"While the guidelines are very valuable, they are an aid to risk assessment and not a substitute for professional judgment," he advises. "You must always assess the real risks and mitigate those risks appropriately."

Researchers assign each organism they work with to one of four biosafety levels (BSL). BSL-1 means that scientists have proven that an organism cannot infect normal, healthy humans. BSL-2 agents are infectious only if you inject yourself with them or if you ingest them. HIV, for example, is considered BSL-2 because, although it is deadly, it is relatively difficult to transmit from one person to another.

BSL-3 organisms are much more dangerous because they spontaneously form aerosols, like smoke rising from an ashtray. They must be handled within the confines of a cabinet.

The highest level of protection is required for BSL-4 organisms that spread the same way as those rated BSL-3, but are much more dangerous. To work with those organisms, such as Ebola virus, respirators and pressurized suits are required, as well as cabinets within negative-pressure labs.

"If you get infected with a BSL-4 organism, not only will you get sick, you have a high probability of dying, and you may infect other individuals as well," says Tillman.

Tillman stresses that the BSL levels are the best starting point in assessing risk, but they can't be used as the last word.

Influenza virus, for example, does not spontaneously aerosolize, so it is listed as BSL-2. But if a researcher were to infect mice with influenza via intranasal instillation, the animals might sneeze, causing droplets to become airborne. This procedure might require BSL-3 precautions even though the organism was listed as BSL-2.

In the final analysis, there is no substitute for good laboratory practices. Very few organisms can legitimately be classified as BSL-1 because they have not been proven harmless. "Unknowns" and clinical samples are normally handled at BSL-2, the "default" level of handling practices. That means that even the most seemingly benign substances deserve at least standard precautions.

Assessing the Risk

Scientists need to consider eight factors when determining how risky a particular course of research will be, and what steps should taken to mitigate those risks:

• Route of transmission, the most important consideration, Tillman says: Do the organisms readily spread by aerosols? Or must you contact them directly in order to be infected?
• Pathogenicity: How sick does it make you? Will it kill you if left untreated, or is it treatable at all?
• Stability: How long will the organism survive? Many viruses, including flu, hepatitis, and HIV, live no longer than 48 hours outside of a living thing. Anthrax spores, on the other hand, live for decades. "Had someone dropped hepatitis B in the mail in Washington, instead of anthrax, they could have closed up the post office on a hot Friday afternoon and opened it up for business on Monday without a problem," says Tillman.
• Infectious dose: How many of the organisms does it take to make you sick? It typically takes exposure to anywhere from 100,000 to 10 million organisms to infect a person, but someone can fall ill from inhaling just 10 Q fever bacterium.
• Origin: Where does the organism naturally occur? It is much less problematic to release an organism—like the adeno virus, which causes the common cold—if it's out there in the environment already, according to Tillman. The consequences of releasing an agent not present in the local environment, such as Foot and Mouth Disease, would be much greater.
• Availability of prophylaxis: "If vaccines are available to protect researchers against the organism, you might be willing to rely on a lower level of mechanical protection," says Tillman.
• Availability of medical surveillance: Would researchers know if an infectious organism escaped? A medical surveillance program could monitor workers for hepatitis, explains Tillman, so a breach in containment could be detected and fixed quickly. Mad cow disease, on the other hand, can remain dormant and undetected for as long as 10 years. If there was a fundamental flaw in the way the work was being conducted, one might not know for decades.
• Skill and training of the workers: "In many circumstances, you can compensate for a weakness in a facility with a high level of skill in the workers," says Tillman.

"You assign the BSL levels not just by looking them up, but by considering the biology of the organism and the circumstances of its use," he says.

Since a very small percentage of the microbial world is specifically listed in the Centers for Disease Control's Biosafety in Microbiological and Biomedical Laboratories (BMBL), and very few veterinary pathogens are listed, one can't use the BMBL as a sole reference, says Tillman.

For example, the BMBL lists canine hepatitis as BSL-1 because people can't catch it.

"But it's a very nasty disease for dogs," says Tillman. As a veterinarian, Tillman would treat canine hepatitis as BSL-2 because the exposure risk lies not with the workers but with the dogs in the community.

Many years ago UC Davis developed its own list of pathogens, augmenting the CDC BSL levels by adding organisms relevant for animal researchers.

Same Disease, Different Responses

Q fever provides an example of how the same disease might be handled in radically different, yet equally legitimate, ways. This bacterium is common among ruminants and does them little harm. If a young ewe contracts it during her first pregnancy, it might cause her to abort, but she will recover without difficulty. Other than that, infected sheep show no symptoms and are unaffected by it.

In humans, Q fever causes a flu-like illness that cannot be transmitted person-to-person and is treatable with tetracycline. Like the flu, many people improve with no treatment at all, but a small segment of the population—the very young, very old, and people with impaired immune systems—can become seriously ill.

Q fever is considered a BSL-3 organism because it "aerosolizes very easily," says Tillman, and it is very stable, meaning it remains infectious for a long time outside of a living thing. People can become infected by inhaling just 10 organisms, and there can be up to one billion infectious doses in 1 ml of an infected sheep's amniotic fluid.

In past years there have been at least two very significant outbreaks of Q fever in hospitals, associated with research sheep used for fetal research. Many of the infected workers had relatively little contact with the sheep. Infected workers included physical plant staff and even laundry workers.

Yet in agricultural settings, such as the lambing barn at UC Davis, the environment has substantial contamination by sheep amniotic fluid, which spills out onto the barn floor. Such agricultural facilities are virtually always simple outbuildings directly vented to the outside. How is this safe?

First, the disease is endemic in most of the United States, being naturally present in the vast majority of normal sheep flocks. People who live near sheep and those who work with them are largely immune because of their long-term exposure. Preventing the release of the organism into an agricultural environment is not the most important issue. One protects the workers by not bringing vulnerable workers, those who lack any occupational exposure to sheep, to the areas in which sheep are used.

"We would never discharge something like Foot and Mouth Disease into the environment, at any level," he says.

It might be OK to work with Q fever without HEPA filtering exhaust air, provided that the exhaust air was discharged into a rural environment far from susceptible individuals. In urban settings such as a hospital or laboratory, Q fever is treated with the caution afforded other BSL-3 organisms.

Risk is Rarely Clear-Cut

Sometimes assessing risk is easy. Some listings in the BMBL include an "Agent Summary Statement," which is a detailed case study of an organism that has actually caused laboratory infections. Agent Summary Statements are based on experience, not theory, and "they are the gospel," says Tillman.

In all other cases, it is important to balance the judgment of the most experienced researchers against strict safety regulations, he says. Both points of view have pros and cons.

Long-time scientists can offer insight gained from years of personal experience. Safety officials are helpful because they are familiar with the most up-to-date technologies for mitigating risk. Scientists may tend to err by being too relaxed with the organisms, since by long familiarity with the agent they know how far they can push the limits of their facilities. Safety officers may tend to err in the direction of being overly cautious, perhaps by applying every available technology even though the incremental gain in safety may be minimal. The optimal set of practices will represent a balance between the two approaches.

Don't be lulled into complacency even if the design of the lab is state-of-the-art, Tillman cautions. Even the best facility design may fail to protect the workers without ongoing maintenance and monitoring. Years ago at UC Davis, four workers were exposed to Psittacosis, which produces flu-like symptoms that can last as long as a month. The workers were exposed by a group of apparently normal laboratory pigeons. The animal rooms in which the pigeons were housed were designed to be operated at negative pressure relative to the corridor, but an investigation of the outbreak revealed that the rooms were actually positive to the corridor. The exhaust vents from the rooms had become clogged with pigeon feathers.

"The finest design in the world isn't good if you don't have good maintenance," says Tillman.

By Lisa Wesel