COMMENTS TO NRC COMMITTEE CONCERNING NBAF SSRI
By Thomas R. Manney
Professor Emeritus, Department of Physics and Division of Biology
Kansas State University
January 30, 2012
My comments concern an aspect of the risk that pathogens will be released by a tornado. When a strong tornado passes near a structure, two forces must be considered. First, the kinetic energy of the winds can produce catastrophic damage to inadequate structures. Second, the vortex of a tornado produces a sudden, local, severe drop in the atmospheric pressure. The pressure drop may contribute to structural damage, but it may well have additional consequences when the objective is to keep the contents of the building, especially microorganisms, from escaping.
In the SSRA , most of the discussion of risks posed by tornados has focused on the potential for structural failure or collapse. But I have found no consideration of the possibility that a sudden drop in atmospheric pressure could contribute to the release of pathogens from negative pressure containment compartments.
There are relatively few research papers reporting direct measurements of ground-level pressures near tornados. An apparently definitive paper is “Pressure at the ground in a large tornado” (W. P. Winn, S. J Hunyady, and G. D Aulich Journal Of Geophysical Research, 104, 22,067-22,082, 1999). They measured pressures near an F4 tornado in Allison Texas in 1995. They reported, “The instrument closest to the tornado recorded a pressure drop of about 55 mbar (hPa) as the tornado approached and a rise of about 60 mbar (hPa) as the tornado receded.”
Another group has summarized measurements near 24 tornados with a range of pressure deficits from 5 hPa to 194 hPa,(“Near-Ground Pressure and Wind Measurements in Tornados” by Christopher D. Karstens, et. al., in Monthly Weather Review, 138, 2570-2588, 2010).
A fundamental principal of containing microorganisms is to maintain them in a sealed chamber that is kept at a negative pressure. The pressure in the chamber must be kept below the pressure outside the chamber. A typical value for this negative pressure in a Class III biological safety cabinet is quoted as at least 0.5 inches of water. In units of inches of water, a pressure of one atm is about 400 inches, so the pressure deficit measured near the Allison, TX tornado would have been about 24 inches. The magnitude of this pressure drop is about 50 times the small negative pressure difference commonly maintained between a Class III containment chamber and its surroundings.
A Class III biological safety cabinet, for example, is a closed, gas-tight enclosure fitted with arm-length rubber gloves. All supply air is filtered through HEPA filters and exhaust air is filtered through two HEPA filters in series. An exhaust blower maintains negative pressure. Although the normal fluctuations of atmospheric pressure are themselves greater than this negative pressure difference, the fluctuations are relatively gradual so the exhaust blower can easily maintain the differential. The same reasoning would apply to larger containment spaces used for handling live animals, such as cattle, in BL-3 Ag facilities.
In the event of a tornado, however, the pressure drop is sudden. The question then is whether the system can maintain the negative internal pressure in the event of a large, sudden depression? The seals around doors in large chambers have been a historical weakness. Have they been designed to handle such a transient event (and if so, can that be verified)?
In conclusion, the apparent potential for a strong tornado to over-power the containment door seals, thereby releasing pathogens into the tornado, is a serious consideration that I have not seen discussed in the SSRA.