Negative ions are found in areas of moving water (the beach, waterfalls, rivers), dense trees, and high altitudes. It is thought that the negatively charged air in these areas can increase the flow of oxygen to the brain and improve mental alertness. Negative ions are believed to increase serotonin, mood, and energy. Columbia University studies of chronic depression have shown that negative ion generators relieved depression as much as antidepressants. “Ionizing” or “plasma” air cleaning devices claim to replicate the negative ions found in nature in an indoor setting, emitting negatively charged particles to cling to and remove viruses, mold, bacteria, and even foul odors. They also claim to emit no ozone. These devices are the most popular air cleaning devices on the market, featured in stores such as Bed, Bath and Beyond, and are available at relatively affordable prices. Below we examine these and other types of filters for safety and effectiveness. Not safe: Anything Generating Ozone

Ozone is a molecule of three oxygen atoms; two oxygen atoms form the basic oxygen molecule and the the third oxygen atom can detach from the ozone molecule, and re-attach to molecules of other substances. do air purifiers help with dust allergiesSome air purifiers, known as “ozonators” market the ability of ozone to attach to other substances as a way to clean the air. air purifiers babies r usHowever this is precisely what makes ozone dangerous, in that it can react with organic molecules that makes up the body. wearable air purifier reviewsWhen inhaled, ozone can damage the lungs, reduce lung function, increase respiratory symptoms, aggravate asthma or other respiratory conditions. Ozone may worsen chronic respiratory diseases and compromise the ability of the body to fight respiratory infections. 

Even low amounts of ozone can cause chest pain, coughing, shortness of breath, and, throat irritation. Several harmful compounds can also be created when ozone is produced, such as nitric oxides. The FDA requires ozone output of indoor medical devices to be no more than 0.05 ppm (or 50 ppb). For more information see  “Ozone and Your Health“. Several federal agencies have established health standards or recommendations to limit human exposure to ozone. “Good up high, bad nearby” is a term used by the U.S. Environmental Protection Agency (EPA) to distinguish between ozone in the upper and lower atmosphere. Ozone in the upper atmosphere is known as “stratospheric ozone” or the “ozone layer” which helps filter out ultraviolet radiation from the sun. It is the ozone in the lower atmosphere, the air we breathe, that can have harmful effects on the respiratory system. Ozone is used safely to purify water, but it is not safe to emit into your indoor air. Ozone does react with some chemicals and removes the molecules from the air, but new harmful chemical compounds are formed as a result and they can be extremely reactive, irritating, and corrosive.

At levels that do not exceed public health standards, ozone does not remove chemicals such as carbon dioxide and formaldehyde, bacteria, viruses, mold, or particles such as dust and pollen from the air. This is an example of an ozone-generator. Mixed Reviews on Safety and Effectiveness: Electronic particle air cleaners, Ionizers and UV Light Devices These air cleaners can eliminate airborne particles such as mold, bacteria, viruses, and odors at the molecular level using an electromagnetic field to induce charged particles or ions in order to attract air pollutants to collector plates or filters. So-called “ionic” or “plasma technology” creates both negative and positive ions (air in a plasma state). The plasma then combines with naturally occurring water vapor to form hydroxyl radicals (unstable molecules stripped of hydrogen and oxygen atoms). Hydroxyls attack airborne pollutants that include viruses, bacteria, germs, odors, and chemical gases (VOCs). The pollutants are destroyed in the process while the hydroxyls reform into water vapor (H2O) or other harmless air molecules.

We would use extra caution with these devices, as it is the ionization process that converts beneficial oxygen molecules (O2) into highly reactive Ozone (O3) and most of these devices emit some level of ozone. Many of the popular brands, such as the Winix PlasmaWave, use a “corona discharge” purifier, which creates ozone. Since the emitted ozone is less than the 0.05 ppm recommended by the EPA, these devices can be marketed as emitting no ozone. However, it is possible they fall just slightly under the EPA’s limit and furthermore, while the device emission itself may be considered safe, the danger lies in the buildup of ozone in the room while the device is constantly running. Furthermore, in dry air, the hydroxyls may not form back into water vapor, resulting in lingering free radicals which also build up over time. Other devices that emit ozone include ultraviolet air purifiers, which use shortwave ultraviolet light to bombard oxygen molecules in the air. These molecules, once exposed to the UV rays, quickly reform themselves into ozone molecules, which obliterates most of the contaminants in the air.

Below are some popular models of ionic, plasma, and UV light air cleaners emitting low amounts of ozone. As you can see by the abundance of models available (these are only a few) this is the most popular and widely marketed type of air purifier. Some of these do not advertise the ionizing technology, it is only contained in the fine print. Honeywell HFD-010 QuietClean Compact Tower Air Purifier appears to be just a HEPA filter however reading the reviews it appears the manufacturer’s instructions indicate there is an ionizing unit in addition to the HEPA filter. Not recommended for people with asthma. ENVION Therapure UV Germicidal/ HEPA TYPE Filtration  uses a combination of the HEPA filter for dust and pollen; UV light to kill viruses, bacteria, and mold; and a photo catalyst filter for volatile organic compounds (VOCs) such as paint fumes. At least the UV / photo features can be turned on/off. Blueair Hepa Quiet 103 Air Purifier is a HEPA filter combined with ionizing technology.

Winix WAC 5500 True HEPA Air Cleaner with PlasmaWave Technology uses a combination of HEPA, a carbon-pre filter for odors, and PlasmaWave technology to neutralize pollutants at the molecular level. Upon reading the reviews for the previous model, the 5300, we discovered that Winix uses a corona discharge purifier. It is unlikely the new model has any different technology. The plasma feature can be turned on/off. Permatech Tower Air Cleaner With HEPA-Type Filter combines HEPA filtration technology with an optional ionizer. Blueair 203W HepaSilent Air-Purification System is advertised as being a HEPA filter but upon further research we discovered it also uses ionizing technology also. Rabbit Air BioGS uses a HEPA filter that lasts up to 3 years, a pre-filter to trap large airborne particles, and a washable Charcoal Based Activated Carbon Filter for odor and chemical absorption. It claims to destroy organisms that get caught by the filter but the fine print says it generates negative ions (this is not visibly advertised).

The Sharp PlasmaCluster utilizes carbon filter, HEPA, and ionic technology, that employs positive and negative charged ions to clean indoor air of odors, VOCs, and lung penetrating particles. Some models have a humidifying function. A previous model, the FP-N60CX produced “virtually” no harmful Ozone gas (claimed to be the lowest emitter of ozone of all ionic air cleaners, emitting less than 0.01 parts per million of ozone). Prices range depending on size and model $105 – $400 +. Most Effective and Safest: HEPA technology that filters over 99.7% of air particles HEPA stands for High Efficiency Particulate Air was first developed by the military during World War II and is the most widely used air filter (including in all the aforementioned devices). HEPA filters can filter out 99.7 per cent of all particulates in the air, down to 0.03 microns. HEPA filters are effective against dust, pollen, pet dander and can also trap viruses and bacteria (although these organisms would stay on the filter unless destroyed by say an ultraviolet light).

HEPA filters should be replaced, we caution against “permanent HEPA” filters which are cleanable, since it is unlikely all particles can be removed during cleaning. Notice how all the above mentioned plasma and ionic cleaners have on/off switches for these features. That is because the main function is the HEPA filter. Those concerned about the other technologies’ ozone emission are better off purchasing a stand-alone HEPA filter. Although difficult to find and not as sleek in appearance, we managed to source a few HEPA-only air filters. Homedics Ar-10 Hepa Air Cleaner 75 Cadr $79.93 Hunter 30057 Hepa Tech Room Air Purifier features a separate activated carbon pre-filter for odor reduction. HealthPro Plus HEPA Air Filter features “Hyper HEPA” technology and claims to be the first home air purifier to pass the world’s most stringent HEPA filter test, known as EN 1822. Recently selected by the Hong Kong Hospital Authority as the room air cleaner to be used in the fight against SARS and was rated the most effective air cleaner for allergies by Germany’s consumer foundation.