The company explained that with Bacillus anthracis, it is important to differentiate between the vegetive and spore forms. Spore forms are most likely to be used for bioterrorism, the company said, “and are much more robust and difficult to disable with most disinfectants, including ultraviolet light.”

After analyzing the results of three published studies (Sharp 1939, Knudson 1986, and Dietz 1980), as well as prior work completed at Pennsylvania State University relating to airborne pathogens, UVDI’s previous position stating that it is necessary to provide high dosages of UV to “kill” Bacillus anthracis, was confirmed.

Data from the 1986 Knudson study were analyzed and charted. From this, the microorganism’s sensitivity (“rate constant”) to UV light was calculated.

Results indicated that the dose required for 90% disinfection could be as high as 74,000 microwatt seconds/cu cm, or 220,000 microwatt seconds/cu cm for 99.9% disinfection. UVDI explained that “This organism, in spore form, would require about 50 times the UV dose required to disinfect smallpox, tuberculosis, and legionella. It is even more robust than the Aspergillis Niger spore that is considered one of the toughest airborne pathogens.”

According to David Witham, vice president-Technology at UVDI, “This leads us to conclude that control of anthrax by UV alone is not practical.” However, he went on to point out that “due to its relatively large size, about 1.2 microns in its most lethal single-cell size, it can be effectively filtered. ... Once captured on a filter, it can be readily disinfected with UV light.”

Other pathogens, such as smallpox, can be readily handled by UV alone, but may be more difficult to filter, he added. Solutions with the proper UV dosage and filter efficiency can be tailored to most hvac systems.

Publication date: 01/01/2002