Clearing the Air

Air-o-Swiss

We’re used to the drumbeat of warnings each flu season, so it’s little wonder that manufacturers of air purifiers and humidifiers have seized on our microbial anxieties to tout the new germ-fighting technologies that are in their latest products.

These technologies include silver nanoparticles and photocatalytic oxidation (PCO). Unfortunately, these technologies aren’t ready for a showdown with this year’s—or any year’s—strain of flu. And experts with whom we spoke couldn’t say when (or whether) manufacturers’ claims might be validated.
    
HEAVE-HO, SILVER? If you haven’t looked at air purifiers in the past 3 years, you’ll notice that some manufacturers are pushing silver nanoparticles (also called nano-silver or just plain silver) as a germ-fighting tool. There are at least four brands of air purifiers that tout that technology, and the models start at about $150.

These air purifiers typically incorporate the silver nanoparticles into removable filters. This is microscopic warfare, experts tell us. The idea is that as bacteria and other microorganisms make contact with the tiny silver particles, they become sterilized, which prevents the growth of new bacteria.

You don’t need to replace your air purifier to use a silver filter. According to Project on Emerging Nanotechnologies, which studies nanotechnology, the number of air-purifier filters that use silver nanoparticles jumped to 43 in 2009 from four in 2008, although some are model-specific. (The numbers include portable air purifiers.) Figures for 2010 weren’t available at press time, but retailers with whom we spoke predict that the number of air purifiers that have silver will continue to expand.

Replacement filters that are coated in silver tend to be more expensive than filters that aren’t. We found around a $10 price gap, on average, between standard high-efficiency particulate air (HEPA) filters and those that have some form of silver coating. (Prices vary widely because of the difference in models and sizes.)

However, we’re not convinced that you should spend your silver on silver nanoparticles just yet, at least not until further effectiveness and safety tests have been conducted. Let’s be clear: Numerous studies confirm that silver nanoparticles can inhibit the growth of microorganisms. But their effectiveness as a sterilizer in consumer air-purifier filters has not been demonstrated, says Richard Shaughnessy, who is program director of Indoor Air Research at University of Tulsa. In other words, you should take with a larger-than-nanosize grain of salt manufacturers’ claims that your air purifier can wipe out large percentages of bacteria or flu viruses that are in the air.

We couldn’t find any studies that specifically address the use of silver nanoparticles in air-purifier filters, but a related study from April 2010 shows how murky the air is on this topic: Researchers at National Nanotechnology Center in Thailand and at Thailand’s Srinakharinwirot University found that silver’s antibacterial properties ranged from zero-percent to more than 99-percent effectiveness when it was used to coat different textiles. Unfortunately, there’s no way to determine where a particular silver filter falls on that spectrum: The tested fibers were made of cotton; HEPA filters typically are made of glass fiber.

SILVER SOAKERS. Silver nanoparticles also have made their way into humidifiers. Meg Hellerstedt of retailer Sylvane says more manufacturers are adding silver nanoparticles to combat germs. The particles typically come in the form of replaceable silver sticks (sometimes called ionic sticks), which cost between $29 and $49.

Although these sticks typically are designed for specific humidifiers, you can drop them into any model’s water tank as long as they don’t, of course, interfere with moving parts. Manufacturers recommend that you replace the stick once a year. Silver also is used as a coating on water tanks and on demineralization cartridges.