Recommended Practices


Fume Exhaust Hoods


With serious consideration to the Occupational Safety and Health, removal of toxic fumes and gases from the laboratory has become very important. We try to provide useful information on the selection and operation of hoods and exhaust systems which can be used in a laboratory.

The accepted method for containing and removing fumes, odors, and other contaminations emitted from laboratory or chemical testing procedures is to restrict the operations within an enclosed cabinet. Ideally, the best procedure is not to emit, but the next best is to remove or exhaust directly and as close to the point of origin as possible to protect personnel and property. Fume exhaust systems, using properly designed hoods, where room air is drawn across the hood face to capture and remove the contaminations can be very effective.

Compromising Fume Hood Usage

Fume hood efficiency depends on the amount of air exhausted and on the hood design. To ensure flexibility of operation and maximum safety to personnel, a fume hood should be designed to exhaust air at rates ample for complete removal of all contaminants.

Capture Velocity

Condition Hood face velocity
Very low toxic level materials; noxious odors, nuisance dusts, fumes 80 FPM
General lab use. Corrosive materials, moderate toxicity level materials
tracer quantities of radioisotopes. TLV of 10=1000 PPM
100 FPM
Higher toxic level materials. TLV less than 10 PPM 125 to 150 FPM
Pathogenic micro organisms, very high toxic materials. TLV less than 0.01 PPM Enclosed GLOVE BOX should be used

Fume Hood Design - Some Basic Points

It should always be remembered that the purpose of a hood exhaust system is to protect laboratory personnel from exposure. Thus, the heart of the system is the hood and its exhaust devices. Nearly all hood designs currently in use attempt to provide protection to personnel in three ways: via (1) a mechanical shield, (2) direction of air movement, and (3) dilution of contaminants by mixing with large volumes of air inside the hood.

First the hood sash serves as the mechanical shield

It should be in raised position to intake a large volume of air during any chemical process. Should avoid lowering the sash to a very low level, which may cause high speed air draft and it may cause adverse effect on your chemical process if a burner is used inside. Also the sash of of good quality material such as tempered glass may reduce the impact on the operator incase of an accident.

Secondly protection is provided by the direction of air flow

Across the back of the worker and directly into the hood, then passing the equipment within the hood, and finally into the exhaust system. Means, there is no back draft of air, so as the operator protected as there is no contaminated air at hid breathing zone

Thirdly by supplying large amounts of air

Through the hood, dilution of the contaminated air takes place readily, thereby further reducing the hazard of breathing hood air. There are seven basic hood designs currently in use. Air Exhausted from Hoods should never be re-circulated.

Makeup air to be supplied inside the laboratory to balance the air that is being exhausted is the most essential design feature of any hood exhaust system or the laboratory design.

Material of Construction –An overview

Although basic fume hood design has changed very little, many advances have been made in the materials. From which hoods are constructed. Here are some of the basic materials and their distinctive features.

Wood Generally poor chemical resistance In expensive to fabricate and modify in the field. Can present a fire hazard in applications involving heat and flame Poor light reflectivity causes a dark interior.
Sheet metal (CRCA) Requires secondary treatment for chemical resistance. Demand extreme care to avoid damaging the coating. Since corrosion can occur in damaged areas "Oil cleaning" due to light gauge metal causes noise in operation Usually heavy and cumbersome to install. Relatively Expensive.
Fibre glass Excellent chemical resistance Light weight for ease of installation or relocation Easily modified in the field with readily available tools. Sound dampening because of physical construction. Some grades cause fire hazards and not chemically inert. Better Optical properties (Good reflectivity).
Stainless Steel Better chemical resistance than CRCA. Not well suited too many acid applications. Generally provided in type 316 for specific applications to which it is well suited, such as perchloric acid. Heavy construction Expensive. Difficult to modify in the field excellent fire resistance Excellent Fire resistance.
Polyvinyl chloride Excellent chemical resistance except for some solvents Good fire-retardant properties. Particularly well suited to acid digestion application using such acids as sulfuric or hydrofluoric acid. Easy Modification. Not available in Molded configurations. Expensive. Not suitable for direct heat applications.
Polypropylene Excellent chemical resistance Good fire-retardant properties. Particularly well suited to acid digestion application using such acids as sulfuric or hydrofluoric acid. Easy Modification. Not available in Molded configurations. Expensive. Not suitable for direct heat applications.

Exhaust System

Roof Fan: Since the exhaust system is under negative pressure, any air leakage will be drawn into the system, thus confining contamination. The best location for an exhaust fan serving hoods is on the roof, then all exhaust ductwork will be on the suction side of the fan and will be indoors. But this is not always possible. If the fan location must be indoors, for example, above the hood, then careful attention must be paid to duct tightness on the discharge side. When flammable material is handled, mounting the fan on the roof is highly advantageous because an explosion- proof fan motor may not be required. However, the fan wheel should be non-ferrous and the inside casing should be protected (for example by use of an epoxy coating) against corrosive attack.

Exhaust duct material should preferably be of non-corrosive materials such as: galvanized iron, stainless steel, PVC or FRP. Depending upon the corrosive characteristic of the emission gas the duct material to be selected

Seven basic hood designs
  • Conventional Hood
  • Conventional Hood with Reduced Face Velocity
  • Conventional Hood with Use Factor
  • Internally Supplied Hood
  • Externally Supplied Hood
  • Perforated Ceiling Supply Hood
  • Horizontal Sliding Sash Hood
Laboratory Safety Guidelines

Hoods: Equipment in use should be completely enclosed in a hood with adequate space allowed for experimental procedures. When the apparatus is too large to be housed in a hood and there is no possibility of toxic or flammable materials being released, anchored shields of safety or wired glass should surround the equipment. Hoods are not designed to be used as storage areas; remove unused equipment and chemicals and store them in their proper places.

Emergency Equipment and Procedures: Well-equipped chemical laboratories have eyewash fountains, deluge safety showers, fire blankets, fire extinguishers and emergency exists. This equipment should be tested periodically. In addition, being familiar with the locations and uses of the equipment may save needed time during an emergency

Personal Protection: Rubber aprons, asbestos gloves, safety glasses, full face shields and approval respirators must be available to protect personnel from spills, burns, spattering chemicals, flying fragments, and irritating fumes. In addition, the laminated safety glass doors on chemical fume hoods protect personnel from mishaps in the hood.

Health Monitoring: When biological agents or carcinogens are used in the laboratory, special medical control programs are necessary to monitor the workers health. If radioactive materials or radiation-producing equipment such as an X-ray defraction unit are used, dosimeters or film badges should be worn to monitor exposures.

Labeling: Chemicals must be prominently and accurately labeled. When a small quantity of material is removed from a large storage container, immediately affix label on the smaller container. Containers for hazardous chemicals should have precautions such as "Poison" or "flammable" indicated directly below the label. After having completed working with a particular material, return the container to storage or dispose of the material. Nothing should be left in open containers.

Eating, Drinking and Smoking: Food, beverages, cigarettes, pipes, and cigars should not be permitted in the chemical laboratory under any circumstances. Chemical glassware should never be used to hold food.

Using pipettes: Never pipette toxic, corrosive, or radioactive chemicals by mouth; always use a rubber bulb or syringe.

Glassware: Cracked or chipped glassware should be discarded to prevent cuts or scratches which can cause further complications if chemicals contact the injury. Always place a towel or cloth over glass tubing being cut or broken, and fire polish sharp ends. When inserting a rod or piece of glass tubing through a perforated stopper, wrap a towel around your hand for protection.

Waste Disposal: Disposal of hazardous waste materials requires special handling: Place all broken glass in specially marked metal containers, never in wasted baskets or containers used for paper or rags. Waste baskets or containers used for paper or rags. Flush dilute acids and alkalis down the drain with large quantities of water. Never pour flammable liquids not miscible with water, compounds that give off toxic vapors, or corrosive materials down the drain, special disposal containers are needed for each of these wastes.

Storage: An efficient placed storage room is essential for safety. Chemical storage rooms should be equipped with fire doors, safety lights, fire extinguishers, as well as good ventilation and sprinkler systems. Remember careful grouping of liquid reagents to prevent hazardous combinations which may produce fumes, fire, or explosion. Remember to segregate incompatible materials. Remember to keep volatile liquids away from ignition sources such as heat, flames, or electric sparks / switches. Remember to store all solvents in safety cans. Remember to store and frequently vent drummed chemicals according to the supplier's instructions. Remember to secure compressed gas cylinders. Remember to replace valve caps when not in use. Smaller laboratories that don't have separate storage rooms should have noncombustible storage cabinets. Large quantities of flammable solvents should be placed outside in ventilated, noncombustible buildings.

Housekeeping: Good housekeeping is essential for safe laboratory operation. All passages, exits, safety showers, fire extinguishers, electrical controls, and stairways must be kept clear of equipment and obstructions. Remove unused equipment or chemicals from work spaces. Clean up spilled chemicals immediately to prevent dangerous chemical combinations, burns, or spills and falls.