Jeffrey Perkel has been a scientific writer and editor since 2000. He holds a PhD in Cell and Molecular Biology from the University of Pennsylvania, and did postdoctoral work at the University of Pennsylvania and at Harvard Medical School.
In tissue culture, aseptic practice is the name of the game. Rich media can easily be contaminated, and so too can culture dishes. At the same time, lab workers must be protected from potential biohazards in their plates.
The solution, of course, is the tissue-culture hood, or more properly, the “biosafety cabinet” (BSC). These boxes, usually about 4 feet long and perhaps 2.5 feet deep, are ubiquitous in cell-culture laboratories, providing a physical and airflow barrier between researchers and their research subjects.
“The biological safety cabinet is considered the primary engineering control [in the tissue-culture lab],” says Cybelle Guerrero, North American sales director at Baker Co. “When a scientist is doing tissue work and generating aerosols, the work is contained in the BSC.”
BSCs aren’t particularly expensive—they can cost up to $10,000, depending on features, says David Phillips, technical application specialist at Thermo Fisher Scientific. But they are high-use items, and they must last a long time. It therefore makes sense to consider some key variables before your next purchase.
A question of class
According to Phillips, BSCs are available in three classes. Class I cabinets use inflowing air to protect the user, but not the sample; they are used, for instance, to house instruments that may generate aerosols or to check suspect mail for potential hazards. Class III cabinets are sealed glove boxes used in maximum-contamination (biosafety level 4) labs.
The most common cabinets, by far, are Class II devices—specifically Class II, Type A2, hoods, which use flowing air and HEPA filters to protect the user from the sample and vice versa. According to Phillips, 90% to 95% of all BSCs sold are Class II cabinets. “These … protect both the user and the cell culture, or whatever it is they are working with,” he says.
A Class II, Type A2, cabinet uses filters to block aerosolized biologicals from escaping the hood. But chemical vapors are not retained. To capture those, users need to connect their hoods to building exhaust. Type A2 cabinets can be connected to the building ventilation system via an exhaust canopy or “thimble,” says Phillips. But researchers working with higher levels of, for instance, radionuclides should consider Type B1 or B2 cabinets, which are hard-connected into dedicated exhaust vents rather than out into the lab.
According to Guerrero, Type A2 cabinets should only be used for "minute traces" of volatiles. "When working with higher concentrations, a lab will usually move to a B type cabinet because it has higher vapor handling characteristics." (According to Baker's web site, Type A2 cabinets recirculate 70% of the air in a BSC and exhaust 30%; Type B1 recirculates 40% and exhaust the rest into building ventilation; and Type B2 cabinets recirculate none of the air, exhausting 100%.)
Type B2 cabinets cost more than A2 cabinets, says Phillips. But the real difference is in cost of ownership. He says it costs perhaps $4.50 per cubic foot per minute per year to run a Type B2 cabinet, just because of the cost of exhaust. “So for the B2 cabinet, you spend close to $4,000 per year conditioning the replacement air—and that’s not including the electricity.”
The four Cs
Phillips recommends users consider “the four Cs” when selecting a biosafety cabinet.
The first C, containment—“keeping the nasties in and the contamination out”—is reflected in the NSF certification. NSF is not the National Science Foundation, but rather an organization, like Underwriters Laboratories, that certifies biosafety cabinets to ensure they work as advertised.
There are other aspects to containment, too. For instance, some hoods have compensation systems that can adjust fan speed as filters become clogged, to keep airflow constant. Baker’s hoods feature negative pressure inside the side walls to prevent leaks, say from loose plumbing fixtures, from contaminating either the lab or the hood. “If any air migrates inside that side wall, it’ll get drawn into the HEPA filter,” Guerrero says.
Comfort refers to ergonomics. “Some users are in hoods six to eight hours a day,” says Guerrero. “That can be very tiresome.”
Does the unit have a vertical sash or a tilted one? What about noise and illumination levels? How easy is it to clean the unit? Thermo Scientific hoods feature a cleaning position for the sash that enables users to easily clean the side walls and top of the hood. Some Baker hoods come with hydraulic lifts to adjust the cabinet’s vertical height.
Convenience involves things like electrical ports, air lines and so on. Do users need to string power cables and tubing under the sash and through the front of the hood, or can they pass them through ports in the rear of the BSC? Baker can modify the front sash to include a microscope optical dome, if users need to keep a microscope in the hood.
And finally there’s the fourth C, cost. BSCs tend to cost about the same, but their operating costs can be substantial. According to Phillips, BSCs used to consume about the same amount of power as a small house (800 W). Today, hoods draw far less—180 W, in the case of some Thermo Scientific units.
Many hoods also feature reduced-flow modes for when the hood is not in use but work is not yet complete. For instance, says Guerrero, users of Baker hoods can simply close the sash, and the cabinet automatically lowers the fan speed to maintain Class II conditions while using less power. Thermo Scientific BSCs offer the same feature, Phillips says, which increases the life of the blower motor and the filters, thereby decreasing the cost of ownership.
In fact, says Phillips, if you really want to keep your costs down and keep your BSC working for years, there’s one sure-fire solution: Turn the cabinet off when it’s not being used.
“If you turn off the cabinet when not in use and then turn it on in the morning and take, say, five minutes to wipe down the inner surface, the air in the cabinet would’ve been replaced about 100 times,” he says. “That cabinet is as clean inside as it would’ve been if we had left the blowers on all night.”