Can Indoor Herbs Purify House Air? (Lies And Reality)


If you look for “Herbs that purify the air,” you will be flooded with hundreds of articles listing many herbs. However, I noticed that hardly ever any of those articles reported the source of their information. So, I decided to dig dip into the topic to identify reliable sources to understand if this really true or not. Surprisingly, science tells a different story.

Hence, do indoor air purify the air? A small number of herbs (like rosemary, lavender, etc.) have been tested and have proven to be able to reduce, under lab-test, the concentration of a dozen (out of 1000+) chemicals. Herbs do not purify the air as they cannot totally remove all chemicals in it.

Is Indoor Air Pollution a Problem?

Let’s start with the basics. If you need herbs to purify your air, that means your house air is polluted, right? But polluted by what?

This is a whole science by itself. Many sources can be found. Still, I will rely on the Environmental Protection Agency (known as EPA) of the United States, a well-known and trusted source on the subject.

EPA clearly states that it can be quite common to have air pollutants at home due to 1) Indoor polluting sources, 2) poor ventilation, and 3) Temperature.

What many ignores is that a large variety of our daily activities and objects are responsible for indoor air pollution. In the ENA website, you can find a detailed list. For simplicity, we can identify 4 sources of air pollution:

  1. Fire: anything that burns involves a reaction during which chemicals are released. From smoking to wood-fired, and even your gas stove;
  2. Household cleaning products: have you ever noticed how strong some cleaning products do smell? Well, this is because perfumes might have been added to cover the real smell of those chemicals;
  3. Household products: how many times you have unboxed a new appliance, or even a sofa to notice that a strong smell (often pleasant, at least to me) coming out from the box? What about when you step into a newly bought car? Those are smells produced by solvent and paint used by the manufacturer. These are definitely not meant to be helpful for your health;
  4. Outside air: very often, let some outdoor air coming in is going to improve your home air quality. However, this is not the case if you live close to a trafficked area or a source of chemicals.

Here you can find a small list of the most common chemicals (for more, have a look at EPA) that you can encounter in your house produced by the sources listed before:

  • Carbon Monoxide (CO): this gas is produced by a large variety of appliances (water heater, clothes dryer), as mentioned in this study. Having them in a closed space with no aeration might not be the best idea. In such a case, just leave the door open or allow some breeze to circulate in your home by opening doors and windows at least once a week;
  • Volatile Organic Compounds: these pollutants are pretty famous as many talks about them. These are the one studies in most of the research analyzing the “purifying” effect of herbs. As defined by EPA, such chemicals are emitted by a large variety of products (wood products, aerosol spray, pesticides, paints, etc…).

    The list is massive (around 1000), here for your curiosity. Studies found that their concentration inside can be many times higher than outdoors. Needless to say that some of them are quite harmful chemicals with short and long term effects. Among these volatile organic compound, a special mention goes to:
    • Formaldehyde: this is by far the most (in)famous among the hundreds of volatile organic compounds. This is something that most of us have “smelled” without realizing. Think of the smell of freshly bought (often cheap) wood products. Everything there, from solvent to glue, emits such pollutants.

      As detailed by EPA, also cigarette smoke and a large variety of daily objects presenting paintings or solvents. Although low level do trigger only minor issues, large quantity can be the cause of serious chronic diseases;
    • Benzene: this chemical deserves a special mention as widely used in the manufacturing industry for plastics, resins, nylon, synthetic fibers, as well as pesticide, house cleaners, and cigarettes. In liquid form evaporates very quickly and has a sweet odor. Long term exposure to high concentration can seriously affect human health. More info from the Agency of Toxic Substances & Diseases Register here;
    • Trichloroethylene (more info from the Agency of Toxic Substances & Diseases Register here) due to their everyday use in many products such as adhesives, paint removers, and many cleaning products (just read the label to check their presence).

Such chemicals, in case of adequate ventilation (just leave a window open 1-2 hours a day for each room), do not generally represent a severe issue. However, in case of the absence of air exchange, these chemicals can build up in the air making their level higher than what is retained safe.

Moreover, temperature is another factor. Higher temperatures increase the rate by which such chemicals are released, building higher pollutant levels in your home way quicker.

So, avoid leaving during hot summer days, your living room or kitchen closed. Indeed, think about the smell of your house after you come back from a long holiday. Such smell is very likely due to a higher concentration of such volatile organic compounds.

Plant To Purify the Air: What the Famous NASA Study Really Says

I challenge you to search on the internet “herbs that purify the air.” I did. The articles that will come out are very likely to be a collection of nice pictures of the same 6-7 herbs. You can immediately notice that, although the “scientific” nature of the topic, almost no reference is provided to back-up this statement.

Moreover, if I would ask you after reading such articles, what does purify the air actually means? Is your house-air “polluted”? By what? How? How many pots do you need to have this “cleaning-air” effect? You will be unable to reply.

Even after reading 50 of those articles, the situation will not change. This is not your fault, and I have been there in searching for information on the same topic. That’s why this article will have you covered with all the information that science provided us.

Let’s start with the pillar study of all claims you can find on the web on air-cleaning plants. This study attracted great attention a few years ago, is a freely available document of 30+ pages (NASA archive, here) published more than 30 years ago.

Let’s break down this important and complex research in easy to digest bits to let you understand what this document really states.

Plant Tested: Here you can find a list of all plants analyzed in this study

  1. Bamboo palm
  2. Chinese evergreen
  3. English ivy
  4. Ficus
  5. Pot Mum
  6. Gerbera daisy
  7. Janet Craig
  8. Marginata Mass cane/Corn cane
  9. Mother-in-law’s tongue
  10. Peace lily
  11. Pot mum
  12. Warneckei

First fact: no herbs were investigated in this study!

Indoor polluting substances investigated: the chemicals that were tested for removal were benzene, trichloroethylene, and formaldehyde. Hence, another fact: this study focused only on three among the 1000+ chemicals (here for the full list) that might be present in your home. Although those three are among the most common, your house air hosts also a large variety of other (harmful if in high concentration) chemicals.

Experiment: The potted plants, taken from the nursery, have been placed, one by one, inside a sealed chamber with adequate artificial illumination, watering, and fertilizer. At the beginning of the test-specific concentration of the three polluting substances were injected in the chamber. Then, their level was monitored after 24 hours.

Results: Regarding these three chemicals, their concentration was reduced in just 24 hours from 10 to up to 90% depending on the considered plant (NASA report, tables 4-5-6).

Benzene is the chemical most effectively removed by plants that achieved a reduction in its concentration from 50% to an outstanding 90% for English Ivy. On the other hand, trichloroethylene is the one that is hardest to remove from the air. In 24 hours, only a reduction of up to 30% was obtained by Peace Lily plant.

For formaldehyde, the plants involved are different as the study was performed during the first of the two years of research. However, again, the results are quite encouraging, with a reduction in the chemical concentration varying from 50 to 70% (obtained by Mass Cane).

Hence, from the study below the most useful plants for those three volatile organic compounds are:

  1. English Ivy (especially for benzene)
  2. Peace Lily (especially for trichloroethylene)
  3. Mass Cane (especially for formaldehyde)

After this, you learned that:

  1. Plants remove some dangerous chemicals (and not completely) among the thousands that exist;
  2. The famous NASA research that everyone talks about covers only 3 among the 1000+ chemicals that might be on your indoor air. Hence, it cannot be considered representative;
  3. Some common house plants can do a great job of removing air pollutants. This has been scientifically proven!

This purification rate you have to remember is in optimal watering and light conditions obtained in a lab. Hence, in your house, you will have a very likely lower level. However, the advantages are still essential and worth it for you to have them at home.

University Studies

What about herbs? The NASA article does not mention them at all! After a bit of digging, I found a few other studies on the subject. Keep reading!

University College of Medicine, South Korea

A detailed university study from the University of Seoul, here, has been published just a few years ago, way more recent than its NASA counterpart. You need to know that Korea is one of the leaders in studies related to air-purifying plants.

Plant Tested: Here you can find a list of all plants analyzed in this study

  • Asplenium,
  • Satsuma mandarins,
  • Gardenia
  • Pothos,
  • Rosemary: unfortunately, this is the only herb!
  • Parlourplam,
  • Money trees
  • Peace lily
  • Pothos
  • Dumb cane
  • Lady palm

Indoor polluting substances investigated: formaldehyde was again among the tested substances. The others are a particular category of a volatile organic compound called BTEX. These are benzene, toluene, ethylbenzene, and xylene. As you can notice, benzene is there again!

Experiment: first, the concentration of the previously mentioned air pollutant was measured in normal conditions inside houses participating in the trial. Then, a specific combination of plant/herb was placed in the three main rooms (living room, kitchen, and bedroom) of the houses participating in the experiment. For the first group of houses, the plants were removed after 3 months (called the withdrawal house group), while the remaining plants were left inside for 6 months more. The concentration level of the chemical was monitored every 3 months.

Results: for the BTEX chemicals, it was noticed that their concentrations dropped from 80% to almost 98%! On the other hand, for those houses that see the plant removed after three months, the chemical levels, after a first dropped, they raised again! This is a clear proof of the purifying power of those plants/herbs.

Table showing the concentration (μg/m3) of harmful chemical before 9 months of introducing plants in the house

Chemical

Before

After 9 months

Variation

Benzene

6

1

-83%

Toluene

79

19

-80%

Ethylbenzene

3

0.3

-90%

Xylene

13

0.2

-98%

The only chemical that shown an odd behavior was formaldehyde as its concentration in the air kept reducing; nevertheless, the plants were removed. The reason might be justified by an improved aeration system during the experiment (as such, an investigation started just before summer).

Again, this study focus on a limited amount of volatile organic compound (although way more than the NASA study). It still proves the point that plants can remove such compounds from the air.

Differently from the NASA study, it is impossible to differentiate the effect of every single plant but rather the combination of multiple plants such as:

  • Living room: Asplenium – Satsuma mandarins – Gardenia
  • Kitchen: Photos
  • Bedroom: Gardenia – Rosemary

Regarding rosemary, the study did not provide any useful information.

University of Georgia, USA

I have to be honest, this is one of my favorite scientific documents (even more than the NASA one) on the topic. Why? Because it is a “Current State of the Art” type. In science, this means that this article provides a summary of all studies on air-purifying plant\herbs. This is a whopping 60+ reports/data that the author (a known professor in the field) has summarised for us. So, let’s pick the critical fact of this great document.

  • 900 volatile organic compounds have been found in public buildings (USA). As stated by the same author here “it is readily evident that a large number of homes and offices have VOC (volatile organic compounds) levels that potentially compromise the health of the occupants”;
  • 200 volatile organic compounds found in Finish houses;
  • In cities, indoor air has been reported in few USA-based studies to be 5 to 1000 times more polluted than exterior air;
  • The research on the ability of herbs to clean the air is still ongoing. Indeed, up to date, only a”minimal number of VOCs that have been tested.” More precisely, of the 900+ compounds, only a dozen have been investigated.

    Herbs and plants can likely remove a large number of dangerous compounds from the air, but the research is not there yet to back-up such a statement. Hence, everyone that claims “herbs that purify the air” (this means totally removing all dangerous compound) are just empty words not backed up by science (for now).
  • The rate by which volatile organic compound is removed varies significantly in the function of the plant and chemical considered. Some plants can be up to 2000 times faster in removing a specific compound from the atmosphere than others;
  • The soil (or the microorganisms in it) contributes to the air purifying effect. Indeed, some experiments noticed a decrease in volatile organic compounds in the air even after removing the plant and just leaving the pot with the soil;
  • Plants remove some of the chemicals studied even at night or in the absence of light;
  • It is not clear what happened to the chemical that the plants absorb. Some are effectively broken down into safe chemicals (like formaldehyde), but for many others, no evidence was found;
  • In the last decades, only around 150 plants have been tested (table 2 in the document). Of those, only a small minority was herbs. These are:
    • Fittonia argyroneura: remove effectively benzene and toluene
    • Fittonia verschaffeltii: specialized in toluene
    • Hemigraphis alternata: can remove benzene, octane, α-pinene, toluene, trichloroethylene
    • Lavender:
    • Melissa officinalis
    • Mentha piperita
    • Mentha suaveolens
    • Nephrolepis exaltata
    • Rosmarinus officinalis
    • Tradescantia sillamontana

They all show the capability, to a different extent, to reduce the presence of pollutants. Even more details in this book.

Urban Agriculture Research Division, South Korea

This is another excellent study from South Korea (leader in this field) in which 86 plants have been investigated. Below some quick takeaways.

Plant Tested: this study deserves a place as it also analyses several herbs (for all the 80+ plants just read the full article, is freely available) listed below:

  • Jasminum polyanthum
  • Jasminum sambac
  • Lavandula
  • Mentha guaveolens
  • Pelargonium
  • Rosmarinus officinalis

Indoor polluting substances investigated: given a large number of plants under study, the authors limited the analysis only to formaldehyde. However, this is one of the most common volatile organic compounds, making such a study still valuable.

Experiment: similar to the NASA testing, every single plant was placed in a small sealed chamber (around 1m height and wide) where air with a defined formaldehyde concentration was introduced. Every hour the concentration of formaldehyde was measured again to verify whether the plants were absorbing it or not.

Results: among all 86 plants, herbs were the second-best performers (after fern) in the ability to remove formaldehyde from the air. Among herbs, the best were Levander (Lavandula) and Rosemary (Rosmarinus officinalis). Another interesting fact is those herbs remove most of the chemical in the first hour, slowing down after. Lavender performed around 5 times better than Jasminum sambac and more than twice than Mint.

Can Herbs Purify Air in a Whole House: What Science Says?

Science has been quite clear on the fact that plants and many herbs can remove, from the air, some chemicals. However, are those herbs going to be effective in our house as they were in the discussed experiment?

I need to be critical here and highlighting some exciting aspects of all studies proposed so far:

  1. In the NASA study, the pots were equipped with carbon filters, as this simulates the conditions plants would be used in space. This is not the case of standard houseplants. The carbon filter was found to affect the removal of toxins. Hence, the plant in typical house-conditions would remove fewer chemicals than declared in the NASA study;
  2. The soil itself (with its microbes) helps in removing chemicals from the air. However, in the majority of cases, the soil alone was removing way fewer chemicals compared to the potted plant version.
  3. All studies analyze the effect of only a dozen chemicals compared to the 900+ that we might be exposed to in our houses/offices. Hence, any statement on the line “herbs purify the air” is inaccurate if not misleading as might tempt you to think that herbs are ideal filters. Simply they are not!
  4. The number of plants you might need to use to experience the same reduction level in the indoor pollutants can be massive. Indeed, NASA studies were based on single plants inside a small sealed chamber. Hence, to experience the same results in a standard kitchen or bedroom (hundreds of times more massive than a glass chamber), you might need dozens (if not hundreds) of potted plants.
  5. The plant per se might not be the source of the entire air purification effects! Indeed, in the same NASA experiment analyzed the cleaning capability of the plants without leaves. Surprisingly, for many of them, their cleaning capabilities were almost unchanged! This very likely means that the cleaning capability is in large due to the roots of the plant. Hence, do not be afraid to prune your rosemary or lavender when needed!
  6. Do not confuse carbon dioxide removal (the famous CO2) with purifying the air. The presence of carbon dioxide, in a healthy environment and in the right amount, is not an issue. Hence, in common indoor conditions, CO2 is not a dangerous chemical as many of the volatile organic compounds can be. So, claiming that an herb is purifying the air just because is removing CO2 does not make much sense.
This is my little indoor garden in the living room. Having a dozen herbs is going to help the reduction of benzene and formaldehyde but do not expect them to purify the whole house

Other Misconceptions On Herb Purifying The Air

Now you know that herbs definitely contribute to removing some chemicals from the air, especially the roots, at a rate probably lower than the one claimed in several studies.

However, the misconception does not stop here. Here three more!

Herbs are not Shrubs

Many articles mention honeysuckle and coffee plants as air-purifying herbs. The problem here is that these are shrubs and not herbs.

What’s the difference? Shrubs have multiple woody stems coming out from the ground and, very often, tend to be way taller than herbs (source). This makes shrubs difficult to fully develop indoor due to their height and so the difficulty in providing adequate light to the whole plant. Also, they are harder to propagate by stem cutting (due to their woody stems) as detailed here.

Herbs and Herb-based Essential Oils are Not the Same

Quite a few articles do not clearly state when you need to use the essential herb oils or only the potted herb. This is a massive difference.

Indeed, you might think that having a parsley plant indoor can “purify” your air, but in reality, what you need are the corresponding essential oils. As detailed in this authoritative university resource, an herb oil is produced after a relatively complex process. Such oils are a very concentrated version of some natural chemicals present in the leaf. Hence, either you produce such oils or buy them, but you cannot expect the same effect of your potted herb.

If someone tells you “the oil of this herb” purifies the air, then this really means that the potted version of it might not be sufficient enough (if you are not going to fill your house).

Having Herbs in the Bedroom: Another Misconception

Some believe keeping herb plants in the bedroom is harmful due to plant emitting CO2 at night. However, this is done in such small amounts that it is not considered dangerous by most in the scientific field.

Ultimately, there is no direct evidence to prove that plants in the bedroom cause negative impacts for people or pets, as discussed here. The benefits are way more significant than the small production of carbon dioxide.

If you are still skeptical, well, you can place them away from your bedrooms at night and put back in place in the morning if not in big pots.

How Many Plants In Your Home? A Controversial Videos Among Gardeners

This TED talk is famous among expert gardeners. The speaker, Kamal Meattle from New Delhi, decided to dedicate his time to research a sustainable way to clean India’s polluted air. He promoted three plants:

  1. Areca palm: the speaker claims that this reduces CO2 and produce oxygen. Of course, it is suitable for an indoor environment, but at the same time, many other plants do the same, although to a less extent. It suggests to have indoor 4 plants for each person that implies around 16 plants for a family of 2 adults and 2 kids;
  2. Mother in law’s tongue: again, only mentioned its capacity to produce oxygen but this time at night time. That’s why it is suggested as a bedroom plant;
  3. Money Plant: generally claimed to remove chemicals.

The author claimed the statistical improvement of the air quality in a building he conducted a test with. He set-up 1200 plants spread in a 50,000ft2 (4645m2) floor surface building (you can check those numbers by yourself watching his TED talk at minute 1:44).

Let’s make the math easy, let’s round. This equates to around one plant each 5ft2 (0.5m2). Hence, if you have a house of 2,687 square feet (250m2), average USA size (data from here), you will need around 500+ plants! Considering around 1kg of soil per pot, this means you need to buy and transport half tons of dirt! Moreover, assuming 10 seconds watering each one, this implies almost 2 hours just to water all those plants every week or so

Hence, it is totally accurate that plants can help the reduction of pollutants. However, for a less extreme case than New Delhi (where air pollution affects more than 2 million people a year, Wikipedia), a significantly smaller number of plants with a proper aeration system can provide similar benefits.

Saying this because it is vital to size expectations to what science really says.

There is a massive number of studies on herbs and plant air pollution cleaner. This article, although way more in-depth than everything you will find online, only scratches the surface. This topic will attract more and more attention in the future, so keep an eye on it!

Where to Find Indoor Herbs?

Most herb plants are found easily at any nursery. Some herb plants are available in supermarkets. From such inexpensive plants, you can then follow my step by step guide to multiply (for free) your herb, in conjunction with the information on propagation by stem cutting (here one of my previous articles).

It’s important to understand that different herb thrives in a nutrient-rich potting mix. As always, you can find detailed information on how to create your best potting soil in this article. If you do not have time, you can buy for a few dollars the best one available in the market (you can check it here in Amazon) or in other retailers.

Further Questions

What are the best plants to clean the air? Osmunda Japonica and Gerbera daisy are the plants, among those tested, with the highest capability to remove formaldehyde (one of the most investigated air pollutants). At the moment no research has studied all plants with and all possible air chemical combinations. Hence, a true final answer does not exist yet.

Does Areca Palm help cleaning the air? Areca palm, also known as bamboo palm, has shown great ability to remove Trichloroethylene and Benzene in relation to its leaf surface.

Further Readings

The famous NASA study from where everything started – https://ntrs.nasa.gov/search.jsp?R=19930073077

Interesting research analyzing 86 different plants effect on air-cleaning – https://greenplantsforgreenbuildings.org/wp-content/uploads/2014/09/Formaldehyde-removal-plant-species.pdf

The most famous TED talk on plant purifying the air – https://www.ted.com/talks/kamal_meattle_how_to_grow_fresh_air?language=en

Illimited herb supply by stem cutting, step by step guide – https://yourindoorherbs.com/3-easy-steps-to-propagate-by-stem-cuttings-free-herb-supply/

How to create your own potting soil – https://yourindoorherbs.com/2-aspects-of-the-best-potting-soils-and-diy-recipe/

21 Tips to grow massive Basil – https://yourindoorherbs.com/21-easy-tips-to-grow-massive-basil-indoor/

Andrea

A young Italian guy with a passion for growing edible herbs. After moving to the UK 6 years ago in a tiny flat, it was impossible to grow herbs outside. So I start my journey in growing indoor and so I decided to share my knowledge.

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