Fume hoods are safety devices, used to contain chemicals with long-term exposure hazards. Fume hoods are not appropriate for protection from substances causing significant health consequences with isolated, short-term exposures.

It is important to note that the technical term for a fume hood is a laboratory chemical hood. However, industry convention remains to call them fume hoods. This catalogue will therefore call them fume hoods.

The exhaust system draws air through all openings in a fume hood, including the face. These entering airstreams prevent chemicals inside the air from escaping. Air in the fume hood is then exhausted, drawing vapors, gasses and particulates in the hood out.

Fume hoods come in different sizes and configurations. There are two basic configurations of fume hood. Benchtop hoods, as the name suggests, are placed on the laboratory bench. Floor-mounted hoods, commonly called walk-in hoods, are therefore mounted on the floor. A floor-mounted hood cannot offer any protection to a user who actually enters the hood. 

Fume Hood Testing

It can be difficult for users to verify proper operation of the hood. Users cannot accurately gauge a hood’s face velocity to determine if it has dropped to unsafe levels. The concentration of chemicals in a hood is low enough to render them invisible. Odor is not a sufficient indicator, because the level of chemical required to be hazardous is independent of our ability to smell the chemical. Suppliers of laboratory equipment and services have therefore developed testing procedures, such as ASHRAE 110,EN14175-3, to help ensure user safety

Initially certifying fume hoods involves three different tests: flow visualization, face velocity and containment. After acceptance, annual face velocity tests are necessary when absent renovations to the HVAC system, laboratory or hood. 

Flow Visualization or Smoking Test  

The first test to measure a fume hood’s performance is to perform a flow visualization, or smoke test. In this test, a smoke generator is moved around the open face of the fume hood. How the smoke enters the hood should be observed. Any areas where smoke does not enter the hood or even comes out should be noted and corrected before continuing with other tests. 

Face Velocity Test

Once the flow visualization test has qualitatively determined that the airflow into the hood is uniform, quantitative testing is performed. In this testing, the open area of the face is divided into sections of approximately 1 square foot. Air velocity is measured at the center of each of these sections. The face velocity at each of these points should be within 20% of the measurement average. Any points falling outside of this range should be corrected before continuing.

Face velocity is sometimes recommended to be between 80 to 120 fpm. However, hood design, HVAC system design and competing airflows will all affect a hood’s ability to contain. Consequently no face velocity will ensure containment. Face velocity is only considered to be an indicator of containment if a containment test has been performed and there have not been significant changes to the laboratory or fume hoods.

Monitors are now required to ensure the long-term performance of fume hoods. These monitors measure face velocity, alarming if it should drop to potentially unsafe levels. 

SF6 Tracer Gas Containment Test

The third test to measure a fume hood’s performance is to measure the containment. Most industry organizations, such as ASHRAE7, AIHA8 and SEFA9, recommend actually testing the hood’s ability to contain a chemical under controlled conditions, recognizing that face velocity alone cannot ascertain a hood’s ability to control exposures.

Containment testing typically is based around releasing a known flow rate of a tracer gas inside a fume hood. A mannequin is positioned in front of the hood to disturb airflow and the concentration of the tracer gas at the mannequin’s mouth measured.