The best way to prevent or reduce exposure to airborne substances is to control their escape into the work environment by the use of laboratory hoods and other ventilation systems.
The two basic types of laboratory ventilation are general dilution and local exhaust ventilation.
A. General Dilution Ventilation (GDV)
General dilution ventilation refers to the quantity and quality of air supplied, for example, exchanging indoor room air with outdoor air. Laboratory air should be replaced continuously, approximately 6 to 12 air changes per hour, so that the concentration of air contaminants are continuously diluted.
General dilution ventilation should NOT BE RELIED ON FOR PROTECTION FROM TOXIC SUBSTANCES RELEASED INTO THE LABORATORY. It provides only modest protection against toxic gases, vapors, aerosols, and dusts. It is an inefficient way to control highly toxic contaminants because of the amount of air exchange necessary to achieve dilute concentrations within acceptable ranges.
Laboratory air should not be recycled. GDV is intended to increase the comfort of the laboratory environment and to serve as a source of air-flow through the ventilation system and through dedicated systems, such as fumehoods.
Typical uses for GDV include:
- Heating, cooling, and humidity control.
- Dilution of products of respiration, pathogens and odors caused by normal human occupancy.
- Dilution of low levels of slightly toxic gases or solvent vapors.
- Dilution of combustible vapors to concentrations below the lower explosive limits.
B. Local Exhaust Ventilation (LEV)
Local Exhaust Ventilation is a system designed to exhaust contaminants captured near their source without allowing them to escape and disperse into the laboratory atmosphere. Laboratory hoods use LEV to prevent harmful dusts, mists, fumes as well as toxic gases and vapors from entering the laboratory.
Laboratory hoods offer other types of protection as well. A chemical reaction system located within a hood, with the hood sash correctly lowered, places a physical barrier between the worker and the chemical reaction system. This physical barrier will provide protection from hazards such as chemical splashes, spills, sprays, fires and minor explosions from an uncontrolled reaction.
C. Other Local Ventilation Devices
Ventilated storage cabinets, canopy hoods, snorkels and other such devices should be provided as needed. Each canopy hood and snorkel should have a separate exhaust duct.
- Ductless Hoods
A ductless chemical hood is useful in areas where a ducted chemical hood is not available or is dedicated to another purpose. These devices should only be used under the conditions for which they are designed. The operation manual should be consulted prior to using such a hood and provisions should be made to ensure that the unit is properly maintained and serviced. In general, a ductless hood should not be used for chemical storage, hazardous operations or continuous operation.
- Special Ventilation Areas
Exhaust air from glove boxes and isolation rooms should be passed through scrubbers or treated by other methods before release into other exhaust systems. Cold rooms and warm rooms should have provisions for rapid escape in the event of equipment or electrical failure.
Any modification of the existing ventilation system is permitted only if the proper function of the system is not compromised, and the laboratory environment continues to be protected from hazardous airborne substances.
Before any modifications are performed, the Laboratory Director shall consult with the Director of Physical Plant and the Chemical Hygiene Officer.
Air flow through the laboratory should be relatively uniform throughout with no high velocity or static areas. Air flow into and within a fume hood should be uniform.
All laboratory fume hoods are tested annually and maintained to ensure adequate flow.
The ventilation system must be evaluated on installation. Physical Plant and the Chemical Hygiene Officer must conduct all evaluations.
Local exhaust ventilation systems should be thoroughly inspected on a regular basis, at least annually, and when any problem is noted. Inspections should include all the associated equipment as well as a review of the operation, exposure level measurements and air flow measurements.
- Installation and Maintenance
With proper design, use and maintenance of ventilation systems, occupational health hazards can be minimized. Consulting engineers, and vendors should be chosen from those having broad experience in designing ventilation systems for health hazard control. Plans for modification or new installations must be reviewed and approved by Physical Plant and the Chemical Hygiene Officer.
Before any new installations or modifications are performed, the Laboratory Director shall consult with the Director of Physical Plant and the Chemical Hygiene Officer.