It is the first responsibility and discretion of the scientific investigator to establish a protective layer between him or herself and chemical hazards using the appropriate personal protective equipment (PPE). However, the laboratory fume hood provides a more robust and quantitative safety, mechanical, and atmospheric barrier against toxic vapors, powders, and spills. Instead of taking for granted the fume hood—seemingly resident in the laboratory since antiquity—one should care deeply about the hood, and think of it as an adaptable partner in hazard mitigation. As such, there are important individual components that must function properly and consistently as parts of a whole, and that can be exchanged or upgraded to restore or improve performance. There are also elements of hood design and construction specifically tailored to different chemical workflows that vary in purpose and scale.
Investigators and lab managers should plan on upgrading or replacing hood systems when it becomes clear that they will have to accommodate laboratory space or personnel changes. Hood system replacements and overhauls are especially requisite when the nature of overall application shifts to a new type and degree of chemical workflow.
According to Kasey Fulmer, a fume hood product specialist at Labconco, “The building-specific chemical hygiene plan will outline standard operating procedures…and whether they can be adapted to accommodate new fume hood equipment, especially if it requires changes in airflow.” At the level of individual components, the most common impetus for replacement is “misapplication of materials or of the hood itself, such as when exhaust is shut off while corrosive material is still there, or materials are used that are incompatible with the hood or blower.”
The work cabinet and surface
Primarily, a fume hood is somewhat of an elevated box, albeit with some caveats. Perhaps the most important caveat is that when the sash is raised (or widened), the work cabinet must be able to maintain a safe perpendicular air speed, or face velocity, at the imaginary plane that marks the interface between worker and working surface. Face velocity correlates to the volumetric flow of air that must be employed inside the hood to contain vapors before they are ducted and exhausted. Although face velocities at 50 percent sash height often exceed 100 fpm, some contemporary high-performance cabinets can now safely run with face velocities as low as 40 fpm, translating to great energy and cost savings by minimizing volumetric flow.
In addition to improvements in airflow, performance, and environmental impact, work cabinet surfaces have evolved to meet and exceed rigorous measures against corrosion and degradation, and to enable clean-up or wash-down. Standard cabinet surfaces are often composed of seamless fiberglass construction to guard against the collection of powders and entry of corrosive vapors into hard-to-clean crevices. Additionally, mobile inserts molded of composite or epoxy resin can aid containment and clean-up.
Modifications for different chemical workflows
Specific chemical workflows have propelled design changes in cabinet size, shape, materials, and integrated equipment. The most familiar and ubiquitous designs are the general purpose standing or benchtop ducted hood, which exhausts vapors outside through building HVAC; and the portable filtered hood, with efficient molecular filters that release clean air back into the laboratory. There are several other specialized types, however, each with their own associated materials and components. Distillation and floor-mounted hoods, common in pharmaceutical formulation assays, contain tall interior cabinet spaces for large equipment, often with walk-in capacity, although the user should always stay out. Acid digestion hoods are often employed by investigators to identify elemental compositions of alloys, and their cabinets usually contain polyvinyl chloride liners instead of fiberglass or steel, to avoid acid etching. Radioisotope hoods contain built-in lead shielding, and seamless corners to allow for thorough reagent clean-up and appropriate disposal. There is, finally, a specific hood design just for work with perchloric acid, which attenuates sparking and friction to mitigate explosion risks, and contains acid-resistant liners, with an integrated washdown system.
How do you know whether the working face velocity is adequate to sequester vapors away from you? The first line of defense is to strictly adhere to regular re-certification paradigms scheduled through chemical safety officers. This will ensure that the hood performs to its standards, and extend its longevity. For daily use, however, an external monitor is an integral element in a new hood system, or a key upgrade in an existing system. Contemporary monitoring devices can function via sidewall sensors that measure flow-through of clean ambient air, and inline or probe sensors that detect velocity within ducting for exhaust air leaving the hood. Alarms notify users when face or duct velocities are compromised, via visual, auditory, and remote app-based notifications. Parameters and thresholds can be changed via touchscreens sensitive to gloved fingers, and performance data can be recorded and analyzed in compliance both with ANSI/NFPA standards, and with GLP/GMP guidelines.
Blowers and ducting
Similar to the face velocity at the plane of the sash, a safe hood must maintain a threshold duct velocity to adequately move vapors out of the hood and propel them through building systems to the outside. This necessitates a symbiosis between hood capacity, blower size, and ducting layout to optimize the airflow appropriate for the size and type of hood. A weak blower exhausting a large cabinet will not sufficiently expel vapors, while a powerful blower can potentially overwhelm the duct system and create turbulence within the cabinet airflow, compromising both face and duct velocities.
A combination of duct diameter, number, and angle of elbow bends, along with cabinet size and working sash height, conspire to determine the appropriate blower size. Additionally, blower construction material should be assiduously partnered to chemical workflow, in concert with standards established by the Air Movement and Control Association. To protect against combustion, blowers often employ non-sparking polypropylene impellers and explosion-proof, fan-cooled enclosed motors. Acidic workflows, and laboratories near coastal areas with high ambient salt concentrations often necessitate corrosion-resistant blower materials such as PVC or fiberglass reinforced plastic.
In all possible iterations of fume hoods and associated componentry, the litany of codes and standards that independently govern each part can be overwhelming. However, as Fulmer explains, the ANSI Z9.5 Standard for Laboratory Ventilation, which is comprehensive, is however “superseded by any local building codes.” Depending on the needs of the investigator, and the capacity of the building, the appropriate fume hood option is always available.