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- Control Methods: Technical Bulletin
Control Methods
A number of control methods are available to reduce the likelihood that science laboratories will create IAQ problems. These measures include selecting and purchasing the safest materials and minimizing quantities that are to be used, storing materials properly, ensuring that appropriate ventilation systems are available, utilized, operated, and maintained, and training laboratory users (teachers, aides, students) about the hazards in the laboratory and in the proper use and disposal of materials. The Material Safety Data Sheet (MSDS) specific to each chemical is an excellent resource document that communicates the hazards of a substance and recommends appropriate handling and control techniques as well as the use of personal protective equipment. MSDS are available, by law, from the manufacturer of the product. An effective laboratory safety program is one that integrates all methods of control. Important aspects of each technique are discussed below.
The best way to ensure that a certain material will not create a problem is to eliminate its use (and presence) in the laboratory. Before using or purchasing chemicals or chemical-containing equipment it is always prudent to evaluate the consequences of its improper use against its value as a teaching tool. Consideration should be given to identification and use of substitute materials that may offer reduced toxicity, reactivity, or flammability; or limiting the material’s use to a well controlled demonstration. As a general guideline, it would be prudent to substitute or limit the use of any material rated 3 or 4 for Health, Flammability, or Reactivity by the commonly used National Fire Protection Association hazard rating criteria. Certain well known hazardous materials have been eliminated, or drastically limited, in many laboratories. The following information summarizes some of the techniques used. Alcohol lamps — These should not be used in schools. Use hot plates with water bath to eliminate the toxicity and flammability hazards presented by the use of alcohol. Asbestos — The interior linings of laboratory hoods and many older heating elements, hot plates, hair dryers, "centers" of wire gauze, and gloves for handling hot items contained asbestos. Replace with non-asbestos insulated equipment. Ether — Typical use has been for killing fruit flies in genetics experiments. Non-toxic methods for controlling the flies include refrigeration and placing flies on a Petri dish over ice for observation. Formaldehyde — This substance was used historically for preserving and storing biological specimens. Schools should no longer purchase formaldehyde-based preservatives. Non-formaldehyde preservatives are readily available and have been found to be effective. Most of these alternatives contain ethylene glycol as a major ingredient. Mercury — This very toxic material may become a hazard if mercury is spilled from a storage container, a barometer, or a broken mercury-filled thermometer. Metal or "spirit-filled" (hexane) thermometers should be substituted. If mercury thermometers must be used for high accuracy, purchase Teflon® coated instruments that will help contain the mercury if broken. Consider carefully the need for a mercury barometer and stored mercury. If used, a layer of water or mineral oil on top of a pool of mercury (barometer, storage bottle) will prevent mercury vapors from being given off. Pathogenic organisms — These may be unintentionally cultured from the environment. Use only nonpathogenic organisms in the classroom for microbiology experiments. Treat all cultures as if they were pathogenic.
Pesticides — If their use is deemed necessary, always identify and use the least toxic material. Read and follow all instructions provided. Table 1 presents some additional materials that have been substituted for toxic chemicals in certain experiments or for cleaning. A technique that offers many benefits is microscale chemistry. It has been defined as a way of carrying out chemical experiments by using sharply reduced amounts of chemicals. Many experiments use as little as one one-thousandth (1/1000) of the traditional amount of chemicals. In addition to greatly reducing the amount of material that may become airborne, lab waste is reduced, the time required to perform the experiment is reduced, clean-up is easier, and less space is required. Microscale laboratory equipment and manuals are available through some supply houses. The National Microscale Chemistry Center at Merrimack College, 315 Turnpike St., North Andover, MA 01845 is promoting this technology through workshops and demonstration projects.
Room Ventilation ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality, recommends 20 cubic feet per minute (cfm) of outdoor air per person for general dilution ventilation of laboratories on the premise of some odor generation. ASHRAE recommends that toxic or otherwise objectionable emissions are contained at the point of generation, as inside a laboratory hood, and exhausted outdoors. As a general guideline, it is recommended that chemicals with an industrial exposure limit (i.e., OSHA Permissible Exposure Limit or ACGIH TLV) of 100 parts per million (ppm) or less (gas or vapor) or 0.1 mg/m3 (solid) be handled inside a laboratory hood. Product MSDS may also stipulate use of local exhaust.
To avoid the spread of odors into other school spaces, the laboratory should be kept under negative pressure when in active use. Also, air from the laboratory should not be recirculated through a central air conditioning system supplying other areas. Negative pressure is best maintained by exhausting more air from a room than is mechanically supplied. How this is accomplished depends on the characteristics of the air conditioning system. To ensure this air imbalance, the supply and exhaust air flows must be compared under the full range of operating conditions. Typically, an inward flow from surrounding spaces into the lab to make up 10 to 15% of the air exhausted is sufficient. Since the corridor door should be kept closed whenever there is risk of offensive emissions, transfer grilles or ducts sized for a velocity of 100 feet per minute (fpm) can supplement leakage around doors to facilitate inward air flow from adjacent space. This imbalance is only necessary when lab activities require it. Since a positive pressure may occur when the laboratory exhaust fan is off, it should operate as long as inward flow of air into the room is necessary. Fire protection regulations in most jurisdictions do not permit transfer of air from a corridor through an unprotected opening such as a floor or wall grille. In these cases it may be necessary to transfer the make-up air through duct work from spaces not used as a means of egress. Laboratory Hoods A wide range of laboratory hood types are available, including variable air, volume, auxiliary air, and horizontal sash. However, for most school requirements a vertical sash, air foil, bypass type hood will be the preferred choice. Air foil design minimizes eddy currents at the sash opening and the bypass feature assures a constant exhaust volume and controls face velocities through the range of sash positions (See Figure 1). Continuous operation of the laboratory hood, when not in use, can be costly if outdoor make-up air must be heated and cooled. For this and other sound reasons laboratory hoods should not be used to store materials requiring continuous exhaust.
Based upon recommendations of the American National Standards Institute and the American Industrial Hygiene Association (ANSI/AIHA Standard Z95-1992), a face velocity of 80 fpm should effectively remove fumes produced within the hood. This rate is based upon the full open sash position and is conditional upon proper use. It is also conditional upon: Location near the room entrance should be avoided to prevent disturbing air flow and to avert a possible fire hazard in the path of egress. If the hood is dedicated for nonhazardous use, a low face velocity (60 fpm minimum) may be acceptable. Laboratory Hood Exhaust Systems Each laboratory hood should be exhausted by an independent system for simple, safe and flexible operation. Corrosion resistant (e.g., plastic protected steel) centrifugal fans, with outboard bearings, should be located on or immediately below the roof to keep the duct within the building under negative pressure to prevent outward leakage. Either direct or belt driven fans can be utilized. Direct drive fans avoid degradation of performance due to drive belt slippage or breakage, but do not offer the flexibility of increased capacity should design estimates of air flow resistance fall short. Belt drives should have two or more belts rated at 200% capacity. Regardless of drive type, a corrosion resistant volume damper with tamper proof adjustment should be installed for each hood. A variety of corrosion resistant duct materials are suitable, but most popular are Type 316 stainless steel and glass fiber reinforced polyester. Ducts within the building must be sealed against both inward and outward leakage. The duct run should be short and direct, sized for approximately 2000 fpm. Assuming objectionable materials will be conveyed by the laboratory hood exhaust system, the discharge arrangement must minimize entrainment into air intakes or open windows and contact with maintenance personnel or others on the roof.
Details of self draining discharge stacks and location guidelines can be found in the Industrial Ventilation manual (American Conference of Governmental Industrial Hygienists, Technical Information Office, 1330 Kemper Meadow Drive, Cincinnati, OH 45240). Specific design considerations include:
Laboratory Air Negative Pressure By design, the laboratory should be maintained under slightly negative pressure whenever production of contaminants, undesirable odors, or fumes might occur. Laboratory hood fan operation may be needed to create this negative condition. Inward flow of air is observable at the crackage around the doors to transfer grilles by means of artificial smoke (as described below under qualitative testing of laboratory hoods). Check monthly for new installations, since the negative pressure is affected by the air conditioning system control and outdoor weather changes. Smoke testing can also reveal construction faults, especially above ceilings or other concealed penetrations, through which contaminants and odors could pass. After the first year of operation, semiannual checks — and on complaint occasions — should suffice. Windows and exterior doors should not be operated in the labs except to air out after accidental spills and then only with precautions to prevent odor passage outward through doorways, transfer grilles or other apertures to occupied spaces. Laboratory Hood Performance Elements of the exhaust system, fan, motor fan drive, and discharge stack drain duct work (where accessible) should be inspected semiannually. Overheating of the duct could cause sealant loss or distortion resulting in excessive leakage and reduced hood effectiveness. Volume damper positions should be checked to verify no unauthorized adjustments have occurred. Worn belts should be replaced as a set, not individually. The performance of the hood should also be verified on a regular basis, semiannually or as directed. Inspect the hood to assure the sash is in good operating condition, internal damage has not occurred, refuse in not restricting air paths, internal baffles are in originally adjusted position (which should be marked), and that it is not being used for material storage. Pilot lights, if provided to indicate fan operation, should be checked. There are two approaches to testing laboratory hood performance. Quantitative testing, as detailed in ASHRAE Standard 110-1985, is costly and rarely would be appropriate for school applications. Qualitative testing to determine face velocities and air spillage is appropriate. Test equipment includes a calibrated velometer (a hot wire thermal anemometer preferably), smoke sticks (glass tubes containing artificial smoke producing chemicals) or a smoke powder gun (a most useful diagnostic tool for many HVAC investigations including negative pressure verification), and a supply of 30 second smoke bombs. This testing procedure is described in an inexpensive user guide (Saunders, G J, "Laboratory Fume Hoods, A User’s Manual," 1993, John Wiley & Sons, NY). See also the American National Standard for Laboratory Ventilation (ANSI/AIHA Z95-1992) published by the American Industrial Hygiene Association, 2700 Prosperity Avenue, Suite 250, Fairfax, VA 22031. Should this procedure be beyond the capability of school districts, it can be performed by qualified air balance or industrial hygiene firms. Such testing should occur prior to initial use and on a regular basis or whenever degradation of performance is suspected. The need for testing may be reduced by use of a simple manometer or diaphragm actuated dial gauge to measure the pressure in the duct downstream of the hood. The device could be permanently mounted on the hood with the initial pressure marked after the initial hood commissioning. Pressure change will advise the user of any malfunction or degradation in face velocity. Monitors are also available with alarms that provide audible warnings that the exhaust system is performing below the set criteria. If a permanently mounted pressure indicator is deemed impractical due to concern over tampering, the maintenance or teaching staff could remove and reinstall the instrument with relative ease for periodic checking. The importance of proper chemical storage must not be overlooked. The low levels of routine emissions expected in chemical storage areas are easily controlled with proper ventilation (see above). A well designed storage system can also minimize the potential for larger releases.
The first consideration for proper storage is one of inventory management. This step is a must. Computerized inventory management programs are available. The inventory should include the chemical name and the estimated amount in the container. Each chemical container should be checked periodically for rust, corrosion, and leakage and have the following information written on the container. An up-to-date inventory can ensure that only needed purchases are made and that chemicals will not be stockpiled needlessly. It is important to consider storage limitations when placing orders for chemicals. Common practice in many school systems is to place one large order for the entire year’s supply or chemicals. This practice leads to improper storage upon arrival because storage areas are quickly overwhelmed. To control this problem, smaller orders should be placed throughout the school year. Next, the storage area should be organized so that only "compatible" chemicals are stored together, and that acids and flammables each have their own dedicated storage cabinets, Incompatible chemicals are those that, upon mixing, can react to produce explosions, fires, or large amounts of heat. It is also important to consider design and construction materials used for shelves and cabinets to minimize safety concerns (incompatibles, fire ratings, strength, spill containment). A recommended storage pattern is to divide and separate the inventory into the eighteen groups of compatible chemicals presented in Figure 2 and Figure 3 below. The basic philosophy to follow is that every chemical should have a definite storage place and should be returned to that place after each use. There are a number of other storage practices that are either required by regulations or generally accepted as prudent practice. For example, the National Fire Protection Association publishes standards for the storage and handling of flammable liquids, the Nuclear Regulatory Commission regulates the use of radioactive materials, and various trade and professional organizations have developed guidance that has become standard in the area of laboratory health and safety. A resource book that provides an excellent overview of this subject is the National Research Council’s "Prudent Practices for Handling Hazardous Chemicals in Laboratories" (National Academy Press, Washington DC, 1981).
Laboratory waste accumulation and disposal can also present IAQ concerns. Each school should have a procedure for collecting wastes and arranging for their disposal. First, each school should ensure that they fall under the "conditionally-exempt small quantity generator" status accorded by the Resource Conservation and Recovery Act (RCRA; 40 CFR, Parts 260-266). This status is for those generating less than 100kg/month of hazardous waste. Exempt sites may accumulate hazardous waste indefinitely by law; however, prudent practice dictates that the quantities of waste generated and stored should be minimized.
Great care must be taken for disposal of chemicals to the sewer system (i.e., "down the drain"). Environmental damage and upset of the sewage treatment processes can result. Local wastewater discharge regulations should be reviewed to ensure that disposal restrictions are not violated. In general, the following guidance should be followed for disposal via laboratory drains: Highly toxic, flammable, or malodorous chemical should not be disposed of down the drain. Drains are typically interconnected; a substance that goes down one sink may come up as a vapor in another. There is also a potential hazard of chemicals from two sources contacting one another. Some simple reactions can cause explosions or create extremely noxious vapors.
In the biology laboratory, sterile plastic Petri dishes should be used only once. Following their use, the dishes should be taped shut, bagged, and properly disposed. Glass Petri dishes must be sterilized before using. Following use, they can be soaked in strong disinfectant and washed. Alternately, they can be autoclaved, the culture medium disposed in a sealed container, and the dishes then washed in detergent. Likewise, specimens should be carefully bagged and properly disposed. These wastes will not ordinarily be considered infectious or medical wastes which require special disposal. A laboratory Chemical Hygiene Plan (CHP), such as that required under the Occupational Safety and Health Act (OSHA, 29 CFR 1910.1540), can maintain good IAQ through a comprehensive program of work practices, procedures, and policies.
The CHP includes the following elements:
Specific requirements for a CHP vary from one locality to another. In Maryland, for example, the employees of public school systems are covered by the regulations administered and enforced by Maryland Occupational Safety and Health (MOSH) which require CHPs; in addition to paralleling most other OSHA requirements. Consult with health and safety support staff or regulatory offices in your area. | |||||||||||||||||||||||||
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