Engineering controls for nanomaterials

Engineering controls are an important set of methods for controlling the health and safety hazards of nanomaterials. Engineering controls protect workers by removing or reducing hazardous conditions such as hazardous dust or excessive noise or by isolating the worker from the hazard. Examples include local exhaust ventilation (LEV) to capture and remove airborne emissions or machine guards to shield the worker. Well-designed engineering controls can be highly effective in protecting workers.  Many engineering controls developed for other industries can be used or adapted for protecting workers from exposure to nanomaterials, and research is ongoing as to what engineering controls are most effective for nanomaterials.

Control measures for nanoparticles, dusts, and other hazards should be implemented within the context of a comprehensive occupational safety and health management system. 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.

They include ventilation and filtering techniques through common laboratory fixtures such as fume hoods, as well as non-ventilation controls such as sticky mats. Many engineering controls developed for other materials can be used or adapted for protecting workers from exposure to nanomaterials, and research is ongoing as to what engineering controls are most effective for nanomaterials.

Background

Engineering controls are the third most effective member of the hierarchy of hazard controls. They are preferred over administrative controls and personal protective equipment, but are less preferred than elimination or substitution of the hazards.

Controlling exposures to occupational hazards is the fundamental method of protecting workers. Traditionally, a hierarchy of controls has been used as a means of determining how to implement feasible and effective controls, which typically include: elimination, substitution, engineering controls, administrative controls and personal protective equipment (PPE).

The idea behind the hierarchy of controls is that the methods at the top of the list are generally more effective in reducing the risk associated with a hazard than those at the bottom. Following the hierarchy normally leads to the implementation of inherently safer systems, ones where the risk of illness or injury has been substantially reduced. The primary means for reducing exposures should be to implement process changes and engineering controls (e.g., LEV). Use of PPE should be the approach of last resort.

Engineering controls are physical changes to the workplace that isolate workers from hazards by containing them in an enclosure, or removing contaminated air from the workplace through ventilation and filtering. They are the third of five levels of the hierarchy of hazard controls, and are used when hazardous substances and processes cannot be eliminated or substituted with less hazardous substitutes.

Well-designed engineering controls are typically passive, in the sense of being independent of worker interactions, which reduces the potential for worker behavior to impact exposure levels. They also ideally do not interfere with productivity and ease of processing for the worker, because otherwise the operator may be motivated to circumvent the controls. The initial cost of engineering controls can be higher than administrative controls or personal protective equipment, but the long-term operating costs are frequently lower and can sometimes provide cost savings in other areas of the process.

The types of engineering controls optimal for each situation is influenced by the quantity and dustiness of the material as well as the duration of the task. For example, stronger engineering controls should be used if dry nanomaterials cannot be substituted with a suspension, or if procedures such as sonication or cutting of a solid matrix containing nanomaterials cannot be eliminated.

Ventilation controls

Ventilation systems are distinguished as being either local or general. Local exhaust ventilation operates at or near the source of contamination, often in conjunction with an enclosure, while general exhaust ventilation operates on an entire room through a building's HVAC system.

Local exhaust ventilation

A fume hood is an engineering control using local exhaust ventilation combined with an enclosure.

Local exhaust ventilation (LEV) is the application of an exhaust system at or near the source of contamination. If properly designed, it will be much more efficient at removing contaminants than dilution ventilation, requiring lower exhaust volumes, less make-up air, and, in many cases, lower costs. By applying exhaust at the source, contaminants are removed before they get into the general work environment.

Examples of local exhaust systems include fume hoods, gloveboxes, biosafety cabinets, and vented balance enclosures. Exhaust hoods lacking an enclosure are less preferable, and laminar flow hoods are not recommended because they direct air outwards towards the worker.

Ventilated Enclosures

Many different types of commercially available enclosures can be employed to reduce exposure during the handling of powder nanomaterials. The controls described below include chemical fume hoods, nanomaterial handling enclosures, biological safety cabinets [BSCs], and glovebox/isolators. In 2006, a survey was conducted of international nanotechnology firms and research laboratories that reported manufacturing, handling, researching, or using nanomaterials. All organizations participating in the survey reported using some type of engineering control. The most common exposure control used was the traditional laboratory fume hood with two-thirds of firms reporting the use of a fume hood to reduce exposure to workers.

Fume hoods should have an average inward velocity of between 80 and 100 feet per minute (fpm) at the face of the hood. For higher toxicity materials, a higher face velocity (between 100–120 fpm) may provide better protection. However, face velocities exceeding 150 fpm may not improve performance and may increase hood leakage. New fume hoods specifically designed for nanotechnology are being developed primarily based on low-turbulence balance enclosures, which were initially developed for the weighing of pharmaceutical powders. These nanomaterial handling enclosures may provide adequate containment at lower face velocities and typically operate at an average face velocity between 65–85 fpm.

Fume hoods should have an average face velocity of 80–120 ft/min, and when used with nanomaterials, air should be passed through a HEPA filter and exhausted outside the work environment, with used filters being handled as hazardous waste. Turbulence can cause nanomaterials to exit the front of the hood, and can be avoided by keeping the sash in the proper position, keeping the interior of the hood uncluttered with equipment, and not making fast movements while working. High face velocities can result in loss of powdered nanomaterials; while as of 2012 there was little research on the effectiveness for low-flow fume hoods, there was evidence that air curtain hoods were effective at containing nanoparticles.

Glove boxes provide a high degree of operator protection but at a cost of limited mobility and size of operation. Gloveboxes are sealed systems, but are more difficult to use and care must be taken in transferring materials into and out of the enclosure. Some gloveboxes are configured to use positive pressure, which can increase the risk of leaks.

Biosafety cabinets are designed to contain bioaerosols, which have a similar size to engineered nanoparticles and should be acceptable, although common biosafety cabinets are more prone to turbulence. As with fume hoods, they should be exhausted outside the facility. Special powder handling enclosures are smaller than fume hoods, and have lower flow rates and thus less turbulence. They are useful for weighing operations, which disturb the nanomaterial and increase its aerosolization.

Dedicated large-scale ventilated enclosures for large pieces of equipment can also be used.

General exhaust ventilation

General exhaust ventilation (GEV), also called dilution ventilation, is different from local exhaust ventilation because instead of capturing emissions at their source and removing them from the air, general exhaust ventilation allows the contaminant to be emitted into the workplace air and then dilutes the concentration of the contaminant to an acceptable level. GEV is inefficient and costly as compared to local exhaust ventilation, and given the lack of established exposure limits for most nanomaterials, they should not be relied upon for controlling exposure, although they can provide negative room pressure to prevent contaminants from exiting the room.The use of supply and exhaust air throughout the facility can provide pressurization schemes that reduce the number of workers exposed to potentially hazardous materials. Production areas should be kept at a negative pressure with respect to nearby areas. Exhaust air volume from the production area should be slightly greater than the volume of supply air.

For general exhaust ventilation in laboratories, a nonrecirculating system should be used with 4–12 air changes per hour when used in tandem with local exhaust ventilation, and sources of contamination should be placed close to the air exhaust and downwind of workers, and away from windows or doors that may cause air drafts.

Control verification

It is important to confirm that the LEV system is operating as designed by regularly measuring exhaust airflows. A standard measurement - hood static pressure - provides important information on the hood performance, because any change in airflow results in a change in hood static pressure. For hoods designed to prevent exposures to hazardous airborne contaminants, the ACGIH Industrial Ventilation: A Manual of Recommended Practice for Operation and Maintenance recommends the installation of a fixed hood static pressure gauge.

In addition to routinely monitoring the hood static pressure, additional system checks should be completed periodically to ensure adequate system performance, including smoke tube testing, hood slot/face velocity measurements, and duct velocity measurements using an anemometer. A dry ice test is another method of evaluation designed to qualitatively determine the containment performance of fume hoods. These system evaluation tasks should become part of a routine preventative maintenance schedule to check system performance. Several control verification techniques can be used to assess room airflow patterns and verify the proper operation of fume hoods. Pitot tubes, hot-wire anemometers, and smoke generators can be used to qualitatively measure air velocity, while tracer-gas leak testing is a quantitative method. Standardized testing and certification procedures such as ANSI Z9.5 and ASHRAE 110 can be used, as can qualitative indicators of proper installation and functionality such as inspection of gaskets and hoses.

Non-ventilation controls

A sticky mat in a nanomaterials production facility. Ideally, other engineering controls should lessen the amount of dust collecting on the floor and being tracked onto the sticky mat, unlike this example.

Nonventilation engineering controls cover a range of control measures (e.g., guards and barricades, material treatment, or additives). Nonventilation controls can be used in conjunction with ventilation measures to provide an enhanced level of protection for nanomaterial workers. Examples include placing equipment that may release nanomaterials in a separate room, or placing walk-off sticky mats at room exits. Antistatic devices can be used when handling nanomaterials to reduce their electrostatic charge, making them less likely to disperse or adhere to clothing. Standard dust control methods such as enclosures for conveyor systems, using a sealed system for bag filling, and water spray application are effective at reducing respirable dust concentrations.

Nonventilation engineering controls can also include devices developed for the pharmaceutical industry, including isolation containment systems. One of the most common flexible isolation systems is glove box containment, which can be used as an enclosure around small-scale powder processes, such as mixing and drying. Rigid glove box isolation units also provide a method for isolating the worker from the process and are often used for medium-scale operations involving transfer of powders. Glove bags are similar to rigid glove boxes, but they are flexible and disposable. They are used for small operations for containment or protection from contamination. Another nonventilation control used in this industry is the continuous liner system, which allows the filling of product containers while enclosing the material in a polypropylene bag. This system is often used for off-loading materials when the powders are to be packed into drums.

External links

• http://www.osha.gov/Publications/safety-health-management-systems.pdf