There is a growing market demand for the aerospace industry to decrease product development flow time while at the same time utilizing materials that meet increasingly stringent, and sometimes contradictory, performance criteria. Accelerated product testing and production may cause material and process development to outpace the parallel development of environmental health technologies. The result may be products which have been performance tested and approved but for which there exist insufficient data on health and environmental impact. Regulations such as the Toxic Substance Control Act (TSCA) in the United States require (1) testing of new materials; (2) the development of prudent lab practices for research and development testing; (3) restrictions on the import and export of certain chemicals; and
(4) monitoring of health, safety and environmental studies as well as company records for significant health effects from chemical exposures.
The increased use of material safety data sheets (MSDSs) has helped provide health professionals with the information required to control chemical exposures. However, complete toxicological data exist for only a few hundred of the thousands of materials in use, providing a challenge to industrial hygienists and toxicologists. To the extent feasible, local exhaust ventilation and other engineering controls should be used to control exposure, particularly when poorly understood chemicals or inadequately characterized contaminant generation rates are involved. Respirators can play a secondary role when supported by a well-planned and rigorously enforced respiratory protection management programme. Respirators and other personal protective equipment must be selected to offer fully adequate protection without producing undue discomfort to workers.
Hazard and control information must be effectively communicated to employees prior to a product’s introduction into the work area. Oral presentation, bulletins, videos or other means of communication may be used. The method of communication is important to the success of any workplace chemical introduction. In aerospace manufacturing areas, employees, materials and work processes change frequently. Hazard communication must therefore be a continuous process. Written communications are not likely to be effective in this environment without the support of more active methods such as crew meetings or video presentations. Provisions should always be made for responding to worker questions.
Extremely complex chemical environments are characteristic of airframe manufacturing facilities, particularly assembly areas. Intensive, responsive and well-planned industrial hygiene efforts are required to recognize and characterize hazards associated with the simultaneous or sequential presence of large numbers of chemicals, many of which may not have been adequately tested for health effects. The hygienist must be wary of contaminants released in physical forms not anticipated by the suppliers, and therefore not listed on MSDSs. For example, the repeated application and removal of strips of partially cured composite materials may release solvent-resin mixtures as an aerosol that will not be effectively measured using vapour-monitoring methods.
The concentration and combinations of chemicals may also be complex and highly variable. Delayed work performed out of normal sequence may result in hazardous materials being used without proper engineering controls or adequate personal protective measures. The variations in work practices between individuals and the size and configuration of different airframes may have a significant impact on exposures. Variations in solvent exposures among individuals performing wing tank cleaning have exceeded two orders of magnitude, due in part to the effects of body size on the flow of dilution air in extremely confined areas.
Potential hazards should be identified and characterized, and necessary controls implemented, before materials or processes enter the workplace. Safe usage standards must also be developed, established and documented with mandatory compliance before work begins. Where information is incomplete, it is appropriate to assume the highest reasonably expected risk and to provide appropriate protective measures. Industrial hygiene surveys should be performed at regular and frequent intervals to ensure that controls are adequate and working reliably.
The difficulty of characterizing aerospace workplace exposures necessitates close cooperation between hygienists, clinicians, toxicologists and epidemiologists (see table 1). The presence of a very well informed workforce and management cadre is also essential. Worker reporting of symptoms should be encouraged, and supervisors should be trained to be alert to signs and symptoms of exposure. Biological exposure monitoring may serve as an important supplement to air monitoring where exposures are highly variable or where dermal exposure may be significant. Biological monitoring can also be used to determine whether controls are effective in reducing employee uptake of contaminants. Analysis of medical data for patterns of signs, symptoms and complaints should be performed routinely.
Table 1. Technological development requirements for health, safety and environmental control for new processes and materials.
|Airborne levels of contaminants
||Analytical methods for chemical quantification Air monitoring techniques|
|Potential health impact||Acute and chronic toxicology studies|
|Environmental fate||Bioaccumulation and biodegradation studies|
|Waste characterization||Chemical compatibility test Bioassays|
Paint hangars, aircraft fuselages and fuel tanks may be served by very high volume exhaust systems during intensive painting, sealing and cleaning operations. Residual exposures and the inability of these systems to direct air flow away from workers usually require the supplemental use of respirators. Local exhaust ventilation is required for smaller painting, metal treating and solvent cleaning operations, for laboratory chemical work and for some plastics lay-up work. Dilution ventilation is usually adequate only in areas with minimal chemical usage or as a supplement to local exhaust ventilation. Significant air exchanges during winter can result in excessively dry interior air. Poorly designed exhaust systems which direct excessive cool air flow over workers’ hands or backs in small parts assembly areas may worsen hand, arm and neck problems. In large, complex manufacturing areas, attention must be paid to properly locating ventilation exhaust and intake points to avoid re-entraining contaminants.
Precision manufacturing of aerospace products requires clear, organized and well controlled work environments. Containers, barrels and tanks containing chemicals must be labelled as to the potential hazards of the materials. First aid information must be readily available. Emergency response and spill control information also must be available on the MSDS or similar data sheet. Hazardous work areas must be placarded and access controlled and verified.
Health Effects of Composite Materials
Airframe manufacturers, in both the civilian and defence sectors, have come to rely increasingly on composite materials in the construction of both interior and structural components. Generations of composite materials have been increasingly integrated into production throughout the industry, particularly in the defence sector, where they are valued for their low radar reflectivity. This rapidly developing manufacturing medium typifies the problem of design technology outpacing public health efforts. Specific hazards of the resin or fabric component of the composite prior to combination and resin cure differs from the hazards of cured materials. Additionally, partially cured materials (pre-pregs) may continue to preserve the hazard characteristics of the resin components during the various steps leading to producing a composite part (AIA 1995). Toxicological considerations of major resin categories are provided in table 2.
Table 2. Toxicological considerations of major components of resins utilized in aerospace composite materials.1
|Resin type||Components 2||Toxicological consideration|
|Epoxy||Amine curing agents, epichlorohydrin||Sensitizer, suspect carcinogen|
|Polyimide||Aldehyde monomer, phenol||Sensitizer, suspect carcinogen, systemic*|
|Phenolic||Aldehyde monomer, phenol||Sensitizer, suspect carcinogen, systemic*|
|Polyester||Styrene, dimethylaniline||Narcosis, central nervous system depression, cyanosis|
|Silicone||Organic siloxane, peroxides||Sensitizer, irritant|
|Thermoplastics**||Polystyrene, polyphenylene sulphide||Systemic*, irritant|
1 Examples of typical components of the uncured resins are provided. Other chemicals of diverse toxicological nature may be present as curing agents, diluents and additives.
2 Applies primarily to components of wet resin prior to reaction. Varying amounts of these materials are present in the partially cured resin, and trace quantities in the cured materials.
* Systemic toxicity, indicating effects produced in several tissues.
** Thermoplastics included as separate category, in that breakdown products listed are created during moulding operations when the polymerized starting material is heated.
The degree and type of hazard posed by composite materials depends primarily on the specific work activity and degree of resin cure as the material moves from a wet resin/fabric to the cured part. Release of volatile resin components may be significant prior to and during initial reaction of resin and curing agent, but may also occur during the processing of materials which go through more than one level of cure. The release of these components tends to be greater in elevated temperature conditions or in poorly ventilated work areas and may range from trace to moderate levels. Dermal exposure to the resin components in the pre-cure state is often an important part of total exposure and therefore should not be neglected.
Off-gassing of resin degradation products may occur during various machining operations which create heat at the surface of the cured material. These degradation products have yet to be fully characterized, but tend to vary in chemical structure as a function of both temperature and resin type. Particles may be generated by machining of cured materials or by cutting pre-pregs which contain residues of resin materials which are released when the material is disturbed. Exposure to gases produced by oven cure has been noted where, through improper design or faulty operation, autoclave exhaust ventilation fails to remove these gases from the work environment.
It should be noted that dusts created by new fabric materials containing fibreglass, kevlar, graphite or boron/metal oxide coatings are generally considered to be capable of producing mild to moderate fibrogenic reaction; so far we have been unable to characterize their relative potency. Additionally, information on the relative contribution of fibrogenic dusts from various machining operations is still under investigation. The various composite operations and hazards have been characterized (AIA 1995) and are listed in table 3.
Table 3. Hazards of chemicals in the aerospace industry.
|Chemical agent||Sources||Potential disease|
|Beryllium dust||Machining beryllium alloys||Skin lesions, acute or chronic lung disease|
|Cadmium dust, mist||Welding, burning, spray painting||Delayed acute pulmonary oedema, kidney damage|
|Chromium dust/mist/fumes||Spraying/sanding primer, welding||Cancer of the respiratory tract|
|Nickel||Welding, grinding||Cancer of the respiratory tract|
|Mercury||Laboratories, engineering tests||Central nervous system damage|
|Hydrogen cyanide||Electroplating||Chemical asphyxiation, chronic effects|
|Carbon monoxide||Heat treating, engine work||Chemical asphyxiation, chronic effects|
|Oxides of nitrogen||Welding, electroplating, pickling||Delayed acute pulmonary oedema, permanent lung damage (possible)|
|Phosgene||Welding decomposition of solvent vapour||Delayed acute pulmonary oedema, permanent lung damage (possible)|
|Ozone||Welding, high-altitude flight||Acute and chronic lung damage, cancer of the respiratory tract|
|Aliphatic||Machine lubricants, fuels, cutting fluids||Follicular dermatitis|
|Aromatic, nitro and amino||Rubber, plastics, paints, dyes||Anaemia, cancer, skin sensitization|
|Aromatic,other||Solvents||Narcosis, liver damage, dermatitis|
|Halogenated||Depainting, degreasing||Narcosis, anaemia, liver damage|
|Phenolics||Interior components, ducting||Allergic sensitization, cancer (possible)|
|Epoxy (amine hardeners)||Lay-up operations||Dermatitis, allergic sensitization, cancer|
|Polyurethane||Paints, internal components||Allergic sensitization, cancer (possible)|
|Polyimide||Structural components||Allergic sensitization, cancer (possible)|
|Asbestos||Military and older aircraft||Cancer, asbestosis|
|Silica||Abrasive blasting, fillers||Silicosis|
|Tungsten carbide||Precision tool grinding||Pneumoconiosis|
|Graphite, kevlar||Composite machining||Pneumoconiosis|
|Benign dusts (possible)|
|Fibreglass||Insulating blankets, interior components||Skin and respiratory irritation, chronic disease (possible)|
|Wood||Mock-up and model making||Allergic sensitization, respiratory cancer|