AUSTIN, Texas—Devices that make particles electrically charged to remove them from circulating air don’t do much good and can generate substantial amounts of ozone, according to a study of five commercially available air cleaners tested by a University of Texas at Austin architectural engineer.
“We should be careful about what we are willing to call an air cleaner,” said Dr. Jeffrey Siegel. “If it’s not that effective and it creates chemicals that are harmful to people without removing them, then the technology in the devices needs improving.”
Dr. Siegel stands in the indoor air quality testing chamber. To his left are a HEPA filter and an ion generator. Equipment that measures ranges of particle sizes sits on the right.
|Photo: Jennie Trower|
Siegel presented the research findings at the 10th International Conference on Indoor Air Quality and Climate held Sept. 4-9 in Beijing, China. The findings are the most comprehensive review of this type of air-purification device to date, and were peer-reviewed before publication in papers that form the proceedings of the meeting.
The air-purification devices he studied are a small, but growing, part of the portable-air-cleaning industry that makes about $500 million in sales annually. Siegel’s review of industry trends suggests at least 1 percent of American homes have ion generators. These devices propose to take advantage of the fact that particles of opposite charges attach to each other in the same way that magnets of opposite polarity attract.
In ion generators, a particle in the air that has picked up a negative or positive charge (become ionized) will become attached to metal plates in the ion generator that carry the opposite charge. But the high-voltage wire used to ionize air particles also converts oxygen in the air into ozone, a highly reactive molecule that irritates the lungs and can produce other harmful byproducts.
The five air cleaners Siegel tested in stainless steel rooms in his laboratory emitted an average of 2.9 milligrams of ozone per hour. When used in a typical home, this translates to what would be an increase in outdoor concentration of ozone of between 4 and 20 parts per billion (four to 20 molecules of ozone for every billion air molecules).
A separate device sold as an air cleaner/ozone generator that was used for comparison produced an even higher level of 31 milligrams of ozone per hour.
The results alarmed Siegel because a 2004 study of 95 urban U.S. communities suggested exposure to an increase in outdoor ozone of 10 parts per billion was associated with an increased risk of death the following week of 0.5 percent. That increased risk would amount to about 317 additional premature deaths a year in New York City, based on 2000 statistics when there were roughly 8 million city residents.
“There’s substantial evidence that ground-level ozone is a bad thing,” Siegel said, “so why would you want to bring something that generates ozone into your home?”
Though one of the ion generators tested only emitted 0.75 milligrams of ozone per hour, a High Efficiency Particulate Air (HEP) filter-based device worked an average of 10 times better than the ion generators at removing pollutant particles in a separate test—without producing any ozone.
In part, the HEPA filter tested alongside the others at Siegel’s laboratory on the J.J. Pickle Research Campus won because it could handle a larger volume of air in the first place. A device that takes in more of a room’s air can remove more contaminants from it. The midrange HEPA filter device Siegel evaluated in a separate test for air flow could handle 377 cubic feet of air per minute. The best ion generator tested the same way, which involved attaching a custom-made chamber to its air outlet, topped out at 44 cubic feet per minute.
A typical house receives at least 100 cubic feet of fresh air from outside every minute. An air-cleaning device needs to have a flow rate that can handle more than that volume to effectively remove airborne particles.
“You could have the best air cleaner in the world, but if it had a low flow rate, it wouldn’t do any good because it’s not moving the air,” Siegel said.
To ensure the accuracy of the tests of each device’s ozone-generating capacity, Siegel and colleagues determined how much air exchange occurred between the stainless steel chambers and the laboratory space surrounding them. They also factored in how much of the ozone that ion generators produced reacted with chamber walls, fans that keep the chamber air circulating and other surfaces.
The same care was taken with the third test that the HEPA filter excelled at: measuring how well each device removed pollutant particles. This was done by testing the air in a chamber after incense was burned for 15 minutes in the presence and absence of an air cleaner.
The presence of fine particles produced from the incense was evaluated using an optical-particle counter. These particles were about half a micrometer wide (100 to 200 times thinner than a human hair), and were measured by how they scattered laser light produced by the machine. Siegel’s lab also tested for larger particles (up to 1 micrometer) using an aerodynamic particle sizer, which measures particle size from the movement of particles between two beams of light.
Air pollutants of about one tenth to 1 micrometer in size include those produced from ozone reacting with chemicals in carpets or other indoor material, and from smoking or cooking. Mid-size pollutants of between 1 micrometer and 10 micrometers include dust, mold spores and the allergen-producing remnants of dust mites. Pollen tops the list with a size range of 20 to 40 micrometers.
As in similar studies done previously, the HEPA filter worked about five times better at removing particles than most of the ion generators. The clean-air-delivery-rate of the filter based on removal of particles of different sizes was about 118 cubic feet of clean air per minute, versus about 44 cubic feet per minute for the ion generator that did much better than the others.
Some of the ion generators were able to efficiently remove particles when exposed to chemical remnants of incense for a single pass in a fourth test, but their poor air-flow-handling meant they couldn’t match the HEPA filter.
Updating ion generators with ozone scrubbers, which are typically attached to photocopiers and other devices to soak up any ozone they produce, would be one way for air cleaning manufacturers to address ozone concerns. However, the one ion generator with an ozone scrubber Siegel’s group tested still released 70 percent as much ozone as an identical unit without a scrubber. Siegel also said he hoped manufacturers make other advances based on future scientific findings.
“The manufacturing industry has a very active and important role in helping consumers find ways to clean up indoor air,” Siegel said, “and I definitely want to encourage their innovation in developing better technologies and standards.”
Siegel’s research is partly supported through the International Society of Exposure Analysis with funding from the American Chemistry Council. He has collaborated closely with civil, architectural and environmental engineering Professor Rich Corsi and their ion-generator research team, which includes graduate students Xiaorui Yu and Ping Zhao.
For more information contact: Becky Rische, College of Engineering, 512-471-7272.