Archive for the ‘Filtration’ Category

Plants as indoor air cleaners

December 9, 2009

Here we go again.  WebMD reported this week that HortScience had an article in August about research on using house plants to remove indoor air pollution.  Interestingly, the article did not report anything about been there, done that.  Well, not me personally.  But, a few years ago, NASA was doing similar research.  At that time, a bunch of people were proposing that plants could be used to clean indoor air.

The research seems to be clear.  Yes, some plants appear to be able to remove some indoor pollutants.  The truth is in the practicality.  Usually, these studies are designed using injection of an initial concentration of a chemical inside an airtight container that contains the plant.  The chemical’s concentrations are then monitored over time to determine how well the plant removes the compound.  The first possible problem with this kind of research is that some of the compound could be lost to surfaces inside the container.  If this research is like most of this kind of research, such losses are not included in the study details.  Another problem is that, over time, chemicals can also break down, depending on the environment and the chemical.  Again, study details don’t usually describe how this loss is accounted for.  And a final detail about the study methodology is sometimes overlooked, and that is loss of chemical to the study.  Each time a sample is removed, the chemical concentration inside the container decreases.

Those facts were about the practicality of the study design.  Reality is probably the least practical aspect of the study results.  We don’t usually have only one chemical floating around in the air.  Does that mean if a whole bunch of plants would be needed if they each have their own chemical they like to dine on?   Realistically, chemicals are not put into the environment in one squirt and then left to decline over time.  Some chemicals are emitted into the air over time, such as those from building materials.  Even more complicating is the fact that chemical concentrations can vary over time because the emissions from materials vary over time, depending on a range of environmental factors.  I have collected this kind of sample data myself, as have many other researchers.  The changes in chemicals floating around a building are pretty amazing to study, actually.

In our buildings, air is constantly being exchanged–moving in and out.  No building is airtight like the containers used for most plant studies.  Air coming in also can carry chemicals into a space and air leaving can taken them out, which is also another reason why chemicals and their concentrations can vary dramatically over time.  Changes in the rate that air enters and leaves a space can be controlled by factors inside and outside the building.  Wind direction and speed, house design, sun and many other factors can influence air movement inside a home.  Operation of the air handler, people movement, heat sources, interior design and many other factors can influence air movement inside a home.  As vapors, chemicals are also influenced by the Ideal Gas Laws, in particular the part about partial pressures which says that vapor pressure of any one chemical in one area will equilibriate with vapor pressure of the same chemical in an adjoining space if the chemical can move between them.  So, chemicals at a higher concentration inside compared to outside a building will eventually equilibriate with concentrations in the outside air.

Materials inside a building also absorb and re-emit chemicals.  Ever smell cigarette odor on non-smoker who has been in a smokey bar?  The fixed things in buildings, the things of which the building is constructed, as well as the solid materials floating around in the air and the other things moving through the building, such as people, pets and their related materials, all absorb and re-emit chemicals.  The researchers call things that absorb sinks and the things that emit sources.  A dynamic exchange is going on between these things all the time.  Further, something that is a sink at this time can become a source when a chemicals concentration changes.  So, what is a plant to do?  Then again, how many plants should a person have when the emission rates could be constantly changing?

But, let’s dig into that question a little more.  How many plants would be needed.  Past research has shown that plants absorb a definable, but minute amount.  Such research predicted that a small forest would be needed to make a significant dent in the chemical concentrations.  Again, we are talking about static chemical concentrations, whereas most buildings have dynamic concentrations.  That brings me to another subject that seems to often overlooked.

Past “Plants Eat Chemical Compounds” researchers also took a lot of criticism for one major overlooked factor.  Plants grow in soil.  Soil contains–dare I say that M word–yes, mold!  In fact, many plants have a symbiotic relationship with certain molds to help them absorb nutrients.  But, many molds themselves emit chemicals into the air.  So, a person has to ask, is it practical to use plants to absorb one chemical when the soil in which they grow could be emitting a whole bunch of other ones?  Probably not.

NASA had a good reason to research whether plants absorbed chemicals or not.  NASA was interested in using plants on spaceships making long journeys where plants might be used for food and to help control the air quality inside the ship.  Currently, NASA uses chemical filters to control chemicals emitted into their ships.  On a long journey, those filters could become saturated, and in some cases then start re-emitting chemicals back into the air.  Having something that absorbs and holds the chemicals would be nice.  Having something that absorbs those chemicals and then turns them into a product that the spaceship residents can use would be great.  Now, given we are kind of like astronauts flying along on this great ship Mother Earth, we still have enough air to take care of plants, chemicals and us.  The amount of air around us is currently adequate to support a recycling system that maintains a reasonably (although I admit debatably) healthy air quality.  Other systems inside out buildings do a better job than plants.

For solid materials floating around in our building air, particle filters do a very good job–as long as air is moving from the building through the filter.  For gaseous materials, chemical absorption filters can work well; but in most cases, they are impractical for many buildings.  More effective, though, is dilution where outside air is brought indoors to reduce chemical concentrations.  And that is where I am going to leave things for right now.  I promise to cover more about control of gaseous chemicals in a future blog.  Suffice to end this blog this way:  use plants to make you feel good mentally; but use dilution or filters to make you breathe well.

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