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Mitigating Health Risks with Laboratory Fume Hoods (Part II) : Fume Hood Design

Fume Hood and Its Safety Requirement

Reducing the exposure of chemical is the key factor for laboratory personnel’s safety. Fume hoods have the primary function to lower and eliminate the chemical concentration inhaled by the chemist. For that reason, it is essential to have a good understanding on how a fume hood works and provide safety.

In terms of its function, there are two crucial parts in any type of fume hoods. The first one is the enclosure which plays an essential role to retain any toxic vapours or fumes. As demonstrated by the Graham Law, air molecules collide with the toxic gas molecules and distributed them at a high speed throughout the laboratory air. Toxic gas molecules escaping from a container such as bottle or beaker will travel very quickly in your laboratory: in a fraction of second, a molecule can travel several meters away from the source depending on its molar mass.

The enclosure shall play the role of a shelf to stop toxic molecules from travel and contain them. But the enclosure is only partially closed with the front sash opening, offering toxic molecule an escape route which could affect the chemist. That’s the reason why the ventilation is also crucial. The ventilation is creating an airflow entering the fume hood at the front sash opening and taking chemical vapours handled inside to outside through the exhaust duct (or through filter in ductless fume hood).

The ventilation of fume hoods have been strictly defined by International Safety Standard (Such as in USA with ANSI/ASHRAE 110:2012, ANSI/AIHA Z9.5-2011 and in Europe with EN 14175) to prevent harmful exposure of user to hazardous substances.

These International Safety Standards have determined two main safety criteria to obtain an efficient fume hood ventilation :

  1. The Air Face Velocity
  2. The Containment Efficiency

The face velocity is related to the speed of air being at the sash plane level and is expressed in meter per second (m/s) (or feet per minute, fpm depending on said country). According to the International Safety Standards for fume hoods, the air face velocity shall be set between 0.4 and 0.6 m/s on average at the sash opening.

It is designed to create an air barrier between the chemical evaporations present in the enclosure and the possible external turbulences around the fume hood, which can eventually provoke Venturi effects and draw air inside the hood into the laboratory room through the front sash opening. Such turbulences can be generated from a person walking along the hood, by the air draft of the HVAC system or simply by open/close of a door. A face velocity of 0.4 to 0.6 m/s at the sash opening surface has been proven to have the capacity to counter the effects of external turbulences in the laboratory room. Air face velocity can be easily measured by a simple and inexpensive device with hot wire anemometer.

For long time the face velocity set at around 0.5 m/s was the only criteria to consider a fume hood as safe, but in the eighties, researchers showed with chemicals could easily escape through the front sash, even with 0.5 m/s face velocity, mainly due to strong air turbulences inside the fume hood enclosure. By 1985, the OSHA in USA introduced in the ASHRAE 110 Standard a second safety criteria called the “Containment efficiency” of the fume hood, or the ability of a fume hood to keep the chemicals vapours inside the enclosure and avoid them to escape through the front sash opening. A new test was introduced consisting in emitting a tracer gas, namely Sulfur Hexafluoride (SF6) at 4 litres per minute in the enclosure and verifying at the nose level of a manikin placed in front of the hood that a minimised quantity of SF6 will be detected. To pass the containment test, the concentration measured at the manikin level shall be less than 0.05 ppm of SF6 (ANSI/ ASHRAE Z9.5 Standard).

Although this containment test is as important – if not more important than the face velocity test – it has been ignored by many fume hood manufacturers since it is more complicated and much more expensive to perform. This test shall be performed to test fume hood samples in ideal conditions in a testing laboratory to verify that the fume hood’s design is appropriate to obtain the 0.05 ppm of SF6, but also after it has been installed at the customer’s site where less than 0.1 ppm level of SF6 is accepted.

Consequently, laboratory managers shall first request the fume hood manufacturers to provide test reports, confirmed by an official independent testing party, for air face velocity and containment efficiency before any purchase. Secondly, they shall request the manufacturers to proceed to those tests after the installation of the fume hoods onsite to verify that they are well installed, with adequate auxiliary air, in a non-turbulent area and can provide full safety to the chemists.

 

▶︎ article continue in Part III

(This article is contributed by Teddy Pitiot, Erlab)

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