How Portable Air Purifiers Work

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How Portable Air Purifiers Remove Gases and Odors from Indoor Air

Source removal and increased ventilation are excellent ways to reduce concentrations of chemical compounds from indoor air. However, as stated above, these strategies have their drawbacks.

Mechanical air purifiers may also have additional filters designed to capture odors and gaseous pollutants. These filters are most often activated charcoal (sometimes referred to as carbon or active carbon filters), which is used to remove gases and odorous substances from the air by adsorption.

Adsorption is a process whereby substances stick to the outside surface of an adsorbent medium without a chemical or physical transformation of either the adsorbent or the material being adsorbed. This is in contrast to absorption in which a substance is taken in and made part of an existent whole.

Activated charcoal is made by treating charcoal with oxygen, which has the effect of creating countless holes or pathways in the charcoal. The result is a charcoal with an amazingly large surface area. As air passes through the activated charcoal, gases are caught in countless bonding sites.

There are other sorbents also used for gas adsorption that have been developed to remove specific gases. These are typically "chemisorbents" which have been specially treated with various chemically active materials such as potassium permanganate or copper oxide. These chemicals will generally react with a specific or limited number of gaseous contaminants.

There is currently very little scientific data to assess the overall effectiveness of portable air purifiers in terms of reducing gaseous pollutants. One of the reasons for this is the sheer number of factors that influence the effectiveness of filters designed for removal of gases.

These include: the type of pollutants in the environment; the concentration of the pollutants; the airflow through the filter; other chemicals and the humidity in the environment; the physical and chemical characteristics of the pollutants and the filter material; the configuration of the filter material in the air purifier; the amount of adsorbent and the depth or thickness of the "bed" of material that the gases pass through.

Filters made with sorbents like activated charcoal have a limited capacity to capture gaseous pollutants. As the filter "fills up", the ability of the filter to capture additional pollutants decreases and will stop working once all the bonding sites are filled with trapped impurities, requiring that the filter be replaced.

There are also concerns that activated charcoal may reemit pollutants after they are captured. Some reputable studies have shown that practically all of the gaseous pollutants captured by such filters may be slowly reemitted back into the environment.

Activated charcoal does, however, remove residential odors to imperceptible levels, even in fairly small quantities. And for odor removal, this type of filter is an excellent choice.

But the lack of odor does not mean that the pollutant has been removed from the environment because many chemicals may produce negative health effects at levels that do not produce odor.

And, testing of gaseous pollutant removal using activated charcoal has been done using only high concentrations of the contaminants. There is little information on how effective such filtration is on low levels of contaminants, which are the concentrations normally found in indoor air.

This doesn't mean that such filters are ineffective. What it does mean is that more testing is required to definitively state the ability of such filters to remove chemical compounds from indoor air and to what degree.

It is also clear that the amount and configuration of the sorbent material is very important and that the useful lifetime of such filters may be much shorter than the marketing claims of air purifier manufacturers.

There are currently several reputable manufacturers of air purifiers that are taking removal of gaseous contaminants very seriously, with various models of air purifiers and gas removal filters designed specifically for various chemical contaminants. These filters often contain large amounts of sorbent as well as special additives for improved performance against specific contaminants.

However, in many cases, these filters are designed for reduction of only low concentrations of the specified pollutant and/or no laboratory testing has been performed to substantiate the removal claims.

This is further complicated by the fact that actual indoor air quality improvements depend not only upon the performance of the air purifier, but also on a host of factors specific to the environment the unit will be operating in, such as, the source and concentration of the contaminant; the size of the space; the operating speed of the air purifier; the number of air cleaners placed in the space and the level of saturation of the filters at any given moment.

Therefore, while an air purifier may reduce or control various odors and gases to some degree; it should not be expected to adequately remove all of the gaseous pollutants present in the typical indoor air environment.

If there are gaseous contaminants in the indoor environment, it is very important to determine and reduce or eliminate the source, if at all possible. Increased ventilation to dilute such pollutants may also be helpful.

How Portable Air Purifiers Remove Gaseous Contaminants with Ozone

Once again, the U.S. EPA states that, "for many of the chemicals commonly found in indoor environments, the reaction process (with ozone) may take months or years. For all practical purposes, ozone does not react at all with such chemicals." EPA further states that "for many of the chemicals with which ozone does react, the reaction can form a variety of harmful or irritating by-products."

Again, it is clear that ozone generators are unsuitable for use as air purifiers.

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