There are many tools available to industry to help protect workers from exposure to contaminants. One of the most common protection methods used is the fumehood. Fumehoods can protect workers from vapours, dusts, gases and fumes generated while handling or working with products. It is important that assuring this protection does not stop at the design and installation phase. Annual inspections of fumehoods are very important to ensure that they continue to draw the air necessary to maintain protection of workers.
Face Velocity
Fumehoods work by creating a draw of air that directs contaminants through the fumehood exhaust and either outside the building or to an additional control system. The effect is to draw contaminants away from the breathing zone of the person using them and limit their potential exposure. The best way to determine if a fumehood is working correctly is to measure the face velocity. This measures the draw of the fumehood at the opening where the employee will be handling products within the fumehood.
The face velocity can be influenced by a number of factors including the design of the fumehood, the size and capacity of the fan and the area of the opening. In addition, any equipment, products, etc. that are kept within the fumehood can affect the flow. Outside factors such as activity and any negative air pressure within the building containing the fumehood can also affect flow. The draw from the fumehood must be sufficient to overcome these outside variables to draw sufficient air to protect employees. Fumehoods also often have a sash which can be moved up and down in order to move things in and out of the fumehood and to access equipment and products for employees to work with. The height of the sash during these activities should be designed in such a way that they maintain sufficient face velocity to ensure that contaminants do not escape from the fumehood.
Standards for Face Velocity
There are a few standards used for face velocity average readings. ASHRAE 110 is one method that outlines both a qualitative and quantitative method for measuring face velocities. The American Conference for Governmental Industrial Hygienists (ACGIH) also outlines a recommended face velocity for fumehoods. The widely accepted and recommended face velocity to maintain at the opening of a fumehood is 0.5m/s (100 fpm) in order to maintain proper function and protection of the fumehood.
How is Face Velocity Measured?
The most effective way to measure face velocity is by using an anemometer. Trinity technicians will use a grid approach on the opening plane of the fumehood. Multiple measurements are collected typically using a hot wire anemometer and each of the velocity measurements are recorded individually and averaged over the opening. Sash opening height is recorded and any locations that fall below the recommended face velocity will be noted. Typically, there are limiters on sash height above which sashes cannot be raised during general use. These are only overridden during maintenance conditions and often an alarm is set should the sash be raised to above the design levels. Trinity technicians will measure the face velocity of the fumehood during the maximum sash height conditions. The face velocity can also be measured during variable heights should these be used during normal operation.
Once the average face velocity has been measured, Trinity will confirm that this is above the recommended level. For the validated fumehoods, Trinity will provide a log that indicates the following information:
- Confirmation of Pass/Fail status
- Due date for follow up inspection
- Sash height conditions during testing
Annual reverification can be planned to ensure ongoing compliance with recommended face velocities.
Other Methods
Trinity offers an additional qualitative measure of airflow during each fumehood validation. By using smoke tubes, a visual representation of airflow can be determined. Smoke tubes are used to conduct a visual representation of the airflow. By blowing smoke at locations around the sash opening, an understanding of how well the fumehood is able to draw air can be determined.
Other methods including setting up an industrial hygiene monitor can be used to determine effectiveness of the fumehood. This entails setting up a fixed monitor at a location that is representative of employee positioning during use. A known compound is placed in the fumehood and the exposure level is measured throughout the day. While this method can be useful, it is often a more expensive and time-consuming method.
Conclusion
Fumehoods are a common control method used in all types of industrial settings. From controlling chemical fumes, to particulate and hazardous compounds, they are an effective tool to ensure that employees are protected from potential exposure hazards. This is why it is critical for facilities to ensure that their fumehoods are maintained and working as intended. Testing the functional performance of each fumehood annual is a key part of the maintenance of the fumehoods.
Trinity can assist your Facility by inspecting, designing, and implementing a program that effectively and economically ensures compliance with recommended face velocity levels to ensure that employees are protected from potential exposures.
Want to learn more? Please join us for a complimentary webinar on January 22, 2026, from 1-2pm, on the topic of ‘Safeguarding Workplaces: Mastering Industrial Hygiene Compliance for Sustainable Operations. Click here to register!
If you have any questions on any of the information discussed in this article or are looking for any fugitive contaminant or odour emissions related support, please contact Steve Babaris from our Trinity Toronto Office at 647-391-2527.
