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Why do Fume Hoods Fail? A Guide to Fume Hood Testing and Maintenance

Fume hoods are essential pieces of equipment in any laboratory or workspace where hazardous materials or chemicals are used. They work by drawing in air from the room and expelling it outside, preventing dangerous fumes and vapors from escaping into the workspace. However, like any piece of equipment, fume hoods can fail over time, which can put users at risk. That’s why regular fume hood testing and maintenance is essential.

A fume hood is a type of ventilation system that is designed to contain and exhaust fumes and vapors generated by laboratory processes. They work by drawing in air from the room, containing the fumes and vapors, and then expelling the air outside. Fume hoods typically have a sliding sash that can be opened or closed to adjust the airflow.

Fume hoods have a sliding sash that can be opened or closed to adjust the airflow, and standard fume hoods typically have face velocities between 80-120 FPM (Feet Per Minute). This range provides the optimum balance between containment and exhaust flow. Fume hoods that exhaust at less than 80 FPM do not adequately contain fumes and vapors, while anything over 120 FPM can cause significant containment problems.

In order to determine if your fume hood is operating within standard range, regular fume hood testing is essential. At H2I Group, we use the standard SEFA air flow test when testing most fume hoods. This involves measuring the face velocity at the sash plane of the fume hood, but how many sample points taken at the sash is determined by the size and model of your fume hood. These tests are conducted using a hot wire anemometer, which ensures accurate readings of air flow velocity in FPM.

If your fume hood is not performing at its best, it could be due to various reasons. While replacing the hood might be necessary if it has reached its end of life, there could be other underlying issues. Here are five common reasons why your fume hood might be failing:

Exhaust fan belt: The belt that drives the exhaust fan can stretch and slip over time, resulting in a lower face velocity. Belts typically have a life expectancy of 1-2 years and may suffer from stretch and dry rot. 

  • Lab exhaust fan: If the lab exhaust fan has exceeded its life expectancy or hasn’t been properly maintained, it can cause issues. Rusting, deteriorating bearings, pulleys, and belts can lead to decreased performance.
  • Chemicals and procedures: The hood may have been designed to handle specific chemicals or processes, and changes in these can damage the ductwork and lab exhaust fan, leading to lower face velocity. 
  • Blockage in the duct work and fan: It is very common to find blockages from items like chem wipes and napkins. This can reduce airflow and lower face velocity.
  • Balance issues: Changes in pressure due to remodeling or new construction can throw off the airflow balance, requiring rebalancing of the system. Airflow can be sensitive and will find the path of least resistance.


H2I Group can help identify any issues with your fume hood by testing and certifying your fume hoods. We’ll provide a compliance report for each hood tested, indicating any necessary repairs or replacements. However, any repairs or replacement of materials and parts are excluded from our certification services.

In conclusion, regular fume hood testing and maintenance is essential to ensure that they are functioning properly and keeping users safe. At H2I Group, we can help you with your fume hood testing needs, from the initial design and installation process to testing and certification. Contact us today to learn more about how we can help keep your workspace safe.

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