Aerospace manufacturing evokes a high-tech image. But the materials and processes used to create today’s cutting-edge airplanes and their components can result in an old-fashioned problem: poor indoor air quality (IAQ). Fortunately, plenty of options are available for aerospace manufacturers when it comes to air quality mitigation.
Aerospace manufacturing encompasses a range of parts and processes – from engine rework to extrusion of aluminum support components. Each of these processes has its own set of IAQ challenges.
In many ways, these challenges are common to all manufacturing industries. Poor IAQ is linked to lower productivity, poor worker satisfaction, lower retention rates and poor employee health outcomes. Particulates generated by welding, blasting, grinding, machining and other processes can also cause excess wear and tear on production equipment and product quality issues if not properly controlled.
And of course, aerospace manufacturers face steep fines and potential litigation if they fail to comply with minimum air quality standards defined by OSHA and other applicable local clean air standards.
But the aerospace industry also faces some unique challenges due to the types of materials used and, in some areas of the manufacturing process, the size of the components.
Aerospace material risks
In the aerospace industry, weight matters. This reality has led the industry to continually seek new materials that offer improved performance characteristics at a reduced weight. Each of these materials has its own health and safety risk profile.
Aluminum has long been used for airplane components and fuselages. While welding aluminum is sometimes considered less dangerous than stainless steel, welding aluminum still results in the formation of aluminum oxide particles that can be inhaled deep into the lungs.
Exposure can lead to respiratory irritation and pulmonary aluminosis, irreversible lung damage that results in reduced lung capacity and, in severe cases, death. Welding aluminum also can result in high concentrations of ozone, which is classified as a human carcinogen.
Aluminum alloys contain additional metals, such as nickel, titanium and lithium, each of which has its own health risks. Nickel is considered toxic and associated with chronic bronchitis, reduced lung function and cancer of the lung and nasal passages. High levels of lithium exposure are associated with reproductive and neurotoxic effects and can lead to vomiting and diarrhea. While titanium itself is not considered toxic, breathing in titanium aluminide dust can result in respiratory irritation.
Furthermore, high-tech coatings used on aerospace components can result in increased levels of smoke or toxic dust during welding, grinding or machining. The health risks are specific to the chemical makeup of the coating. These coatings may contain toxic compounds or elements, such as chrome, formaldehyde and polyurethane epoxies.
Chrome-based coatings, commonly used for corrosion resistance, have known health risks, including cancer, eye and skin irritation, pulmonary damage and asthma. Newer coatings may contain nano-materials such as carbon nanotubes, which have unknown health risks when ground into dust or fumed during thermal processes.
High-tech composite materials, including reinforced thermoplastics and carbon nanofiber materials, are gaining ground in the aerospace industry. These composite materials are often pre-formed into the final component shape, reducing the need for welding or machining of components, but joints are still needed for many applications. Composite materials can be joined using a variety of thermal methods, including electric resistance welding, ultrasonic vibration, hot plate, electromagnetic induction and infrared radiation.
Depending on the makeup of the material, fumes generated by these thermal processes may contain toxic chemicals, such as formaldehyde, styrene or isocyanates, which can cause eye, nose, throat and skin irritation and respiratory distress. Formaldehyde is also classified as a human carcinogen.
The health risks associated with new carbon nanofiber materials are not yet well understood, but there is concern that the small size of the nanoparticles may allow them to pass from the lungs into the blood stream or even cross the blood-brain barrier.
Advanced ceramic materials are increasingly used in the manufacturing of both commercial and military aircraft as well as space shuttle components due to their light weight and heat resistance. In addition to their use in electronics and sensors, ceramic composites, such as silicon carbide are making their way into structural components that are subjected to high heat, such as engine parts and jet engine turbine blades. Silicon carbide is regulated by OSHA as a hazardous material due to risks of eye, nose and lung irritation and pneumoconiosis, a chronic decrease in lung function.
In addition to the chemical makeup of the material, other factors influence the health risks associated with exposure to particulates.
The higher the concentration of the particulate in the air and the longer workers are exposed, the greater the health risks. OSHA sets permissible exposure limits (PELs) for toxic airborne materials. These are usually given as a time weighted average (TWA), or the exposure averaged across a standard 8-hour shift. There may also be a total exposure limit, which defines the highest allowable concentration in the air at any time regardless of exposure length.
Smaller particles are considered more dangerous than larger ones because they are pulled deeper into the lungs and are more likely to cross over into the blood stream where they can make their way to other body systems. Larger particulates associated with cutting and grinding cause immediate lung irritation that triggers coughing, which helps in expelling particulates from the lungs. Welding and other thermal processes produce smaller particulates that are considered more problematic.
Understanding IAQ options
OSHA mandates that engineering controls, including dust collection and filtration, should be the first line of defense against air quality problems in aerospace manufacturing. Air quality system design depends on a number of variables, including:
- The processes you are using
- The type and size of particulates (e.g., large, heavy particulates that fall downward vs. thermal fumes that rise)
- The overall volume of particulates produced
- The toxicity of the particulates produced
- How well fumes and dust from those processes can be contained
- The exposure levels for humans directly involved in processes and in the wider facility
- The physical layout of the facility and airflow patterns
Regardless of the processes you are using, IAQ mitigation options all fall into a few broad categories.
Filtration vs. exhaust: Many aerospace manufacturers rely on exhaust and makeup air systems. These systems simply push dirty air out of the facility and pull clean(er) air in. If particulate volumes are low and heating and cooling costs are not a consideration, this option is cheap and easy to implement.
However, if makeup air must be heated or cooled to indoor temperatures, exhaust systems can be a real drain on the energy budget. Depending on the type and volume of particulates, they may also put the facility out of environmental compliance.
Filtration systems pull dirty air into a dust collector where particulates are filtered out before clean air is returned to the facility. Filtration is usually the better option for facilities with high volumes of particulates and temperature-controlled indoor environments.
Source capture vs. ambient: Source capture systems collect particulates close to the source as they are generated, before they escape into the ambient air in the facility. Ambient systems turn over air for the entire facility. For some aerospace manufacturing processes that do not produce a large volume of particulates, ambient air quality control may be enough.
For processes that produce larger volumes of particulates, source capture will be the cheaper option when it is feasible; the less air you need to move, the lower your equipment and operating costs will be.
For most welding applications, a source capture solution is the best option. Hoods are generally used for robotic welding of small to medium-sized components. A hood contains fumes in a small area for easy collection. Hoods can be ducted to small individual dust collectors, such as the RoboVent Spire, or several robotic cells can be ducted to a larger centralized system.
Manual welding requires source capture solutions that keep weld fumes out of the welder’s breathing zone. These may include:
- Fume guns, which add fume extraction right to the weld torch to collect fumes as they are generated
- Backdraft or sidedraft tables
- Fume arms and portable extension booms
Backdraft tables and fume arms are best used for bench-scale welding of smaller components that do not require welders to move around much. Fume guns can be used for a broad range of MIG welding applications and really shine in environments where a high degree of welder mobility is required.
Special considerations come into play when working on very large aerospace components that make source capture challenging or impossible. Ambient systems may be needed to clean air for the entire facility. If particulates are highly toxic, personal protective equipment might be needed for workers directly exposed.
A comprehensive approach
Selecting the right approach to air quality control requires taking a comprehensive look at the facility’s attributes, goals and challenges. There are several approaches that aerospace companies can take to reduce exposure risks for workers and control costs for air quality control.
Begin with an air quality evaluation to determine your initial starting point. Dust concentration meters are used to measure overall particulate concentrations. During an evaluation, they should be placed in the breathing zone at different points in the facility to develop a map of how fumes propagate through the facility and show where concentration levels are highest.
It may also be helpful to get a chemical analysis of your dust to determine the relative concentrations of toxic chemicals and elements, especially if you are working with composite materials or engaged in multiple processes. If your processes and materials are well understood, the evaluator may be able to estimate exposure levels to specific toxins based on your total particulate concentrations.
Determine your air quality goals. Are you simply trying to meet minimum regulatory requirements or do you want to meet more stringent IAQ targets? Many toxicologists and industrial hygienists believe that OSHA PELs for a number of substances do not go far enough to protect workers from negative health impacts.
The American Conference of Government Industrial Hygienists has set voluntary guidelines for manufacturers with lower exposure limits for many compounds based on the latest science. Better air quality also pays dividends through higher productivity, lower employee turnover and recruiting of highly sought skilled workers.
Look at materials and manufacturing processes to find opportunities to lower exposure risks. Can less toxic materials be substituted – such as chrome-free corrosion coatings or formaldehyde-free resins? Can processes be adjusted to reduce the amount of particulates produced?
Physically contain fumes and particulates where possible to reduce human exposure and make dust easier to collect. Put robotic processes under hoods or use partitions (often combined with negative air pressure) to separate dust-producing processes from areas of the facility where humans are working.
Ensure that dust collectors have adequate airflow (measured in cubic feet per minute, or CFM) and filter media to handle the volume of dust you are producing and the volume of air you need to turn over. A qualified air quality systems engineer can help you size your dust collectors for your application.
Consider total cost of ownership when evaluating air quality equipment, including energy use, filter replacement costs and preventative maintenance needs. Look for energy- and filter-saving features, such as smart control systems, variable frequency drive motors that compensate for filter loading and dynamic filter pulsing systems. Also consider maintenance costs, including labor and downtime requirements.
A well-designed air quality system can protect companies from legal liability and government fines and sanctions while improving worker health, satisfaction and productivity. A qualified air quality system designer can help aerospace companies find solutions that balance costs, regulations and goals.