A fume hood is a ventilated enclosure that is used to contain vapors and fumes generated inside its work surface. They are one of the most effective devices used to protect laboratory personnel from hazardous chemicals. Because of its function as a protection device, proper performance is essential in providing safety to laboratory personnel. The level of protection provided is affected by the manner in which the fume hood is designed, constructed and operated. This has prompted several regulatory agencies to develop standards, codes and recommended practices for the safe operation of the fume hood.
There are several types of fume hoods found in modern labs and they all have a specific purpose:
Constant Volume Fume Hood - This is the most popular device in laboratories today. A constant volume fume hood is connected to an exhaust fan on the roof that runs at a constant speed. The fan speed is balanced to ensure the face velocities at the hood are adequate at the operating sash height. These fume hoods exhaust the amount of air all the time, regardless of the sash position. As the sash is lowered or raised, the velocity at the face of the hood changes. This change in velocity can result in less-than optimal hood performance.
Variable Volume Fume Hood - This hood provides the most optimal protection. A variable volume fume hood has a controller that regulates the amount of air exhausted to maintain a constant face velocity regardless of the position of the sash. The speed of response to the controller, as well as face velocity at different sash positions is tested in these fume hoods to ensure adequate protection.
Ductless Fume Hoods - Ductless fume hoods provide great benefits in terms of lower infrastructure costs and lower energy costs. As its name implies the ductless fume hood is not connected via a duct to an external discharge outside of the building. Instead the ductless fume hood, uses a combination of carbon and/or HEPA filters to capture fumes vapors and recirculates clean air back into the laboratory.
Codes, Standards and Recommended Practices
As mentioned above several organizations have developed standards to regulate the design, performance and testing of the fume hoods:
ANSI/ASHRAE 110:2016 "Method of Testing Performance of Laboratory Fume Hoods" - This standard is published by the American National Standard Institute (ANSI) and the American Society of Heating, Refrigerating and Air conditioning Engineers (ASHRAE). This standard provides guidelines to conduct qualitative tests to evaluate hood performance and quantitative tests to measure air velocities and containment capabilities. The standard provides method for:
Inspecting the hood and operating environment.
Airflow visualization (smoke test)
Measure face velocity.
Tracer gas containment tests.
Limited evaluation of variable air volume operation.
ANSI/AIHA Z9.5 :2003"The American National Standard for Laboratory Ventilation" - This standard is published by ANSI and the American Industrial Hygiene Association (AIHA). This standard covers the exhaust requirements, face velocity in fume hoods and expectation to hood monitoring systems. This standard provides non-regulatory guidelines and recommendations.
OSHA Part 1910.1450 - This code is published by the Occupational Health and Safety Association and addresses several topics to include aspects of laboratory design & operation and face velocity, exhaust system, airflow monitoring and maintenance of the fume hood.
SEFA 1.2:2010 "Laboratory Fume Hoods" - This recommended practices document is published by the Scientific Equipment & Furniture Association. It provides a guideline that compiles the available standards and codes into a more standardized document.
These standards focus on three (3) main important attributes for an optimal performance for the fume hood as follows:
There are common misconceptions around airflow requirements for a fume hood. While intuitively one might think that high velocities will create a better airflow and hece remove any harmful fumes from the fume hood work surface, the reality is that when the face velocity exceeds 150 FPM, the potential of creating eddy currents is increased. Eddy currents can cause harmful contaminants to escape the hood increasing the workers exposure.
While the ANSI/ASHRAE:110:2016 does not specify a target face velocity and relies mainly on the qualitative airflow visualization test and the quantitative tracer gas test to determine proper airflow, other agencies have more definitive airflow requirements.
(ACGIH) Industrial Ventilation (24th Edition) p. 10-40 “Supply Air Distribution – For typical operations at a laboratory fume hood, the worker stands at the face of the hood and manipulates the apparatus in the hood. The indraft at the hood face creates eddy currents around the worker’s body, which can drag contaminants in the hood back to the body and up to the breathing zone. The higher the face velocity, the greater the eddy currents. For this reason, higher face velocities do not result in as much greater protection as might be supposed.”
Federal Register – OSHA p. 3332. Paragraph G, Quality “…airflow into and within the hood should not be excessively turbulent.” (200) “…hood face velocity should be adequate (typically 60 – 100 lfm).” (200,204) Note: Reference to page numbers in Prudent Practices for Handling Hazardous Chemicals in Laboratories are given in parenthesis i.e., (200)
Prudent Practices - p. 178 “In most cases, the recommended face velocity is between 80 and 100 feet per minute (fpm). Face velocities between 100 and 120 fpm may be used for substances of very high toxicity or where outside influences adversely affect hood performance. However, energy costs to operate the fume hood are directly proportional to the face velocity. Face velocities approaching or exceeding 150 (fpm) should not be used, because they may cause turbulence around the periphery of the sash opening and actually reduce the capture efficiency of the fume hood."
ASHRAE Handbook Applications 1999 - p. 30.10 Face Velocity. “If the face velocity (design and operation) must be maintained at 100 fpm (0.5/s) + 10%, this average may be allowed to deteriorate to 85 fpm (0.47 m/s) before correction and then the face velocity must be returned to 100 fpm (0.5/s). Individuals reading may not vary more than + 15% with the hood empty or + 25% with research equipment in the hood.
NFPA 45: 2000 - P 45 – 28 Appendix “A-6.4.6. Laboratory fume hood containment can be evaluated using the procedures contained in the ASHRAE 110, Method of Testing Performance of Laboratory Fume Hoods. Face velocities of 0.4 m/sec to 0.6 m/sec (80 fpm to 120 fpm) generally provide containment if the hood location requirements and laboratory ventilation criteria of this standard are met.”
Monitoring and Alarms
Many older laboratories' fume hoods are not equipped with airflow monitoring devices. As the codes and regulations have evolved over the years to ensure proper laboratory personnel protection, the use of airflow monitoring is now a requirement. If adding additional equipment to your laboratory the procurement official must ensure the new or remodeled equipment is fitted with an airflow monitoring device.
ASHRAE Handbook Applications 1999- p. 30.5 “All laboratory fume hoods and safety cabinets should be equipped with visual and audible alarms to warn the laboratory workers of unsafe airflows.”
NFPA 45:2015 - "p. 45-13 A measuring device for hood airflow shall be provided on each laboratory hood. The measuring device for hood airflow shall be a permanently installed device and shall provide constant indication to the hood user of adequate or inadequate hood airflow.
OSHA 1910.1450 - p. 484. Paragraph (B) Hoods “…each hood should have a continuous monitoring device to allow convenient confirmation of adequate hood performance before use (200, 203).”
Maintenance and Inspections
As is true for all equipment, regular maintenance and inspections are necessary to ensure the equipment is operating in the optimal conditions. Fume hoods are not an exception. Regular maintenance for a fume hood is paramount to ensure the safety of the laboratory personnel. Most fume hoods have ancillary equipment such as exhaust fans, volume dampers, air valves, etc. These also need to be included in a regular maintenance program. The maintenance checks should be performed at least annually, or more frequently depending on the results of the inspections.
ANSI/AIHA Z9.5:1992 - "Routine performance tests should be conducted at least annually or whenever a significant change in the hood system occurs."
OSHA 1910.1450 - "Quality and quantity of ventilation should be evaluated on installation, regularly monitored (at least every 3 months), and re-evaluated whenever a change in local ventilation is made"
NFPA 45:2015 - When installed or modified and at least annually thereafter, laboratory hoods, laboratory exhaust systems and laboratory special exhaust systems shall be inspected and tested."
Typically, the owner establishes the acceptance criteria based on the chemicals handled by the facility, engineering designs and any of the regulations described above. Such programs may include the allowable average and maximum exposure levels, allowable fue hood face velocities and required speed of response for variable volume operations. To assist with this task the owner can engage the help of a certified contractor in this subject.
The National Environmental Balancing Bureau (NEBB's) Fume Hood Performance Testing Program provides certification of firms and individuals that meet the stringent training and quality systems criteria established by the NEBB accreditation board. A firm certified by the NEBB offers hospitals, laboratories, and educational facilities proof of a firm and individual's technical knowledge, skills and instrumentation to test fume hoods per the latest regulatory standards.
Fume hoods are installed in laboratories to protect the workers. This level of protection is affected by the manner in which the fume hood is designed, installed and operated. No fume hood can guarantee adequate containment unless it is periodically verified by a trained professional.
Micron has NEBB Fume Hood Testing certified professionals on staff that can perform onsite testing on your fume hoods. Call us today for a consultation.