Is Rockwool Harmful? Yes, But…


Rockwool has long been a popular media for growing hydroponic fruits, vegetables and herbs. However, I’m going to make the case against rockwool and argue why you should never use rockwool again because rockwool is harmful.

This post has gotten a lot of attention recently, and as a result is in the process of being updated to include more information. I cite studies and in no way reference any particular company – I am talking about mineral wool as a growing media in this post.

If you want better alternatives to rockwool, please check out my hydroponic media guide.

It’s Not Environmentally Friendly

I believe in environmental sustainability – it’s one of the reasons I’ve chosen to grow hydroponically. Rockwool doesn’t score well on the environmental scale. It’s not a natural material. Manufacturers use combine chalk and rock and then heat them up to around 3,000 degrees Fahrenheit. Next a stream of air is blown through it, resulting in extremely thin fibers of the rocky material. As the strings are blown out, they bunch together and form the material that you see for sale at the local hydroponics store.

Basically, they are taking two materials that are 100% natural (chalk and rock) and turning them into a hybrid material that will remain in that form forever. When you throw away your old rockwool it’s going to sit in a landfill looking just like that for a long, LONG time. If you absolutely insist on using it, try to save your rockwool in between your growing season and reuse it.

It’s Not Healthy To Be Around

Not only is rockwool unfriendly to the environment – it’s also potentially harmful to your health. New blocks can contain a lot of dust and loose fibers that can get in your eyes, mouth, skin and lungs. It’s similar to asbestos in the sense that the little fibers can lodge themselves in your lungs if you’re working with it a lot. It may not be as toxic as asbestos, but why take the risk? Not something that I’m willing to gamble with if I don’t have to – there are plenty of other hydroponic media choices! If you’re using rockwool, you should be using a mask, goggles and gloves when you work with it to protect yourself.

Here is what a 2002 study on man-made mineral fibres found:

The International Agency for Research on Cancer (IARC) has reviewed the carcinogenicity of man-made mineral fibres in October 2002. The IARC Monograph’s working group concluded only the more biopersistent materials remain classified by IARC as “possibly carcinogenic to humans” (Group 2B). These include refractory ceramic fibres, which are used industrially as insulation in high-temperature environments such as blast furnaces, and certain special-purpose glass wools not used as insulating materials.

The important part:

In contrast, the more commonly used vitreous fibre wools, including insulation glass wool, stone wool and slag wool, are considered “not classifiable as to carcinogenicity in humans” (Group 3). – Wikipedia article on mineral wool safety

A note on the IARC study:

Some manufacturers of insulation products have cited this volume while making erroneous claims that “IARC scientists confirm safety of mineral wool insulation”. These claims are just false. The findings in this volume are not a determination of non-carcinogenicity or overall safety.

What this means is that the study was not able to determine if it these mineral wools caused cancer or not. Again, my original point: why bother with it when there are better options?

If you’d like to read the original study, click here.

More Resources on Mineral Wool Safety

It Has a Naturally High pH

If you use rockwool right out of the package, you will likely have a problem once you plant seeds or seedlings in the material. It’s pH is much higher than other media, so it requires treating before it can be safely used with plants. Not only is this annoying and a hassle to deal with, it just slows down your entire process and puts a barrier in front of your growing efforts. Even when you get the pH correct, it can fluctuate much more than other types of growing media. You’ll have to watch the pH levels of your rockwool like a hawk to make sure there’s no nutrient blockage to your roots. The last thing you want is for your media to actually slow down the growth of your plant instead of speed it up.

If you’ve been convinced to try something else out, you might be wondering what options are out there. I’ve put together a hydroponic growing media guide that breaks down the advantages and disadvantages of some of the most popular growing media, but if you need some quick guidelines, here they are:

  • Ability to reuse the material helps the environment, and saves you money
  • No potentially negative health effects
  • Easy to use, little maintenance required

​What do you think? Do you use rockwool?

Header image courtesy of ilovebutter

The Green Thumbs Behind This Article:

Kevin Espiritu

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58 thoughts on “Is Rockwool Harmful? Yes, But…”

    • I’m sorry about your fish!

      Depending on what fertilizers were used on the plant before you got it, leftover chemicals in the rockwool have messed up your fishtank’s water chemistry.

      I usually recommend taking plants that were started that way out of the rockwool before adding them to the fish tank, just to be on the safe side. Many people will attach their tank plants to driftwood or similar things instead, because it looks better inside the tank. Of course, that all depends on what type of plant you’re growing!

  1. I have used Rockwool, but after reading this article I am no longer wanting to. I use a Tower Garden, and am wondering whether felted natural wool fabric would allow roots to penetrate and then support them with expanded, and reusable clay pellets. As a fabrication c artist, I have plenty of felted wool fabric. I intend to experiment with it growing lettuce for my next crop. May try it with micro greens as well and use the same nutrient solution as Tower Garden. I would welcome any ideas.

  2. Just because “man is natural”, doesn’t make the things that man makes, natural. I have been toying with the idea of using J*ff* pellets (compressed coco coir things that expand in water) instead of rock wool. Renewable, and I have re-used them when seeds don’t germinate. Worried about algae and fungus? soak the used pellet in vinegar for a bit, then rinse with clean water. I think there is a smaller sized net pot they would fit into perfectly.

  3. I am not an expert on using rock wool for planting or as a planting medium. However, I can tell you that Holland has become one of the largest tomato producers by using this very product. I believe it originally comes from Denmark and is a mix of basalt rock and chalk. You can go to the active lava fields in Hawaii and see the same product, albeit not cmpressed, floating around in the air. I know it is used by the people of Holland, and the tomatoes consumed by them, so it obviously hasn’t had any ill effects thus far.

    Although rockwool was originally made for building insulation, its properties are quite different. You couldn’t grow a plant in rockwool insulation as it doesn’t hold water. Think about it: if rockwool insulation were the same as rockwool planting medium, people would be screaming for a refund on their insulation jobs and hme builders would be sued by all. Not only would it hold water from any leaks or from a lightly sealed home in a humid climate, but it would grow mould, maybe algae, cause the wood trusses and attic walls to warp, among other things. It would also be the bane of homeowners who love ivy growing on their exterior walls. It would happily find the nearest tiny cracks, worm its way inside and happily grow to fill the attic. It would also be attractive to any critters that eat algae, or the plants that might take root in there. Not sure if rats eat plants, but I wouldn’t want to find out by experience!

    It sounds to me like one should use gloves safety goggles and a mask over one’s mouth and nose due to its fibrous dust. Just like you would were you using bonemeal or diatomaceous earth.

    I cannot tell you how many times I have seen friends or family members using diatomaceous earth without anything covering their mouth, let alone their hands or eyes. I’ve even seen many of them use it inside their homes to read them selves of ants or other crawling insects. That is a horrible thing to do. Every time your air conditioner or heater kicks in, every time you are a pet walks by the area quickly you are kicking it up into the air.

    I think if one uses common sense when dealing with a natural product or combination of natural products that have not been shown to be cancer causing, or bad for the environment, we should be fine. I agree with those who say having Rockwall, that looks like an earth product or like dirt, in a landfill, is far better than having piles of plastics .

    If you do use it, it is recommended you recycle it by either reusing the block or similar type rockwool or, reshred it and use it in your compost pile.

    • I definitely wouldn’t expect them to handle it well, although I’m not knowledgeable enough to say it’s super harmful. It’s inert, but their digestive tracts may not handle the fibers well.

  4. Life is harmful. It’s 100% fatal and sexually transmitted. All things are natural, even things that are made by humans, because humans are natural.

  5. “Basically, they are taking two materials that are 100% natural (chalk and rock) and turning them into a hybrid material that will remain in that form forever.” looks like someone skipped chemistry

  6. You can literally eat rockwool up to a certain amount per hour and be fine. Msds on each kind should be observed, however, I use it everyday without ppe and rarely the least bit aggravated.

  7. I know many people as well as myself who install rockwool as insulation for 15+ years and are 100% healthy. Interesting how you bash inaccurately rock wool at the same time as selling something else lmao… I’ll continue using my free supply.

  8. Replace rockwool with any medium that basicly supports the plant, the more neutral the better.

    Water and Food for the plant needs to be added anyway.

    Most people waste tons of money on “dirt” you can buy in the store and thinks the plant eats that stuff, then after a month or so the plants dies anyway despite them watering them. (some people think water is also the only thing a plant needs along with “dirt” 😛

    I mean you could probebly get a hydro farm going in styrofoam if they could make it like rockwool desnitypropertys.

  9. I like the fact that the PH of rock wool is higher. Plus it’s less prone to insects like coir or hydroton. All you have to do is wet the rock wool before using it and it won’t become airborne. wear a dust mask if you want.

  10. My father worked with Rockwood on production/manufacturing insulation. For over 30 years since 1972, he recently died of kidney and lung cancer. In America and Poland there are hundreds of lawsuits against the company. Here in the UK studies state that they can not trace family members to continue their studies which I find strange as I myself
    and ten other ex employees have contacted the union to log cancer deaths in our families.

  11. I don’t understand the infatuation with recyclable materials that break down and put chemicals like BPA’s and phalates etc. into our bodies and the ground water. Since rockwool does not break down chemically then i am all for . Any plastic that is made to be recyclable is starting to break down when it is storing your food. It is such a dumb idea to contaminate our bodies and ground water to save the environment visually but not chemically.

  12. How about you guys grow in soil or hydroponics.Hydroponic growing greatly reduces pest that breed from soil. Hydroponic water can be filtered and reused,or pour onto trees in soil,preferably you use organic nutrient solutions.Soil grow foods taste so much better than hydroponic and or chemically fertilized foods. The taste difference is night and day. you guys have never tasted an organic pineapple or cantaloupe,until you have tasted one grown 100% organically. No man made sugar in this world can replica the sweetness of nature’s sugars,when grown organically.

  13. Thankyou for the information, you just helped me with my homework. And Rockwool really sounds harmful!!!!

  14. I have to object to what you said about Rockwool.

    Is it natural?
    In that man makes it? No. But is it ever produced naturally? Yes. It’s called “Pele’s Hair”, google it, it’s made by volcanoes which is how we learned how to make it. Now, we make a much more consistent type of rockwool than a volcano does, but it’s the same principle.

    Is it environmentally friendly?
    I think so. I mean…it’s can be recycled indefinitely with much less work than it takes to recycle a lot of other things. How is that not environmentally friendly? And if it does by chance end up in a landfill, so what? Would you care if rocks and dirt were in a landfill? Edit: Dirt and rocks, they’re never going to biodegrade either.

    Is it harmful?
    Toughy, but let me categorize it.
    To breath? No.
    To eat? Unknown.
    To inject into your body cavity? Some of it is.

    Now you mentioned the fact that CRF is classified as 2b, which is possibly carcinogenic. Rockwool, however, if not CRF. CRF is class 3, which I believe you indicated, which means they have found no link. What you suggested is that this doesn’t mean anything other than they found no link “YET”.
    This is wrong. Rats, in 3 studies, were subjected to high doses of rockwool in their air. They had high amounts of rockwool in their lungs shortly after. Within 18 months they had no significant amounts.
    Asbestos would have still had 30% of the material after 18 months.
    Aside from that, edit: no study that I read where rats breathed in rockwool ever showed the rats to develop cancer.
    In addition the workers at rockwool plants in Germany were monitored for decades and there was no significant link found between cancer and rockwool workers.
    The exception here, of course, is that if you inject some variants of rockwool into the body cavity (yes, it’s as controversial and stupid as it sounds) that some variants will cause cancer. So if you ever get an open pneumothorax (sucking chest wound) don’t try to stop it up with rockwool…..

    Finally, I would like to know more about how dangerous it is to digest. This is the kicker, isn’t it? I mean, this shit is being used to grow food and dogs eat it all the time. How dangerous is it for them and us to eat?

    As an insulation for housing, however, I think it’s one of the greener products we have.

    Just so no one says it I am not in any way affiliated with any rockwool manufacturer. I discovered this blog and all the information I know about rockwool while researching home insulation for a future home I am going to build.

    Anyways, if you have any questions or rebuttals I am open to discussion or debate.

  15. Rockwool is entirely and completely safe; there is no scientific or empirical data anywhere that show any potential for harm to humans or the environment. Also the fibers do begin to fall apart and break down within a short frame of time outside. Regardless, just because something doesn’t quickly decompose doesn’t mean it’s suddenly dangerous. Your driveway isn’t decomposing, but I’m pretty sure everyone is perfectly fine right now.

  16. whats wrong with rockwool again?
    sitting like that in landfills? its still organic and plants can grow in it and nothing is destroyed producing it either so why is it not eco friendly?

  17. I am shocked that if your concern is sustainability, you recommend highly (and use) sphagnum peat based starter plugs! These are really terrible for the environment, and the only people who say otherwise are the Canadian Peat Grower’s Association. Please reconsider your recommendation and use of peat products and use/recommend more sustainable choices.

  18. Kevin, I do not use rock wool nor clay pellets. Instead I use bio balls. They are more stable, can wash in a washing machine to keep them clean of pathogenic materials. They were originally made for the gas/alcohol distilling process in the 60’s. But over time they used them in the aquarium business. The reason I like them, they interlock and keep the plants stable. IF you write me at I shall send you photos to show you how easy it is, also photos of the fruit we have grow with our new fertilizer. Waiting for your email

    • Hi Snake,

      Bio balls sound amazing. Would you mind linking me to the product? My email is kevin [at] epic gardening [dot] com so feel free to send!

  19. Google: “Rockwool Arsenic” and you will find that Rockwool Industries is the proud owner of an EPA super fund site in Bell, County Texas. Next google: ” Plant metabolization of arsenic”. This should paint a real clear picture of how dangerous Rockwool can be as a growing medium.

    • Rockwool industries isn’t the manufacturer of hydroponic rock wool.. Yeah I know the name can fool you. They make insulation for homes and not hydroponic rock wool. I know confusing right… They have been in the industry before hydroponics really caught on.

    • Kris – Unfortunately, you have jaundiced the thread by speaking of rockwool based on the Bell County, Texas Rockwool Industries site. If you investigate why the site was shut down in 1987 and cleaned up by 2005 is because they produced rockwool using copper slag as a raw material. This is what produced the shot that contained arsenic. Today, rockwool is manufactured and tested continuously with absolutely no danger to humans as a hydroponic growing medium.

  20. Not a lot of objectivity here. I’d challenge you to actually show how it is dangerous before accepting any of your claims.

    • I work with rock wool on a commercial level. Reading this certainly brings some light alarms. I always get itchy and sometime have minor hives when I work with it. I never thought about the airborne fibers breathed in every day. Its one of those things like debating whether GMO food is safe or not.. I guess time will tell. On the other hand maybe long sleeve shirts and masks wouldn’t hurt if you are always around it. Also, I do disagree with the sustainable claims in this article. I have been experimenting with rock wool and combining certain elements to form a nutrient rich compost within a 90 day period. The rock wool does in fact retrieve near back to its natural element but a bit more fine. Almost like a moist sandy but dense mush.. Its a great way to produce a filler for compost and circulate the current soil in your conventional garden.

      • ” Like debating if GMO is safe or not” There is no debate, it’s safe. The safety of rockwool is questionable because of its fiberous anture, like fiber glass. It can cuase irritation and, like fiberglass, propper PPE should be used whenever handeling it. If you work as a hydroponic farmer on a commercial levle and are not wearing PPE then you have failed to take your own safty into consideration, and your superiors have failed to meet what is probably basic OSHA safety standards.

        • I don’t know much about the industry but my father was never given any PPE to ware he just wasn’t aware of it 30 years ago, having worked on production for 30 years he died 18 months ago from kidney and lung cancer he was riddled with it and there are other too we just can not get the union or the company he worked for to acknowledge their deaths. I strongly agree with Kevin’s article.

  21. Dear Kevin and all the good folks following this thread.
    First of all, on behalf of the GRODAN team, I, as the Manager, would like to offer our sincerest apologies for the manner in which our colleague, Shane Hutto, reacted to you. This is not how we as GRODAN Inc. communicate and we regret how this situation was handled. We will ensure that this type of communication does not take place again in future.

    As far as I can see, there has been some discussion regarding the safety of our products on your website and on the Reddit thread. I would like to stress that mineral wool is probably one of the most investigated products outside of the medical industry. Since the publication of the European directive, IARC has classified mineral fibers as not classifiable as to its carcinogenicity to humans (category 3). Newer materials (which includes Grodan) (Nota Q Fibers) are found not to be carcinogenic.
    This is the newest international classification, based on the update of many epidemiological and animal studies done by worldwide recognized experts.

    We are, however, open to any other questions you may have, so please don’t hesitate to send your questions to
    I sincerely hope you all accept our apologies.

    Kind regards,
    Grodan Inc.
    Business Manager North America.

    • Vee, way to stay on top of things. Its sad that every industry needs to compare itself to the medical industry these days.. however isn’t the food industry another whole ballpark? I heard it only takes 6 months for a biotech company to submit a “proven” test of a safe food product.. what about the utilities involved to create the product? I am a hydroponic gardener.. I would like to know that I am safe dealing with rock wool everyday. If it is can you send me a link of the entire study through a PDF?

    • Pardon me.. not a gardener.. a farmer. I literally transplant thousands of seedlings and nursery’s every day containing rock wool.

  22. Great article – quick, informative, and easy to understand. Rockwool has some upsides in hydro (mainly the fact that it’s a deterrent for mold) , but it is definitely not the perfect hydroponic growing medium by any stretch of the imagination. There are lots of other options out there and I would not be surprised to see rockwool completely replaced in the future. Thanks for posting!

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