fumes, diesel engine exhaust, crystalline silica dust and organic dusts such as wood dust and materials associated with micro-organisms such as endotoxins.

The following examples are not considered to be process-generated contaminants:

  • Evaporation of volatile agents from a mixture, as this represents a change of state of a substance that is present in the mixture.
  • Particles released from handling, scooping, mixing, etc. of powders.

In neither of these examples is the activity or process causing a chemical or major physical transformation in agents present in the mixture prior or during the emission, although due to differences in vapour pressure and dustiness, the relative composition of the released materials may be different from that in the original liquid mixture or powdered materials.

Examples of process-generated substances

All combustion processes will result in the production of a complex mixture of gases, vapours and solids depending on the fuel, the combustion conditions (e.g. temperature, level of oxygen) and any control measures adopted to reduce emission of hazardous materials.

One commonly encountered example is diesel exhaust fumes, arising from the combustion of diesel fuel in compression ignition engines. Emissions from diesel engines are complex mixtures of gases, liquids and solids. Many of the individual components have their own specific toxicity, and some have exposure limits assigned to them. The gas phase comprises of carbon monoxide, nitrogen oxides and volatile organic compounds such as benzene and formaldehyde. The particle fraction consists of elemental and organic carbon, ash, sulphates and metals. Polycyclic aromatic hydrocarbons and nitroarenes are distributed within both the gas and the particle phases. Diesel engine exhaust is classified as a human carcinogen by the International Agency for Research on Cancer (IARC)[1].

The exact composition of diesel engine exhaust depends on the type and age of the engine, the fuel composition, the performance of the engine, and the duty cycle. Diesel engine emissions have changed (both qualitatively and quantitatively) substantially over time due to changes in engine technology (to improve efficiency and reduce emissions) and due to changes in composition. The sources of occupational exposures to diesel engine exhaust include motor vehicles, locomotives, and diesel-powered equipment such as tractors, portable generators and forklift trucks.

Any process or activities that involve heating, cutting or soldering or welding metal will produce a fume that again will contain a mixture of gases and particulate materials.

The composition of welding fume depends on the type of welding process, the composition of the welding rod and the material being welded. The majority of the fume from metal welding is generated by the consumable (welding rod) rather than the substrate being welded. Information on the composition of the fume should be given on the safety data sheet provided with the consumable. Stainless steel, and other specialist alloys containing high levels of chromium, nickel and manganese carry a particularly high risk. Manual metal arc (stick) welding generates more fume than other techniques, such as MIG and TIG welding (metal inert gas, and tungsten inert gas). Although welding is most commonly used to join metals, other materials, such as plastics, are also welded and these processes can also generate toxic fumes which must be controlled.

The physical and chemical composition of aerosols generated from metal cutting depends on the composition of the metal and any products applied to the surface of the metal. High exposures to welding and other metal fumes can cause metal fume fever, which can present itself as non-specific flu-like symptoms such as fever, chills etc.

Other processes involving heating materials can lead to generation of process-generated contaminants. One example is Rubber fumes. There is a wide range of different base rubbers, both natural and synthetic, each with a unique chemical structure. In addition, during the production of rubber products, a wide range of varying chemical agents are used as fillers, vulcanising agents, accelerators and inhibitors, antidegradants and antioxidants, plasticisers, etc. Rubber fumes have been defined by UK HSE as fumes evolved in the mixing, milling and blending of rubber and in the processes which convert the blended rubber into the finished products. The composition of rubber fumes varies depending on the production process and temperatures, as well as on the raw materials used in the production of the rubber product. Chemical agents present in rubber fumes may include various volatile agents (e.g. benzene, toluene, xylenes, ethylbenzene, dimethylbenzenes, diisopropylbenzenes), polyaromatic hydrocarbons, and other agents[2]. As is generally the case for process-generated fumes, many of the individual components have their own specific toxicity, and some have exposure limits assigned to them. The composition of rubber fumes has also changed over the years. For example, concerns related to PAH- and N-nitrosoamine levels in rubber fumes have resulted in changes in the raw materials used, decreasing the levels of these compounds in rubber fumes.

Many activities within the construction sector, as well as in the mining and quarrying sector, produce dusts which are released into the air. The exact composition of the dust will depend on factors such as: the types of activities; the materials/products that are being used; the materials used in the buildings that are being built, repaired or demolished; or the composition of the ore that is being mined or quarried. Dust generated in these activities often includes some amount of respirable crystalline silica (RCS). Similarly, asbestos exposure in Europe now mainly occurs during the repair, maintenance or demolition of buildings and hence could be considered a process-generated contaminant.

Organic dusts may also be considered to be process-generated contaminants as they are emitted from organic materials that have undergone some degradation, resulting in a release of a complex mixture that can include a range of viable micro-organisms (e.g. fungal spores, bacteria, viruses) and their by-products including toxins, constituents of their cell-walls (e.g. endotoxins and glucans) and parts of living organisms. Exposure to organic dusts can occur in occupations and industry sectors that involve working with animals, plants and organic materials, including farming and collection and processing (e.g. composting) of household and other waste. Other organic contaminants may be released as aerosols, for example during the slaughtering and butchering of animals.

An example of an organic dust is Wood dust, which is generated from sawing, sanding and other woodworking processes, and carries a variety of health risks. This includes the dust from hard and soft woods, and also composite materials such as medium density fibreboard (MDF) and chipboard, wood chippings used for animal litter or in the paper and pulp industry, or mulch made of wood chippings. Wood dust can cause asthma and is also classified by IARC as a carcinogen[3].

Health risk from exposure to Process-Generated Substances

As is clear from the above there are a wide variety of process-generated contaminants, with widely varying physical, chemical and biological composition, to which workers might be exposed. Consequently, any health risks will also vary. However, many commonly encountered process-generated substances have been linked to malignant (lung cancer) and non-malignant respiratory disease (e.g. COPD). Exposure to organic dusts, consisting of viable micro-organisms and agents such as endotoxin and glucans have been associated with infectious diseases as well as syndromes such as Organic Dust Toxic Syndrome, which is a potentially severe flu-like syndrome which has been observed amongst others in farmers and mushroom workers.

IARC has classified several process-generated substances as Group 1 – Carcinogenic to humans (e.g. diesel engine exhaust, coal combustion, soot, and wood dust) and Group 2a – probably carcinogenic to humans (e.g. welding fumes, bitumen, biomass fuel emissions, combustion of coal, gasoline engine exhaust). In addition, several occupations and industries are classified as Group 1 (e.g. rubber industry, painter) or Group 2b (e.g. firefighter, petroleum industry) where the causal agent has not been established, but where the process-generated emissions are likely to play an important role.

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