This article was originally published in the November/December 1998 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online November/December 1998
Clean Ducts with Elbow Grease, Not Toxic Gas
by Ed Chessor
Some builders and contractors use large ozone generating devices, believing that ozone will effectively disinfect, clean, and deodorize HVAC ductwork. I do not recommend this--my experience is that ozone is too dangerous for use in air purification.
Most of the ductwork that I've seen contains accumulations of house dust and tobacco smoke solids, as well as loose pieces of fiberglass duct insulation, dead bugs, and mold growing in a damp mess. Ozone does not have a significant impact on this dirt--the ducts simply won't be decontaminated until their inside surfaces are bright and shiny. If a clean damp cloth still looks clean after wiping it over the duct surfaces, then the duct is clean. Ozone can't clean ducts like this--only scrubbing with soap and water can.
Furthermore, using ozone to fight bacteria in ductwork is not effective, even if it does kill them. If there are bacteria in the building (and there always are) they will get back into the ducts as soon as the fans start turning. We all live with some bacteria all the time.
As to mold spores, the numbers of these will depend mainly on how clean the building is and on how many spores are floating around outdoors. If the inside of the duct system is clean and dry, it will not be a source of mold. At concentrations well above what humans can survive in, ozone may do some harm to some molds, but dead mold is as big a health hazard as live mold in the sense that people have pretty much the same allergy responses to dead mold spores as to live ones. Keeping the ducts clean and dry also prevents Legionnaire's disease, which needs water in a fairly narrow temperature range to become a problem.
My experience is that ozone is dangerous to use. When--not if--the ozone gets out of the duct into an occupied space, it can cause severe asthma and other breathing problems. In a test I conducted, an ozone generator produced ozone at a concentration of 3.7 ppm in a small washroom in 21 minutes. This is a dangerous concentration. The safe limit for eight-hour exposure in workplaces here in British Columbia was lowered from 0.1 to 0.05 ppm on April 15, 1998. A level of 5 ppm is considered by Canadian authorities to be Immediately Dangerous to Life and Health.
In another case I observed, an ozone generator was used to deodorize an elementary school classroom about 60 miles from Vancouver. The residual ozone triggered a severe asthma attack in the teacher when she reentered the classroom a couple of days later.
Ozone can be safely used to disinfect water for drinking and in swimming pools, but that's the only application for which I would ever use it.
Ed Chessor is a professional engineer and certified industrial hygienist with a specialty in ventilation for contaminant control. Ozone is an industrial contaminant he has been controlling for more than a decade.
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