Abstract

Ozone is a potent germicide that has been used extensively for water purification. The germicidal activity of ozone in water has been reported by many authors (see for example U.S. EPA, 1999); however, there is limited information on the biocidal activity of ozonated air as a treatment for contaminated surfaces. Understanding of the biocidal capability of ozone against the microorganisms primarily responsible for indoor air quality biocontaminant problems is still relatively limited.

In a previous research project, we evaluated the biocidal efficacy of three to 10 ppm ozone on selected microorganisms in laboratory chamber studies under controlled conditions (Foarde et al., 1997). The organism challenge consisted of vegetative organisms or spores dried on surfaces. The study was conducted in two phases. First, an extensive series of tests employing glass slides as the test surface were performed under ideal conditions of ozone exposure in which intensive efforts were made to minimize (or eliminate) ozone losses in the chambers. Second, a short series of tests was performed using building materials as the test surfaces. We found that ozone concentrations of 6 to 10 ppm were required to obtain 3-log reductions in colony-forming units (CFUs). The results from the second phase of the study, where spores of Penicillium spp. were deposited on actual building material surfaces, showed no reduction in CFUs after a 23-hr exposure to 9 ppm of ozone. For the denser materials (ceiling tiles), test levels of ozone were not attained, probably due to the reaction with the substrate.

The objective of this project was to expand on work from the earlier study by testing the effect of ozone at much higher levels (up to 1000 ppm for 24 hours) on a variety of microorganisms. The goal of these experiments was to ascertain the biocidal efficacy of ozone against four organisms - two bacteria (one spore and one vegetative organism) and two fungi (one spore and one vegetative organism). A series of experiments was performed using either glass slides or gypsum wallboard as the test surface. This series of experiments confirmed the results of the earlier experiments that the organisms on glass slides were more readily killed than organisms on building materials for higher levels of ozone. It would be reasonable to assume that the difference in ozone efficacy between the two test surfaces was due at least in part to the ability of the gypsum wallboard to inactivate the ozone and thus to protect the spores deposited on its surface. Also as in the earlier experiments, increasing RH increases the biocidal capability of ozone.

Because adverse health effects differ by organism and susceptibility of the exposure population, no standard acceptable level of contamination exists, nor does any required level of efficacy for decontaminating building materials in the field, therefore, a key issue in evaluating the efficacy of any biocide, including ozone, is to determine the acceptable number of CFUs remaining after treatment. For example, in these experiments the inoculum was usually at least 1 x 10⁶ CFU. A 1 log reduction (90% inactivation efficiency) would mean that 1 x 10⁵ CFU remained after exposure. If a 4 log reduction (99.99% inactivation efficiency) was attained, there would be 100 CFUs remaining after exposure. The acceptability of either of these inactivation efficiencies would depend on the specific situation.

Although the specific results vary depending upon the test organism and the test surface, the overall results of this study indicate that, even at relatively high concentrations of ozone, it is difficult to achieve significant inactivation of organisms on material surfaces. The high ozone concentrations used in this study would probably be difficult to maintain near or at the surface of some commonly contaminated building materials, and even if these concentrations could be maintained in the field, it would be challenging to achieve a significant reduction of surface biocontamination using ozone.

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Marc Menetrez

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