In 1989, NASA performed a study on indoor air pollution abatement. Since then, the research has been displayed in plant marketing and some fancy infographics with all of them claiming that plants clean air. However, in most cases, the performance of a plants ability to reduce air pollution is greatly exaggerated. So, let’s take a look at the actual study and understand what the study actually found out.

My goal is to help you understand the results of this study showed, and help clear up the marketing jargon. You’ll consistently see stuff like the image below. While some of it can technically be construed as the truth, it’s more or less a carefully worded marketing pitch.

Before we jump in, I would like to establish that this study was critical to our understanding of using plants to clean our indoor pollution. The problem lies in how the marketing has misconstrued the results to make it seem like adding a few snake plants around the house might somehow make you healthier and clean your air sufficiently.

Reasoning for the study

The primary reason for this study was for learning how to remove pollutants from indoor areas. The pollutants in the air became a large problem in the 1980/90s due to the changing of building materials that were addressing energy inefficiency concerns.

Since NASA was studying indoor air quality, it cannot be stressed enough that the study was ultimately looking at the closed environment that would be on a space station (or ship). Yes, the study does state this intent at the end of page 2.

Why is motivate for the research an important point?

Since the study was meant for potential use in space, it makes sense that the study might include methods that would otherwise be impractical for the average household. One such exercise carried out was using activated carbon in conjunction with plants to attempt cleaning the pollutants. Below is an example of the set-up they used.

NASA Study Figure 1 – Indoor air purification system combining houseplants and activated carbon

Year One Results

During the first year of the study, they produced four tables which seem to be widely used in portraying the effectiveness of a plant to remove pollutants.

However in the paper they released detailing the study, the researchers state they made mistake in the control for the first year of testing. I applaud the fact the researchers provided details of their mistakes to help prevent future studies from falling trap to the same issues, but I think this is one of the points that a lot of people miss when reviewing the study.

During the first-year studies, the only controls used were chambers free of plants to test for loss of chemicals from chamber leakage and pots with fresh potting soil without plants. It was then assumed that after correcting for controls, the removal of chemicals from the sealed chambers could be attributed to the plant leaves. Because of the low photosynthetic and metabolic rates expected from these plants at light levels of 125 to 150 footcandles, the high chemical removal rates attributed to these low-light-requiring houseplants were puzzling.

Page 13 in the NASA Interior Landscape Plants for Indoor Air Pollution Abatement

The above quote is basically saying after looking at the results and the given amount of light provided to the plants the results didn’t add up to what the expected outcome was. They then brought in a microbiologist to study the soil, and what they found fundamentally changed the path of this study.

So, it suffices to say that while the year one results are intriguing and do offer some insights into the effectiveness of plants. The results do not provide a clear picture as to what is removing the air pollutants. At this point, it could be micro-orgasms living in the soil or the plant itself.

Year Two Results

During the year two tests, they brought in a microbiologist to help study the results. They started finding instances where removing all of the foliage was more effective in removing benzene than pots containing the plant and soil. What they later found was that by removing the foliage around the soil so that air can easily access the soil there was more reduction in the pollutants.

So, in the table below they performed a variety of tests that were aimed at testing the soils ability to remove pollutants with and without the plant involved.

  • Full foliage – They only removed foliage that would be inhibiting the air from reaching the soil.
  • Foliage removed – They removed all foliage from the plant. They do not state that the entire plant was removed so I’m assuming the trunk was left in the planter.
  • Fresh potting soil control – As it sounds, they put fresh potting soil in a planter and place it in the chamber to see how it performs.
  • Leak test, empty chamber control A completely empty chamber to account for any potential leakage in the chamber
Table 7 from the NASA study

As you can see in a few of the results, having the soil that came from the plants made the largest difference in removing the pollutants. For instance, the Marginata jumped from 20.4% to 49.7% removed when going from fresh soil to the foliage removed. Then, when the foliage was added, it only removed an additional 8.3%.

So what does the year two results tell us?

Micro-Organisms are absolutely at play with helping remove the pollutants. However, we still can not safely determine if the removing of pollutants can be contributed to the plants, micro-organisms, or maybe even both. The reason is the experiment never fully isolated the plant from the soil which contained the micro-organisms.

The Micro-Organism Catch

The micro-organisms played a large part in the removal of pollutants in the air. However, in the study, they looked at how the bacterial counts correlated with increased chemical removal as shown in Table 8 below.

What the researchers found was the chemical removal increased with soil bacterial counts. This wasn’t a consistent finding, so the conclusion they made was there maybe unidentified biological factors.

Table 8 from NASA

Results

Overall, the study offers a lot of lessons learned and glimmers of hope. The data itself can be used for further research purposes, but most of it has little direct benefit to the general population that I can see.

Yes, plants and the micro-organisms can remove some pollutants. However, due to the limited scope of the study, it would be hard to conclude any health benefits or major air purification from this study. It’s also hard to say how many plants would be needed to clean up the specified pollutants in an average home. Who can have too many plants anyway right? Just create your personal jungle and enjoy the beauty.

As a final note, this study only looked at three pollutants. So while plants/bacteria did successfully remove some of these pollutants, it’s still unknown how well it’d clean the average home with many different pollutants. On top of this, the three pollutants, while bad, are not the six pollutants that are being regulated by the Center for Disease Control and Prevention (CDC)

What do you guys think? Do you see anything in this study that is immediately useful to the general public?

References

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