Engineering controls for nanomaterials

Engineering controls are an important way to control the heath and safety hazards of nanomaterials. They include ventilation and filtering techniques through common laboratory fixtures such as fume hoods, as well as non-ventilation controls such as sticky mats. Research is ongoing as to what engineering controls are most effective for protecting workers from exposure to nanomaterials.

Background

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 used when hazardous substances and processes cannot be eliminated or replaced 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.

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.

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.

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

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. General exhaust ventilation 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.

Control verification

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.

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.