Batten Down the Hatches

A brief tutorial on strategies for containing biosafety labs The design of high-containment biosafety laboratories is not as simple as it may look. Federal guidelines from the National Institute of Health provide a starting point, but the information may be misleading. The most significant issue is that the laboratory's air pressure can never go positive.

By George Mellen, P.E., Chief Engineer, Iowa State University, Ames, Iowa September 1, 2004

A brief tutorial on strategies for containing biosafety labs

The design of high-containment biosafety laboratories is not as simple as it may look. Federal guidelines from the National Institute of Health provide a starting point, but the information may be misleading. The most significant issue is that the laboratory’s air pressure can never go positive.

To start, the operation must be separated from other facilities, be it by brick and mortar or isolation. On many projects, the biosafety lab is a section of an existing building. Therefore, the best isolation involves concrete walls coated on the interior with a material that can be washed down, but which will not allow the inclusion of any harzardous material. The floor must be covered with a monolithic epoxy coating that extends six to eight inches up the base of the walls. The ceiling must be of a solid material such as gypsum wallboard with no openings except those required for ductwork. The ceiling must be primed with a continuous coating of high-quality paint. Edges of the duct must be sealed. In fact, any opening into the laboratory must be sealed, including conduit, piping, screw holes and fire-detection devices, as this reduces the places where contaminants can collect.

Sealing is also important for the lab decontamination process, which might involve materials that should not be allowed out of the laboratory such as vaporized hydrogen peroxide. The sole exception is a minimal space under the door to yield buffer for the BSL3 pressure.

On the subject of doors, they are to be minimized, and exterior windows are not permitted. That said, an anteroom leading to the laboratory is desirable. This room should be of the same construction as the lab and should also be kept at negative pressure, but not as high as the laboratory.

Air distribution

Air supply should be as dedicated as possible. At Iowa State it’s common to use central plant air with a booster fan. However, this is less than desirable, as there is some compromise adapting to existing conditions. Under this condition a HEPA filter system is installed on the supply side. This ensures that contaminated material cannot be pulled back into the plant and no unknown contaminants can enter the laboratory. The most desirable condition is to have a dedicated air supply with filtration. We modulate the supply air using a flow sensor and damper to maintain flow differential.

As far as the exhaust system, it is perhaps the most important element. The system must track the supply and other exhausts to maintain a negative state. A strobic fan or similar device prevents the exhaust from being ingested into other air inlets and dilutes the air stream. Such a fan also self regulates static pressure. Biosafety hood exhaust is also a contributor to maintaining the negative environment. This must work with the main exhaust fan to maintain negative pressure. All exhaust passes through a HEPA filter.


The control system is critical in a BSL3. We started by using a programmable chip device. This turned out to be a mistake. The controller must be able to be adjusted and reprogrammed on site to compensate for varying conditions. The controller should be set up to monitor supply air, exhaust air and static pressure in the laboratory and anteroom. It also needs to be a reference source. We chose to control the pressure by regulating the damper controlling air from the anteroom and the by-pass damper. Quick-acting dampers were located in the supply duct and on the exhaust of the biosafety hood. The biosafety damper was placed upstream of the exhaust fan to prevent backdraft through the HEPA filter located in the biosafety cabinet. The supply-side damper must respond quickly enough to prevent the spread of air generated during the decay of the fan on shutdown as it may cause the BSL3 to go into a positive pressure scenario.

Pressure management is very important in the safe operation of the BSL3. We found that operating the anteroom at -0.05 to -0.08 in. of water and the laboratory at -0.1 to -0.2 worked well. Readings lower than these proved difficult to control. Alarm points are sounded when the laboratory pressure reaches -0.05. When this occurs the supply fan is shut down, the supply damper closed and the exhaust fan ramped up. Reset is activated by pressing the reset button. This action should stabilize the laboratory; if it doesn’t, experienced building service people are called.

Final considerations

Access to the lab is controlled with magnetic locks that prevent opening of the anteroom and laboratory doors at the same time. The exterior anteroom door is controlled by a card reader. All locks default to open in the event of activation of the fire-alarm system. An emergency push button is also in place to allow exit in the event of excess pressure. The push button drops out the shunt trip breaker that shuts down the fans.

Emergency power is required for the supply and exhaust systems. This is needed to keep the laboratory negative in the event of an electric power outage. We found it desirable to have the lighting, fire and security systems also on backup power.

What About Radioactivity?

HVAC experts should know something about radiation and its control for a variety of reasons. First, planning and designing effluent systems for laboratories and other industrial facilities requires some knowledge of this hazardous material and how it is used. Secondly, maintenance and repair of these systems will require some near contact with potentially contaminated components such as fume hoods, blowers, flow dampers and ductwork. Finally, the rare need to dismantle these systems will require the knowledge and training of specialized contractors to properly and legally dispose of system components contaminated by radioactivity.

Although some engineers and technicians may have acquired an understanding of radiation, it is a specialized field. Don’t feel singled out if your knowledge is zero or based on hearsay. Radiation safety specialists are known as health physicists and radiation safety officers (RSO). If the institution that you are working for already uses radioactivity on site, they must, by law, have a designated RSO.

The role of the health physics consultant will change depending on whether a new air-effluent system is to be installed or if an old system is to be dismantled and scrapped. In all cases, however, the RSO can provide basic on-site radiation safety training to HVAC techs and engineers and describe the specific use and hazards of the air-effluent system.

The RSO or HP can provide advice about hood materials, required fume hood face velocities, the need for fan-failure alarms, the need to maintain a supply of fan and motor parts, filtration requirements, if any, and stack heights and stack placement. For many laboratory applications, special materials like stainless steel are not required. Decontamination of low-level radioactivity on accessible surfaces usually only requires an impervious, smooth surface. The chemicals used with the radioactivity would have more of a bearing on the choice of materials. Although not a necessity, if inner surfaces can be detached with relative ease from the hood frame, disposal of permanently contaminated surfaces becomes easier. Special, high-flow rates for most lab uses of radioactivity are also not required. Face velocities of 60 to 100 fpm (sash fully open) are adequate, recognizing that room air patterns and sash heights affect hood performance.

For an article on radiation and ductwork, including decommissioning, visit the HVAC community at