The risk of any potential release of biological material(s) to the community is low to negligible. There is an extreme amount of redundancy of safety equipment at these facilities that would preclude any potential release of material(s). For instance, the reference Biosafety in Microbiological and Biomedical Laboratories [BMBL] (which is an advisory document recommending best practices for the safe conduct of work in biomedical and clinical laboratories from a biosafety perspective, and is not intended as a regulatory document) states that all procedures involving the manipulation of infectious materials must be conducted within a biological safety cabinet or other physical containment devices. Thus, a barrier is placed at the immediate level of a hazard. In addition, a ducted air ventilation system is required. This system must provide sustained directional airflow by drawing air into the laboratory from “clean” areas toward “potentially contaminated” areas. The laboratory shall be designed such that under failure conditions the airflow will not be reversed. The BMBL can be accessed at https://www.cdc.gov/biosafety/publications/bmbl5/bmbl.pdf.
For biosafety level 3 (BSL-3) laboratories (appropriate for agents with a known potential for aerosol transmission, for agents that may cause serious and potentially lethal infections and that are indigenous or exotic in origin), a ducted air ventilation system is required. This system must provide sustained directional airflow by drawing air into the laboratory from “clean” areas toward “potentially contaminated” areas. The laboratory shall be designed such that under failure conditions the airflow will not be reversed (redundancy). The laboratory exhaust air must not re-circulate to any other area of the building and the laboratory building exhaust air should be dispersed away from occupied areas and from building air intake locations or the exhaust air must be filtered through a high efficiency particulate air (HEPA) filter. A HEPA filter is defined by the United States Department of Energy (DOE) standard adopted by most American industries to remove at least 99.97% of airborne particles 0.3 micrometers (µm) in diameter.
To the best of our knowledge, all air exhausted from biosafety level 3 laboratories at Fort Detrick is HEPA-filtered. Notably this level of engineering exceeds the recommendations of the BMBL publication for BSL-3 laboratories.
For biosafety level 4 (BSL-4) laboratories (appropriate for dangerous and exotic agents that pose a high individual risk of aerosol-transmitted laboratory infections and life-threatening disease that is frequently fatal, for which there are no vaccines or treatments, or a related agent with unknown risk of transmission). There are two models for BSL-4 laboratories: (1) A cabinet laboratory - manipulation of agents must be performed in a Class III BSC (the Class III cabinet must have a HEPA filter on the supply air intake and two HEPA filters in series on the exhaust outlet of the unit); and (2) a suit laboratory - personnel must wear a positive pressure supplied air protective suit (the suit is supplied with HEPA-filtered breathing air). In addition to the criteria listed for BSL-3 laboratories, all exhaust air from both the suit laboratory and cabinet laboratory, decontamination shower and fumigation or decontamination chambers must pass through two HEPA filters in series before discharge to the outside environment.
The Code of Federal Regulations (CFR) defines a maximum credible event (MCE) as a hypothesized worst-case accidental explosion, fire, or agent release that is likely to occur from a given quantity and disposition of explosives, chemical agents, or reactive material. One example of an MCE at the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID) follows:
“The first MCE scenario for a BSL-3 laboratory accident occurs during the processing of 1 liter (0.26 gallons) of slurry containing Coxiella burnetii, the causative agent of Q fever, to prepare an experimental vaccine. The infective dose for this species of bacteria ranges from 1 to 10 organisms. During this process, a centrifuge rotor holding six 250-milliliter (8.45-fluid ounce) polypropylene centrifuge tubes is fitted with O-rings; each tube contains 165 milliliters (5.58 fluid ounces) of slurry. The 990 milliliters (33.46 fluid ounces) of slurry contain a total of 9.9 x 1012 (9.9 trillion) human infective doses (HID50) of the organism. One HID50 is the dose that infects 50% of exposed humans. In this scenario, a laboratory worker fails to use rubber O-rings to seal the centrifuge tubes and fails to properly tighten the safety centrifuge caps designed to prevent leakage into the centrifuge compartment that houses the rotor. All six tubes spill slurry into the rotor cups, and some of this slurry leaks into the rotor compartment, which is not sealed against the release of organisms in a small-particle aerosol. It is assumed that 10% of the slurry spills, of which 1% leaks into the rotor compartment, where 0.1% of the leakage is aerosolized. It is further assumed that 90% of the aerosol settles as liquid droplets inside the chamber. Thus, 10% (spilled from tubes) x 1% (leaked from rotor cups) x 0.1% (aerosolized) x 10% (did not settle out) = 0.00001% of the original slurry placed in the centrifuge tubes for processing would be released into the room. The most serious consequence of this laboratory accident would be the release of enough concentrated aerosol to override the air filter system, allowing the subsequent release of a significant number of infectious doses into the surrounding community. Following the assumptions above, 9.9 x 105 HID50 are presented to the filter. Further assuming that the air filter system is 95% efficient, approximately 5 x 104 HID50 (5% not removed x 9.9 x 105 HID50) would be released to the atmosphere from the exhaust stack. Using a simple Gaussian plume dispersion model in HPAC [Hazard Prediction and Assessment Capability (HPAC) modeling system developed by the Defense Threat Reduction Agency] with weather condition parameters of USAMRIID for each calendar month, the worst-case total exposure of a person breathing ground-level air would be less than 1 HID50 of Coxiella burnetii at a distance less than 2 meters (6.56 feet) from the stack. This concentration of organisms would pose no risk to human health.”
Additional scenarios of a maximum credible event are contained within the document Programmatic Environmental Impact Statement - Chemical Biological Defense Program, March 2006, Appendix C, Hazard Analyses, and can be viewed at