Ozone (O3) is a colorless, pungent-smelling gas that is a form of oxygen. It is composed of three atoms of oxygen, rather than the two oxygen atoms of ordinary atmospheric oxygen (O2). Ozone is a highly reactive, unstable molecule that is heavier than air. It has been successfully used for certain types of water purification and for the treatment of residual odors. In controlled circumstances it has been used to react with residual smoke odor to neutralize those odors.
It does not, however, get rid of the soot deposits or the physical damage caused by a fire. Similarly, ozone does not eliminate mold-related particles such as spores and hyphae, nor does it remove the damage caused by water or the growth of mold into materials.
Ozone has been successfully used to treat water for swimming pools and spas to reduce or eliminate the chlorine disinfection. According to the American Conference of Governmental Industrial Hygienists publication "Bioaerosols: Assessment and Control," aqueous ozone has been effectively used to disinfect drinking water, cooling towers and for wastewater treatment. When used to treat water, ozone concentrations less than 1 part per million are reported as being effective. Regardless of its effectiveness for controlling bacteria and other organisms in water, the ACGIH publication states that aqueous ozone is "subject to rapid degradation and certainly is not recommended for use on visible fungal growth."
Ozone in its gaseous phase was studied and found ineffective for use on surface contaminants as early as 1942. In a 1997 study conducted by Karin Foarde, et al, the ability of ozone to decontaminate fungi on a variety of building materials exposed to ozone at 9 parts per million (ppm) for a 23-hour exposure, found ozone to be ineffective.
To put this amount of ozone in perspective, 9 ppm is 90 times the present OSHA Permissible Exposure Limit (PEL) of 0.1 ppm. The 9 ppm used is this study is almost twice the amount presently determined by NIOSH to be Immediately Dangerous to Life and Health (IDLH), 5 ppm.
According to "Mold Remediation in Schools and Commercial Buildings," from the Environmental Protection Agency, "The purpose of mold remediation is to remove the mold to prevent human exposure and damage to building materials and furnishings. It is necessary to clean up mold contamination, not just to kill the mold. Dead mold is still allergenic, and some dead molds are potentially toxic."
The New York City Department of Health's "Guidelines on the Assessment and Remediation of Fungi in Indoor Environments" states that "The use of gaseous, vapor-phase, or aerosolized biocides for remedial purposes is not recommended. The use of biocides in this manner can pose health concerns for people in occupied spaces of the building and for people returning to treated space if used improperly. Furthermore, the effectiveness of these treatments is unproven and does not address the possible health concerns from the presence of the remaining non-viable mold."
The IICRC S520 Standard and Reference Guide for Mold Remediation agrees with the EPA and NYC documents when it states, "The use of treatments, such as encapsulants, sealants, ozone or ultraviolet (UV) light as a substitute for removal and detailed cleaning is generally not recommended."
It is important to remember that ozone has a strong oxidizing reaction on rubber and other materials. As a result, it may be responsible for electrical safety hazards, since it can destroy some types of electrical insulation and damage other materials. Other studies have demonstrated that ozone can actually increase the levels of formaldehyde and other chemicals when used in buildings. Ozone has been shown to increase sub-micron particles and adversely react with many compounds and artwork.
In spite of the strong evidence that ozone is not appropriate for fungal remediation activities, some publications continue to suggest it be used for this purpose. A recent study concluded that for Penicillium citrinum and other organisms, "ozone should be highly effective and provide a reliable safety factor in treating microorganism contaminated surfaces." After reviewing the article, Eugene Cole, DrPH and professor of Environmental Health at Brigham Young University, states that "the study was conducted by exposing mold to ozone on agar plates where the mold was growing. It is not surprising the mold was killed, since the agar acts like an aqueous environment. One should not expect ozone to kill mold on building materials, and even if it did the dead microbial residue would continue to pose a potential allergic or toxicogenic hazard if left in place."
It should be noted that the levels of ozone tested in the study ranged from 8 to 16 parts per million. As noted above, this level far exceeds the OSHA PEL of 0.1 ppm for an eight-hour exposure. Even if ozone was effective in killing mold on building materials, it is considered unlikely that ozone concentrations in buildings could be controlled to achieve such concentrations.
So, is there ever a time when the use of ozone might be appropriate in mold remediation? According to the IICRC S520, "With heavy mold odors, a deodorization process, such as confined use of ozone, may be desirable prior to and/or following laundering or dry cleaning."