Nanomanufacturing: Worker Risks and Protections

In an effort to give employers specific guidance on protecting workers from the potential harmful health effects of manufacturing and handling nanomaterials, the National Institute for Occupational Safety and Health (NIOSH) recently released four guidance documents.

Three documents provide “key tips on the design, use, and maintenance of exposure controls for nanomaterial production, post processing, and use.” The fourth document is a poster with questions that employers and workers should consider before starting work with a nanomaterial. For each question, the poster lists risk-reduction options described in the guidance documents.

Here we provide a brief look at the challenge of protecting workers from health effects from overexposure to nanomaterials in the manufacturing environment and also summarize the main points in the NIOSH documents.

Superior Performance

Engineered nanomaterials are intentionally produced to have at least one primary dimension less than 100 nanometers (nm). These particles have unique shapes and physical and chemical properties—for example, a high level of reactivity—that conventionally sized particles do not possess. Nanomaterials can provide superior products in areas such as medicine, electronics, biomaterials, and consumer products.

The use of nanotechnology to manufacture consumer products has been occurring for more than 20 years. According to unofficial data compiled by the Project on Emerging Nanotechnologies, manufacturers have introduced more than 1,600 nanotechnology-based consumer products into the marketplace. In the current climate of warp-speed technological change, a manufacturing technology that has been in place for over 2 decades may not seem new or emerging. And yet, even with significant experience with nanomanufacturing, a remarkably small amount of information has been developed on risks to workers in the nanoproduction and processing chain.

Two OSHA Exposure Limits

When addressing risks to workers from chemicals, OSHA’s fundamental task is to establish permissible exposure limits (PELs), or the maximum amount of a chemical to which a worker can be exposed over a given period of time without facing significant risks of chronic or acute illness. The agency has been criticized for not revising the hundreds of PELs it has had in place for conventional chemicals since the 1970s. While OSHA has made several efforts to update its PELs, it has failed to do so, citing the “complexity” of the rulemaking process. Given that less is known about the properties of nanochemicals, the challenge of setting PELs for these substances is even more daunting.

“Few occupational exposure limits exist specifically for nanomaterials,” says OSHA. “Certain nanoparticles may be more hazardous than larger particles of the same substance. Therefore, existing occupational exposure limits for a substance may not provide adequate protection from nanoparticles of that substance.”

OSHA has made only two recommendations—not PELs—for specific nanomaterials:

1. Worker exposure to respirable carbon nanotubes and carbon nanofibers should not exceed 1.0 micrograms per cubic meter as an 8-hour time-weighted average.
2. Worker exposure to nanoscale particles of titanium dioxide (TiO₂) should not exceed NIOSH’s recommended exposure limit of 0.3 milligrams per cubic meter.

Protecting Workers

In “Occupational Safety and Health Criteria for Responsible Development of Nanotechnology,” (Journal of Nanoparticle Research, 2014; 16(1)), Schulte et al. identify five “criterion actions” employers should take to ensure worker safety:

• Anticipate, identify, and track potentially hazardous nanomaterials in the workplace.
• Assess workers’ exposures to nanomaterials.
• Assess and communicate hazards and risks to workers.
• Manage occupational safety and health risks.
• Foster the safe development of nanotechnology and realization of its societal and commercial benefits.

NIOSH’s guidance documents expand on these general criterion actions.

Handling Nanomaterials

Protecting Workers during the Handling of Nanomaterials addresses small-scale handling of nanopowders. Activities include working with a quality assurance/control sample, weighing out a specific quantity for mixing/compounding, and processing smaller quantities in downstream industries.

Weighing out nanomaterials can lead to worker exposure primarily through the scooping, pouring, and dumping of these materials. The guidance cites a study that found that the highest exposures at six manufacturing facilities that used carbon nanotubes and nanofibers occurred during dry powder handling tasks, including mixing and weighing operations.

The primary engineering control used in the nanotechnology industry during handling is a ventilated enclosure. These enclosures include fume hoods, glove box/isolators, and biological safety cabinets. In addition, bag in/bag out filter changeout systems can help protect workers removing filters contaminated with nanomaterials. Specific design and operational considerations include:

• Consulting a qualified industrial ventilation engineer, industrial hygienist, or containment specialist to design the new control system.
• Locating the ventilated enclosure away from doors, aisles, walkways, and room air supply registers.
• Avoiding air currents that are 30% to 50% of the hood face velocity, typically between 30 and 50 feet per minute (fpm) as these tend to reduce containment efficiency. Check for an air face velocity between 80 and 100 fpm into the chemical fume hood. For higher toxicity materials, a higher face velocity (between 100 and120 fpm) may provide better protection. However, face velocities exceeding 150 fpm may not improve performance and may increase hood leakage.
• Newer nanomaterial handling enclosures may provide adequate containment at lower face velocities. These hoods are often based on ventilated balance enclosures used for potent powder handling in pharmaceutical applications and typically operate at an average face velocity between 65 and 85 fpm.
• Ensuring that each enclosure has an airflow indicator to show that it is working properly. This may include a flow indicator, flow alarm, or face velocity alarm that alerts users to improper exhaust flow.
• Keeping exhaust ducts short and simple—avoid flexible duct if possible. The duct material and filters chosen need to be compatible with the nanoproducts and by-products generated.

NIOSH also recommends preventive maintenance and system checks, including:

• Develop a written preventive maintenance plan to check system performance and repair identified deficiencies.
• Keep equipment in effective and efficient working order. Review the enclosure manufacturer’s performance specification to know whether the control is working properly.
• Look for signs of damage to the ducting and enclosure. Repair damage immediately.
• Regularly check that the enclosure system and the airflow indicator work properly and that the enclosure has no visible dust leaks.
• Have a qualified industrial ventilation engineer or industrial hygienist examine the ventilated enclosure system and check its performance at least once every year or if it is modified or relocated.

Reactor Operations

Manufactured nanomaterials are often produced from aerosolized precursor materi­als in a variety of enclosed reactors using methods such as furnace flow (hot wall), laser, plasma/discharge, and flame reactors. Harvesting nanomaterials from reactors results in potentially high exposures. In addition, cleanout of reactors contributes to increased facility concentrations and exposures among operation and maintenance workers. Leak­age from pressurized reactors can also contribute to background concentrations and result in exposure to employees throughout the facility. The following equipment and control measures are recommended in Protecting Workers during Nanomaterial Reactor Operations:

• Small reactors. Laboratory fume hoods, enclosures, and glove boxes can be used when the reactor is small (less than approximately 3 feet (ft) long and 2–3 ft wide), such as in research and development (R&D) operations. Studies have shown that when the reactor is housed in a well-designed and operating fume hood, particle loss to the work environment is low.
• Medium and large reactors. Where production reactors are larger, custom-fabricated enclosures (often constructed from a polycarbonate, transparent thermoplastic material) or vinyl curtains can be used to reduce fugitive emissions. A canopy-type hood can be used to collect materials emitted when the reactor is opened for product harvesting. Side baffles or plastic curtains can be used to improve collection efficiency by minimizing the adverse effects of drafts. The size of the canopy hood (especially depth) should be sufficient to contain reactor emissions while allowing access for the operator.
• Cleaning. Cleanout of reactors may involve manual sweeping, brushing, or scraping to remove waste materials that could lead to emissions of nanoparticles, regardless of method. Wet methods result in generally lower emissions, while dry and energetic methods (like scraping, sanding, and use of an air jet) typically cause higher emissions. Use of local exhaust ventilation and baffles and side shields to enclose the process as much as possible typically reduces exposure.

Downstream Processing

After production, many nanomateri­als are further processed. In Protecting Workers during Intermediate and Downstream Processing of Nanomaterials, NIOSH cites research that found that the amount and type of nanomaterial released into the workplace was largely based on process energy.

“High energy processes, including spraying and machining (e.g., ball milling), were found to release nanoparticles,” states NIOSH. “However, low energy processes, such as packing and bagging nanomaterials during downstream processing tended to release relatively large agglomerates of nanomaterials into the workplace.”

NIOSH recommends that manufacturers and downstream users of nanomaterials develop prevention through design strategies to protect workers (including maintenance personnel) during the production and handling of engineered nanomaterials. Recommendations address several downstream processes, including:

• Ball milling, which uses machines to reduce particle size, mix or blend materials, or change particle shape. The top engineering control is a ventilated enclosure for capturing emissions during the pro­cessing in a larger ball mill.
• Spray drying, in which a mixture of liquid and powder ingredients (slurry) is sprayed within a large sealed tank. Local exhaust ventilation can be used to capture emissions during the collection of a product processed in a spray dryer.
• Extrusion, in which the nanomaterials are added to a thermoplastic using an extrusion process to create a nanocomposite with improved properties. An exhaust hood and local exhaust ventilation system can capture emissions during the loading of nanomaterials/fillers and during nanocomposite discharge from the extruder.

Questions

NIOSH’s poster on controlling health hazards in the workplace includes questions employers can ask before working with nanomaterials in three forms—dry powder, suspended in liquid, and physically bound/encapsulated. As noted, dry powder typically poses the highest risks of exposure, while working with nanomaterials that are physically bound/encapsulated presents the lowest.

• Form. Can you reduce exposure to the nanomaterial by changing its form (for example, putting powder into a solution) or reducing the amount you are using?
• Work activity. Could the work activity cause exposure? Can the work activity be changed to reduce the exposure?
• Engineering controls. Which engineering controls will be effective? What are the key design and operational requirements for the control?
• Administrative controls. Is there a plan for waste management? Is there a plan to respond to a spill and how equipment will be maintained?
• Personal protective equipment (PPE). Will the PPE for nonnanomaterials be effective for the same materials in nano form?

The poster lists avenues for answering these questions, which are described in greater detail in the three guidance documents.