One notable reference is by a guy named B. C. Wolverton, who authored
the study for NASA back in 1989. This document is available on the
internet by way of an outfit called the National Foliage Foundation or
the address provided by Sherry; or people can e-mail me and I'll send
them the PDF file. To the credit of the National Foliage people, their
welcoming page features a nice picture of a Peace Lily.
I waded through the evidence described in Dr. Wolverton's paper and
found the data to be questionable. My main objection, at least on
first reading, is that they are testing air samples on an extremely
complex system, involving plants or several types, soils of
undisclosed compositions, sometimes with activated carbon and
sometimes without, and air circulation equipment present. While they
seem to have tested a few controls (the complex system without plants,
for example), one is tempted to be skeptical since what they are
trying to establish is rather subtle. The study bites off too much at
once.
Then there are the data. They describe injecting benzene into one of
their test setups. They say they added 35 microliters of benzene,
which comes out to be roughly 0.03 grams by weight. Then they drew
samples and measured the amount of benzene that was not absorbed by
the setup after 24 hours. The datum for Gerbera daisy, listed in their
Table 2, page 10, is that this particular test absorbed a total of
107,653 micrograms of benzene, which translates to 0.108 grams by
weight. When more material is removed than is added in the first place
it makes one wonder. For benzene, the experiments withdrawing more
than they put in comprise half of the data points. For
trichloroethylene, two of their data points have this problem.
We also have no idea of the reliability of the data points. It is
customary in science to provide some indication of the "plus or minus"
range (standard deviation). Here there is no information on this.
Exactly 107,653? Not credible. Is that 107,653 micrograms a result of
ten experiments, averaged? Or was it just a single experiment?
The experimental plants range all over the map regarding the amount of
leaf area exposed in a test. One plant type has a large leaf surface
area while another has much less. How are we to compare the plant
types? Maybe the amount of chemical removed is dependent on leaf area.
But this does not come up in the discussion. They should at least have
provided a calculation of the amount removed per a given leaf area
standard. It makes it difficult to understand otherwise. The same
criticism applies for listing the amount added as a volume and then
report results as a weight.
And why do we have varying amounts by weight of the three pollutants?
While the amount by volume is said to be the same (35 microliters),
the three pollutants have from low (benzene) to quite high
(trichloroethylene) specific gravities. This means that the amount
absorbed cannot be compared easily. If the amount removed is dependent
on the initial loading then the data cannot be compared at all with
any justification.
And the three pollutants are quite different, chemically. Benzene is
flammable, trichloroethylene is not. Benzene and trichloroethylene are
both quite inert while formaldyhyde reacts with a wide variety of
materials almost instantly (removing it, in a sense, from the system).
Besides that, formaldehyde converts to a polymeric form upon
evaporation. That means it is not dispersed into the system as a vapor
as they describe.
Maybe I've missed some things. If so, please let me know.
Absent from the Wolverton paper is an answer to the question, "Where
does the pollutant go?" Back in 1989 it was probably difficult to
answer that question. With today's techniques it should be possible to
answer it. Is the pollutant merely absorbed, being retained in the
tissues? Is it metabolized or otherwise destroyed? If it is
metabolized and destroyed that would be good. If it's just sitting
around somewhere, like in plant tissues, that's not so good.
Dr. Wolverton is careful to state that the levels of his experimental
pollutants are far in excess of what might be expected in the average
office or home. How good are plants with the removal of vastly smaller
trace quantities? This is outside the scope of the NASA testing, so we
don't really know. Since what we want to know is how good plants are
at removing these normal amounts, the answer is important. If they are
effective on normal (probably non-hazardous) levels it is something to
boast about (regardless of whether there is an actual health threat).
On the other hand, it is quite feasible that plants can do well in the
artificial test conditions, but are useless for normal levels.
Maybe plants can act as efficient pollutant scrubbers in ways that are
not merely suicidal (meaning they adsorb material, probably to the
detriment of the plant). But I am worried about a study that implies
that living plant tissue is as effective or more effective that
activated carbon in the removal of benzene, formaldehyde and
trichlorethylene when the gold standard for the removal of such gases
from the air is, in fact, activated carbon.
On that note, one other thing is the use of their little combination
device with activated carbon. Nice, so far as it seems. But notice
that they did not run a control with these experiments. The graphs
look nice: The pollutants go down in a regular fashion. But one would
expect that with activated carbon alone. Why bother with the plant?
There is no comparable graph showing actibvated carbon alone. Why not?
Finally, there is some discussion about microorganisms in the soil,
etc. While the implication here is that these organisms are
contributing to a kind of bioremediation, this is not explored. The
reader is left to use her imagination about what the work is meant to
show. As a matter of personal experience, I know for certain that the
degree of actual bioremediation of organic pollutants is quite a bit
lower than that implied by Dr. Wolverton. In fact, to suggest that it
happens on any measurable scale in their setup within 24 hours is a
sleight of hand trick at the least. My own work has shown that even
for rather readily biodegraded materials (like corn oil) in ideal
situations preliminary degradation (defined as any portion of the
original molecule having been nibbled off) is only on the order of
about half the original material in a month's time. For more
recalcitrant materials (and benzene and trichloroethylene would fit
that description) the rate is far, far lower.
There is more to say, but this entry has probably put most people to
sleep by now. Just be advised that not everything you read is correct.
That applies even when the work is touted to have been done for NASA.
Maybe plants can do something like what we'd all like to believe. But
this paper does not show it.
Ted Held.
On Thu, Dec 8, 2011 at 6:17 PM, Sherry Gates wrote:
> Hi everyone,
> I remember NASA doing a study about this very thing. Here's a link to a
> little info about it. I'm sure y'all can find other information as wel l.
> I wish every one of y'all and your loved ones the Happiest and Merriest
> Christmas ever!
> Sherry
> http://en.wikipedia.org/wiki/List_of_air-filtering_soil_and_plants
>
> ----- Original Message -----
> From: Geneviève Ferry
> To: Discussion of aroids
> Sent: Monday, December 05, 2011 11:24 AM
> Subject: [Aroid-l] depolluting plants
>
> Dear aroiders ,
>
> I want your opinion on depolluting plants because the aroids family is
> often present (Anthurium, Pothos, Philodendron and especially
> Spathyphillum). There are scientific studies about aroids? If so, which?
> Or that is only a means for commercial sale?
> I 'm very interested.
> All the best ,
>
> Geneviève Ferry
> _______________________________________________
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>
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