Executive Summary

The fume hood is the primary safety device in the laboratory, standing as the only line of defense between a chemist and toxic vapors. It is also, ironically, the single most expensive piece of equipment to operate in the entire facility.

A standard ducted fume hood consumes as much energy annually as three residential homes, pumping conditioned air (heated or cooled) straight out of the building. For the Lab Manager, the purchasing decision is a high-stakes balancing act. A Ducted hood offers universal protection against unknowns but taxes the building's HVAC infrastructure. A Ductless hood offers incredible flexibility and energy savings, but requires rigorous chemical management to prevent filter breakthrough.

Buying the wrong hood is dangerous: a Ductless hood cannot handle acid digestion, and a Ducted hood installed without adequate make-up air will back-draft into the user's face.

This guide outlines the physics of airflow, the standards of containment (ASHRAE 110), and the operational realities of filter management to ensure your lab remains safe and solvent.

1. Understanding the Technology Landscape

Fume hoods are categorized by a single fundamental question: where does the contaminated air go? Traditional systems operate on a "single-pass" philosophy, treating the laboratory like a wind tunnel where conditioned air is used once and then exhausted to the atmosphere. Newer sustainable systems treat air as a recyclable asset, scrubbing it of contaminants and returning it to the room. This choice dictates not just the safety profile of the lab, but the entire HVAC architecture of the building.

Core Hood Types

• Constant Air Volume (CAV) Ducted: The traditional standard found in older labs. The exhaust fan runs at a constant speed regardless of the sash position.
  • Mechanism: As the sash is lowered, the face velocity (speed of air) increases dangerously unless a "bypass" grille above the sash opens to bleed in air.
  • Pros: Simple, robust, and has the cheapest capital cost because it lacks complex electronic controls.
  • Cons: Massive energy waster. It pumps expensive conditioned air out of the building at maximum volume 24/7, even when the sash is closed, and no one is working.
• Variable Air Volume (VAV) Ducted: A "smart" hood linked electronically to the building's HVAC system.
  • Mechanism: As you lower the sash, a sensor signals the exhaust valve to throttle down and the supply fan to slow, maintaining a constant safe face velocity (e.g., 100 fpm) while drastically reducing the total volume of air exhausted.
  • Pros: Huge energy savings and a safer, less turbulent airflow for sensitive experiments (like weighing powders).
  • Cons: Higher complexity; requires fast-acting venturi valves and sophisticated periodic maintenance to ensure response times remain safe (typically <3 seconds).
• Ductless (Filtered) Hood: A self-contained unit that recirculates air through a stack of Carbon and HEPA filters.
  • Mechanism: Air is pulled up from the work surface, scrubbed of specific chemicals, and returned safely to the laboratory room.
  • Pros: Zero ductwork required. Can be installed instantly in any room with a standard electrical outlet. Offers massive energy savings since no conditioned air leaves the building.
  • Cons: Chemical specificity. You must strictly match the filter type to the chemical family (e.g., Acid, Solvent, Ammonia). If you use a chemical the filter isn't designed for (e.g., Methanol on standard carbon), it passes right through and back into the user's face.
• Perchloric Acid Hood: A specialized, stainless-steel ducted hood with a built-in water wash-down system.
  • Why: Perchloric acid vapors can settle in ductwork and form shock-sensitive perchlorate salts, which are explosive. The wash-down system is mandatory to flush the ductwork after every use to prevent this accumulation.

2. Critical Evaluation Criteria: The Decision Matrix

The decision to go Ducted or Ductless is rarely a matter of preference; it is strictly defined by your chemical inventory and your facility's available infrastructure. A ducted hood is a "Universal Receiver"—it can safely handle almost any chemical. A ductless hood is a "Selective Receiver"—it requires a known, stable process. Use this matrix to determine feasibility based on your specific application risks.

Decision Track 1: The Chemical Application

• "We do diverse synthesis, acid digestion, or use unknown chemicals." → Ducted (VAV or CAV)
  • Reasoning: Carbon filters cannot trap everything. Light gases (hydrogen, methane), extremely high volumes of acid fumes (digestion), or unknown synthesis byproducts can break through filters instantly. You need the guarantee of physical removal from the building.
  • Estimated Cost: $6,000 – $15,000 (Hood Only) + $20,000+ (HVAC/Ductwork installation).
• "We do routine HPLC prep, histology, or teaching labs." → Ductless (Filtered)
  • Reasoning: The chemical load is known, low-volume, and compatible with filtration. The "chemical passport" of the process is stable, minimizing the risk of filter mismatch.
  • Estimated Cost: $4,000 – $10,000 (Plug-and-play hardware).

Decision Track 2: Infrastructure Limits

• No Make-Up Air Available: A fume hood removes air. That air must be replaced ("make-up air"). If your building HVAC cannot supply more fresh air, adding a ducted hood will depressurize the room, causing doors to slam shut and potentially sucking toxic fumes outof other hoods via the path of least resistance.
  • Solution: Ductless Hood (Neutral air balance; it recycles the room air).

3. Key Evaluation Pillars

Once the exhaust path is chosen, the specific engineering features determine the hood's safety and usability. A hood is not just a metal box with a fan; it is a precision aerodynamic machine designed to capture a plume of heavy vapor moving against the natural turbulence of a busy laboratory.

A. Face Velocity & Containment (ASHRAE 110)

Face velocity is the speed of the air entering the sash opening, creating a "curtain" of protection.

• The Standard: 100 feet per minute (fpm) is the historic industrial hygiene standard.
• High-Efficiency (Green) Hoods: Modern hoods are designed with advanced aerodynamics to operate safely at 60 fpm. This 40% reduction in air volume saves massive amounts of energy but requires perfect baffle design and airfoils to prevent leakage. Note: Never simply turn a standard hood down to 60 fpm; it will leak.

B. Sash Configuration

• Vertical Sash: The standard single pane that moves up and down. Easiest for loading large equipment, but requires the user to lift the entire sash to work.
• Horizontal Sash: Sliding glass panels on a track. Offers better protection (the glass acts as a physical shield between the user's face and the reaction) and saves energy (smaller opening area), but can create a barrier to lateral arm movement.
• Combination Sash: Can move up/down AND slide left/right. The most flexible and expensive option, favored by research labs.

C. Liner Material

The inside walls of the hood must survive your specific chemistry for 20 years.

• Phenolic Resin: Good general-purpose liner. White, easy to see, moderate heat and acid resistance.
• Stainless Steel: Required for radiochemistry (easy to decon/scrub) and sterile applications. Avoid strong acids like HCl, which will pit the steel.
• Fiberglass / Poly: Excellent acid resistance (even HF) and seamless corners (easy cleaning), but surfaces can stain over time.

4. The Hidden Costs: Total Cost of Ownership (TCO)

For fume hoods, the purchase price is irrelevant compared to the operating cost.

5. Key Questions to Ask Vendors

1. "Has this model passed the ASHRAE 110 containment test at the factory or As Manufactured?" (This measures actual leakage of tracer gas. Do not buy a hood without seeing this data.)

2. "For Ductless: What is the 'Retention Capacity' for my specific chemicals?" (Ask for a chemical assessment. "It traps solvents" is not enough. You need to know: "It traps 500g of Acetone before saturation".)

3. "Does the sash have a 'Stop' or 'Lock' at 18 inches?" (Safety feature. It reminds users not to open the sash fully during operation, which would destroy containment.)

4. "Is the work surface dished?" (Spill containment. A flat surface lets spills run onto the user's lap. A dished epoxy surface keeps the spill inside the hood.)

6. FAQ: Quick Reference for Decision Makers

Q: Can I store chemicals in the fume hood?

A: NO. Storing bottles blocks the rear baffles, disrupting the airflow and causing turbulence (eddies) that can roll fumes out into the room. Use a dedicated Vented Acid Cabinet under the hood for storage.

Q: What is a "Walk-In" hood?

A: A floor-mounted hood designed to hold large equipment racks or 55-gallon drums. Note: The bottom section usually has no airfoil, so they are not as safe for table-top work.

Q: How do I know if the Ductless filter is saturated?

A: Modern units have PID sensors that detect breakthrough and sound an alarm. However, manual "sniffer" tubes or colorimetric badges are often used as a backup safety check.

7. Emerging Trends to Watch

• Automatic Sash Closers: Human behavior is the biggest variable in hood efficiency. Studies show that users leave sashes open 70% of the time they are away from the bench. Automatic sash closers use presence sensors to detect when the user walks away, gently lowering the sash after a set delay. This simple upgrade converts a VAV hood from an energy hog into an efficient device, often paying for itself in under two years through reduced HVAC load.
• Green / High-Performance Low-Flow Hoods: Traditional hoods require 100 fpm face velocity to ensure safety. New "High-Performance" designs utilize advanced aerodynamics—including aerodynamic sills, deeper rear baffles, and vortex-suppressing posts—to maintain robust containment at just 60 fpm. This 40% reduction in exhaust volume (CFM) allows facilities to downsize their roof blowers and chillers, becoming a standard specification for LEED-certified laboratory buildings.
• Smart Hoods & Building Connectivity: Fume hoods are entering the IoT era. Modern units connect directly to the Building Management System (BMS), providing real-time data on sash position, face velocity, and energy consumption. This allows Facility Managers to move from reactive maintenance (fixing a broken fan) to proactive management, receiving alerts if a sash is left open overnight or if a specific lab zone is consuming unusual amounts of energy.

Conclusion: The choice of a fume hood is a choice between infrastructure investment and operational discipline. A Ducted VAV hood offers the ultimate peace of mind for diverse chemistry but requires a heavy HVAC commitment. A Ductless hood offers agility and low energy costs but demands strict adherence to chemical limits. By prioritizing face velocity, sash management, and liner compatibility, Lab Managers can ensure their facility protects its most valuable asset: its people.