Current Strategies for Engineering Controls in Nanomaterial Production and Downstream Handling Processes/Introduction

by the National Institute of Occupational Safety and Health

CHAPTER 1

Introduction

The number of commercial applications of nanomaterials is growing at a tremendous rate. As this rapid growth continues, it is essential that producers and users of nanomaterials ensure a safe and healthy work environment for employees who may be exposed to these materials. Unfortunately, because nanotechnology is so new, we do not know or fully understand how occupational exposures to these agents may affect the health and safety of workers or even what levels of exposure may be acceptable. Given our current knowledge in this field, it is important to take precautions to minimize exposures and protect safety and health.

This document discusses approaches and strategies to protect workers from potentially harmful exposures during nanomaterial manufacturing, use, and handling processes. Its purpose is to provide the best available current knowledge of how workers may be exposed and provide guidance on exposure control and evaluation. It is intended to be used as a reference by plant managers and owners who are responsible for making decisions regarding capital allocations, as well as health and safety professionals, engineers, and industrial hygienists who are specifically charged with protecting worker health in this new and growing field. Because little has been published on exposure controls in the production and use of nanomaterials, this document focuses on applications that have relevance to the field of nanotechnology and on engineering control technologies currently used, and known to be effective, in other industries. This document also addresses other approaches to worker protection, such as the use of administrative controls and personal protective equipment.

1.1 Background

Nanotechnology is the manipulation of matter at the atomic scale to create materials, devices, or systems with new properties and/or functions. Around the world, the introduction of nanotechnology promises great societal benefits across many economic sectors: energy, healthcare, industry, communications, agriculture, consumer products, and others [Sellers et al. 2009].

Some nanoparticles are natural, as in sea salt or pine tree pollen, or are incidentally produced, as in volcanic explosions or diesel engine emissions. The focus of this document is engineered nanomaterials, those materials deliberately engineered and manufactured to have certain properties and have at least one primary dimension of less than 100 nanometers (nm). Nanomaterials have properties different from those of their bulk components. For example, many of these materials have increased strength/weight ratios, enhanced conductivities, and improved optical or magnetic properties. These new properties make nanomaterial development so exciting and are the reason they hold the promise of great economic potential.

Nanomaterials are often classified by their physicochemical characteristics or structure. The four classes of materials of which nanoparticles are typically composed include elemental carbon, carbon compounds, metals or metal oxides, and ceramics. The nanometer form of metals, such as gold, and metal oxides, such as titanium dioxide, are the most common engineered nanomaterials being produced and used [Sellers et al. 2009]. Nano-sized silica, silver, and natural clays are also common materials in use. The carbon nanotube is a unique nanomaterial being investigated for a wide range of applications. These tubes are cylinders constructed of rolled-up graphene sheets. Another interesting carbon structure is a fullerene (also known as a Bucky Ball). These are spherical particles usually constructed from 60 carbon atoms arranged as 20 hexagons and 12 pentagons. As shown in Figure 1, the structure resembles a geodesic dome (designed by architect Buckminster Fuller, hence the name). Nanomaterials are widely used across industries and products, and they may be present in many forms.

Significant international health and safety research and guidance concerning the handling of nanomaterials is underway to support risk management of commercial developments. Both risks and rewards are inherent in these new materials. Scientists around the world are conducting toxicological studies on these nanomaterials, and initial findings are concerning. Animals exposed to titanium dioxide (TiO₂) and carbon nanotubes (CNTs) have displayed pulmonary inflammation [Chou et al. 2008; Rossi et al. 2010; Shvedova et al. 2005]. Other studies have shown that nanoparticles can translocate to the circulatory system and to the brain and cause oxidative stress [Elder et al. 2006; Wang et al. 2008]. Perhaps the most troubling finding is that CNTs can cause asbestos-like pathology in mice [Poland et al. 2008; Takagi et al. 2008].

Figure 1. Atomic structure of a spherical fullerene

1.2 Industry Overview

In March 2006, the Woodrow Wilson International Center for Scholars created an inventory of 212 consumer products or product lines that incorporate nanomaterials (http://www.nanotechproject.org/inventories /consumer/analysis_draft/). These products were broken down into eight categories using a publically available consumer product classification system. As of March 2011, the number of consumer products has increased by 521% (212 to 1,317 nano-enabled products) with products coming from more than 24 nations [WWICS 2011]. These products include acne lotions, antimicrobial treatment for socks, sunscreens, food supplements, components for computer hardware (such as processors and video cards), appliance components, coatings, and hockey sticks. Of the current 1,317 nano-enabled products, the largest product category with 738 products was health and fitness. The most common type of nanomaterial used in these products was silver (313 products), followed by carbon (91 products) and titanium dioxide (59 products).

Roco [2005] reports that worldwide, the investment in nanotechnology has increased from $432 million in 1997 to about $4.1 billion in 2005. In this same time period, the U.S. government investment in nanotechnology has increased to nearly $1.1 billion. Estimates made in 2000 suggested that $1 trillion in products will use nanotechnology in some way by 2015. The National Science Foundation estimates that the number of workers in this industry will increase to 2 million worldwide by 2015.

Currently, most production facilities are relatively small, with lab, bench, or, at most, pilot plant operations [Genaidy et al. 2009]. This is also indicative of downstream users (applications and product development). As new manufacturing processes and technologies are developed and introduced, novel materials with unknown toxicological properties will require effective risk management approaches. As more of these products enter the market, concern about the health and safety of the workers grows.

1.3 Occupational Safety and Health Management Systems

Control measures for nanoparticles, dusts, and other hazards should be implemented within the context of a comprehensive occupational safety and health management system [ANSI/AIHA 2012]. The critical elements of an effective occupational safety and health management system include management commitment and employee involvement, worksite analysis, hazard prevention and control, and sufficient training for employees, supervisors, and managers (www.osha.gov/Publications/safety-health-management-systems.pdf). In developing measures to control occupational exposure to nanomaterials, it is important to remember that processing and manufacturing involve a wide range of hazards. Conducting a preliminary hazard assessment (PHA) encompasses a qualitative life cycle analysis of an entire operation, appropriate to the stage of development:

• Chemicals/materials being used in the process
• Production methods used during each stage of production
• Process equipment and engineering controls employed
• Worker’s approach to performing job duties
• Exposure potential to the nanomaterials from the task/operations
• The facility that houses the operation

The steps taken to perform PHAs for specific operations should be documented to let others know what was done and to help others understand what works. PHAs are frequently conducted as initial risk assessments to determine whether more sophisticated analytical methods are needed and to prepare an inventory of hazards and control measures needed for these hazards. One or two individuals with a health and safety background and knowledge of the process can perform PHAs. As part of the assessment, the health and safety professional should evaluate the magnitude of the emissions (or potential emissions) and the effects of exposure to these emissions. PHAs are an important first step toward developing control measures that can be considered during the planning stage. Essentially, hazard control should be an integral component of facility, process, and equipment design and construction. This includes design for inherent process safety. The use of engineering controls should be considered as part of a comprehensive control strategy for hazards associated with processes/ tasks that cannot be effectively eliminated, substituted for, or contained through process equipment modifications.

The standards for an occupational health and safety management system, as outlined in ANSI/AIHA Z10 [ANSI/AIHA 2012] and BSI 18001 [BSI 2007c], promote a continuous improvement cycle (plan, do, check, act), which does not have an exit point and is the basis for worksite analysis. Figure 2 illustrates how control measures are incorporated into an occupational safety and health management system. The continuous improvement loop is applicable to all hazards in a process/facility (e.g., airborne contaminant exposures, ergonomic, combustible dusts, fire safety, and physical hazards). The hazard assessment should be reviewed during each cycle described by Figure 2 and periodically updated when major changes occur. Although the optimal time to undertake a PHA is during the design stage, hazard assessments can also be done during the operation of a facility and have the benefit of using existing data.

After the PHA is complete, the nanomaterial risk management plan is designed to avoid or minimize hazards discovered during the assessment. The following options should be considered:

• Automated product transfer between operations. A process that allows for continuous process flow to avoid exposures caused by workers handling powdered or vaporous materials.

• Closed-system handling of powdered or vaporous materials, such as screw feeding or pneumatic conveying.
• Local exhaust ventilation. Steps should be taken to avoid having positive pressure ducts in work spaces because leakage from ducts can cause exposures. Ducts or pipes should be connected using flanges with gaskets that prevent leakage.
• Continuous bagging for the intermediate output from various processes and for final products. A process discharges material into a continuous bag that is sealed to eliminate dust exposures caused by powder handling. Bags are heat sealed after loading.
• Minimizing the container size for manual material handling. Minimizing the size of the container or using a long-handled tool is recommended so that the worker does not place his breathing zone inside the container (as shown in Figure 3). NIOSH recommends a maximum container depth of 25 inches [NIOSH 1997]. If large containers are required, engineering controls to provide a barrier between the container and the breathing zone of the worker are recommended.

Figure 2. How control measures are selected, implemented, and managed into an occupational safety and health management system. (adopted from [ANSI 2005])

Photo by NIOSH

Figure 3. Worker reaching into drum

Many good resources are available on the occupational safety and health risk management of nanomaterials. Comprehensive documents have been produced by a number of organizations. Some of these are listed in Appendix A.

1.3.1 Prevention through Design (PtD)

The concept of Prevention through Design (PtD) is to design out or minimize hazards, preferably, early in the design process. PtD is also called inherent or intrinsic safety, safety by design, design for safety, and safe design. When PtD is implemented, the control hierarchy is applied by designers (e.g., engineers, architects, industrial designers) and business leaders (e.g., owners, purchasers, managers) who consider the benefits of designing safety into things external to the worker to prevent work-related injuries and illnesses.

PtD strategies, like the hierarchy of controls, can take many forms. Elimination and substitution measures are desirable, but these strategies may be difficult to implement when working with nanomaterials because these materials are likely being used for their unique properties. The pharmaceutical industry has addressed some of these challenges since their products must be contained rather than removed or eliminated from the process. They have adopted a containment hierarchy of controls that addresses designing inherent safety into the process [Brock 2009]. The initial levels of containment include elimination and substitution as well as product, process, and equipment modifications. Only after efforts have been made to design the process to reduce potential emissions sources should engineering controls be considered.

Other PtD strategies can be considered:

• Limiting process inventories by producing the nanomaterials as they are consumed in the process.
• Operating a process at a lower energy state (e.g., lower temperature or pressure), which typically results in lower fugitive emissions and therefore safer operation.
• Using fail-safe devices where possible. Fail-safe devices are designed so that if they fail, the system reverts to a safer condition. An example of a fail-safe device is a valve controlling a reagent for a reaction. If the safe condition for the system is for this valve to be closed, the fail-safe valve would automatically close in the event of a failure.
• Installing a closed transport system to eliminate worker exposures during transport activities.

PtD strategies typically do not include administrative controls and personal protective equipment (PPE) as the primary controls. These measures require worker interaction with the process or active steps to limit the extent of the hazard. Most effective PtD approaches reduce or eliminate hazardous conditions without relying on input from workers. Humans are generally recognized as being much less reliable than most machines, particularly in emergencies [Kletz 2001]. The use of administrative controls and PPE in PtD strategies is generally for redundancy—further safeguards should the primary control fail. The ideal time to develop a PtD strategy is during the development phase of a process, material, or facility. As the nanotechnology field is still in its relative infancy, there are numerous opportunities to implement PtD in the early stages. The manner in which these materials are handled and processed can largely affect the overall safety of the process, and the health and safety of workers may be significantly improved through the implementation of a PtD strategy.

1.3.2 OELs as Applied to Nanotechnology

Occupational exposure limits (OELs) are useful in reducing work-related health risks by providing a quantitative guideline and basis to assess the worker exposure potential and the performance of engineering controls and other risk management approaches. Currently, no regulatory standards for nanomaterials have been established in the United States. However, NIOSH has recently published two current intelligence bulletins (CIBs) regarding occupational exposures to nanomaterials. In a CIB on titanium dioxide (TiO2), NIOSH recommends exposure limits of 2.4 mg/m3 for fine TiO2 and 0.3 mg/m3 for ultrafine (including engineered nanoscale) TiO2, as time-weighted average (TWA) concentrations for up to 10 hours per day during a 40-hour work week [NIOSH 2011]. In a CIB on carbon nanotubes and nanofibers, NIOSH recommends that worker exposure be limited to no more than 1 µg/m3 [NIOSH 2013].

Other countries have established OELs for various nanomaterials. For example, the British Standards Institute recommends working exposure limits for nanomaterials based on various classifications such as solubility, shape, and potential health concerns as related to larger particles of the same substance [BSI 2007b]. Germany’s Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung, an institute for worker safety, has published similar guidelines [IFA 2009].

In the absence of governmental or consensus guidance on exposure limits, some manufacturers have developed suggested OELs for their products. For example, Bayer has established an OEL of 0.05 mg/m3 for Baytubes® (multiwalled CNTs) [Bayer MaterialScience 2010]. For Nanocyl CNTs, the no-effect concentration in air was estimated to be 2.5 μg/m³ for an 8-hr/day exposure [Nanocyl 2009].

Another approach that may be taken when OELs are absent is the ALARA concept, As Low As Reasonably Achievable. While ALARA is generally the goal for all occupational exposures, this concept is particularly useful when OELs are absent or in the case of contaminants with unknown toxicity.

1.3.3 Control Banding

Control banding is a qualitative risk characterization and management strategy, intended to protect the safety and health of workers in the absence of chemical and workplace standards. Control banding groups workplace risks into hazard bands based on evaluations of hazard and exposure information [NIOSH 2009b]. Note that control banding is not intended to be a substitute for OELs and does not alleviate the need for environmental monitoring or industrial hygiene expertise.

To determine the appropriate control scheme, one should consider the characteristics of the substance, the potential for exposure, and the hazard associated with the substance. Four main control bands, based on an overall risk level, have been developed:

• Good industrial hygiene (IH) practice, general ventilation, and good work practices

• Engineering controls including fume hoods or local exhaust ventilation

• Containment or process enclosure allowing for limited breaks in containment

• Special circumstances requiring expert advice

One basic principle of control banding is the need for a method that will return consistent, accurate results even when performed by nonexperts. Other requirements include having a user friendly strategy, readily available required information (e.g., material safety data sheet [MSDS]), practical guidance on applying the strategy, and worker confidence in the results. With the absence of OELs, control banding can be a useful approach in the risk management of nanomaterials [Maynard 2007; Schulte et al. 2008; Thomas et al. 2006; Warheit et al. 2007]. Several control banding tools are available for use with engineered nanomaterials. The CB Nanotool, for example, bases the control band for a particular task on the overall risk level, which is determined by a matrix that uses severity scores and probability scores [Paik et al. 2008]. The severity score is based on the toxicological effects of the nanomaterial, while the probability score relates to the potential for employee exposure. The health hazard categories for some control banding approaches are based upon the European Union risk phrases, while exposure potentials include the volume of the chemical used and the likelihood of airborne materials, estimated by the dustiness or volatility of the source compound [Maidment 1998].