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Teacher Resources

Ventilation

An Important Step in Improving the Safety of Your Laboratories

One of the most important steps in maintaining a safe laboratory environment is to ensure that the laboratory is properly and adequately ventilated. Unfortunately architects, engineers and administrators who make the design decisions when planning a new or renovated laboratory, frequently leave out plans for proper laboratory ventilation in an effort to save money. Good ventilation is important to protect everyone who uses the laboratory from short-term exposure to toxic substances. Ventilation is equally important for protecting the teacher from the hazardous effects of long-term exposure to hazardous chemical vapors, aerosols and fumes. This article is intended to provide guidelines on how to solve the problem of poor ventilation in your laboratories and preparation/storage areas. We hope that you and your administration will follow these guidelines in order to provide a safer working and learning environment for teachers and students at your school.

Fume Hoods

Fume hoods are an important feature in any laboratory using hazardous chemicals. Fume hoods provide a location for the dispensing and use of hazardous chemicals that are likely to form a vapor or aerosol that could be inhaled by the user of the chemical or others in the laboratory. It is a common misconception that fume hoods provide adequate room ventilation in a school science laboratory. Fume hoods are not designed for room ventilation. Fume hoods are only designed to ventilate the activity taking place in the fume hood itself and to prevent the release of hazardous chemicals into the general laboratory environment.

When functioning properly, a fume hood should have a face velocity of 80-120 feet per minute (fpm)1. This is a measurement of the amount of air moving across the front opening of the hood and can be measured using a device called a velometer (Catalog No. SE4055) available from Flinn Scientific. In order to work efficiently, a fume hood should be located more than 10 feet from any doorway or window and should not be located near major traffic areas. Fume hoods should be checked periodically to make sure that they are working properly and efficiently. The fume hood ductwork should be checked annually to make sure that it is in good condition and venting directly to the out-of-doors, away from building air intakes. Fume hoods should be checked using a velometer or a small smoke generator which will allow you to test the effectiveness of your hood as well as its exhaust location at the exterior of the building. Simply turning the fume hood on and hearing the motor run does not constitute a thorough fume hood test.

A fume hood should never be used as storage space for chemicals or other items. Doing so not only decreases the efficiency of the hood, but also creates an unsafe situation for the user(s) of the hood. When items are stored in the hood they will prevent proper airflow, which decreases the hood's efficiency. Additionally, items stored in the hood force the users to perform their tasks closer to the front of the hood, which increases the possibility that the products or fumes from their activity will enter the general laboratory environment.

Fume hoods generally function most efficiently when used with the individual blower provided with the hood. Connecting multiple fume hoods to common ductwork and using a single, large blower is not recommended as the rate of air exhaust is usually insufficient to provide a face velocity in the acceptable range. Also, ensuring the proper amount of draw from each hood is difficult when using more than one hood at the same time.

Laboratory Ventilation

Good laboratory ventilation is the single most important ingredient in maintaining a safe laboratory environment. The ability to quickly provide a complete air change in science laboratories will greatly minimize the potential for chemical exposure. Good laboratory ventilation will allow teachers and administrators to feel safe and comfortable in doing all of the experiments and activities needed to provide an outstanding laboratory experience for their students. Adequate ventilation will also decrease the risks of long-term exposure to chemicals for teachers and other laboratory personnel. Unfortunately, architects, engineers and administrators often get so concerned about conserving energy, money and HVAC (heating, ventilation and air-conditioning) costs, that they ignore the need for proper ventilation of the science classroom(s). It is up to the science teacher to ensure that all key decision-makers are aware of the need for good ventilation.

So, how much ventilation is enough? The OSHA Laboratory Standard 1910 states that "4 to 12 room air changes per hour is normally adequate general ventilation"2. Air that is exhausted from the laboratory should always be vented to the out-of-doors and should never be recirculated. This will prevent laboratory air from being drawn back into the school building. Ideally, the ventilation system should have a complete air change in five minutes. This is the equivalent of 12 air changes per hour. Such a rapid air change is needed in the event of a chemical spill, or when conducting experiments that generate a hazardous vapor or a particularly strong or long-lasting odor. In general, a five-minute air exchange can be most easily and inexpensively achieved using an auxiliary ventilation system such as a purge fan or ceiling ventilator. Whichever type is chosen, the system must move a large enough volume of air in cubic feet per minute (CFM) to provide a complete air exchange in five minutes. With any auxiliary ventilation system, it may be necessary to open the laboratory doors in order to provide enough make-up air to allow the system to provide the required air change. Make-up air is the air required from outside the room (for example, a hallway) to replace the air being removed from the laboratory by the ventilation system.

Other Thoughts

While it is true that ventilation is a large problem in many, if not most, schools, several steps can be taken to minimize chemical exposure in addition to adequate ventilation. The most important step that can be taken to limit exposure to hazardous chemicals is to store and use minimum quantities. This can be accomplished by purchasing and storing only those chemicals that are used by the school in a short amount of time. Flinn Scientific recommends that no more than a three- to five-year supply of any hazardous chemical be stored in a school's chemical storeroom. This not only assures a safe environment, but also prevents inaccurate results in experiments due to old, stale chemicals and saves valuable budget dollars that are wasted on excess chemical purchases. Also, the amount of chemicals needed for use can be greatly reduced by using microscale techniques whenever hazardous chemicals are used. Finally, exposure to hazardous chemicals can be limited by choosing lab activities that use chemicals that are less hazardous to teach the same concepts as more hazardous chemicals.

1ANSI/AIHA Z9.5-1992, p 13

2OSHA Part 1910-Occupational Safety and Health Standards

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