Paper and cardboard provide a good carbon source for composting, and they are also used for sheet mulching and lasagna gardening, but is it safe to use paper and cardboard in the garden?
Bleached paper contains chlorine and dioxin, a known carcinogen. Inks contain heavy metals, and BPA is used in some types of paper. Much of this paper is recycled, and the recycled paper contains these various chemicals. Is it safe to compost paper and cardboard?
Concerns Over Paper and Cardboard
Paper products contain chemicals that are of concern. The question is, do they pose a problem in the garden? To answer that, you need to break the question down into several questions.
- Are the amounts of these chemicals in the paper high enough to cause a concern?
- If they are high enough, do they survive the composting process? Are they long-lived in soil?
- If they survive these treatments, will they be absorbed by plants?
- If plants do absorb them, do they absorb enough to cause a health concern?
Most discussions about the safety of paper products only address the first and maybe the second question, which is not enough to confirm a problem.
Dioxin in Paper
Paper is made from wood pulp, which is a brown color. For many applications, we require a white paper, so the pulp is bleached with chlorine. This process introduces a very toxic chemical called dioxin, which is a strong carcinogen even at low levels. Dioxin can be found in products like diapers, sanitary napkins, coffee filters, toilet paper, writing paper, and even milk cartons. Many of these products are recycled, and dioxin is found in all recycled papers.
One way to reduce this problem is to buy brown paper, like brown toilet paper and brown coffee filters.
Many online sources talk about the concern of composting paper because it contains chlorine, but there is no concern over chlorine. It is dioxin that is a potential problem.
The EPA looked at this problem and in 1990 concluded that “the amounts of cancer-causing dioxins in milk cartons, coffee filters, and toilet tissues are too small to pose a health problem.” The Ministry of Health in New Zealand reached the same conclusion. A study looking at coffee made with white coffee filters concluded, “They do not present any significant health risk to the coffee consumer.”
These studies looked at the total amount of dioxin and at the amount that leached out of these products when used as intended. I am sure they didn’t test the amounts after composting. However, if pouring hot water over coffee filters does not extract enough into your coffee to cause a health concern, it is highly unlikely that putting that coffee filter in a compost pile and growing food in the compost would cause a problem.
Dioxin degrades quickly in the sun, and its half-life on the surface of soil is 1-3 years (Case Studies in Environmental Medicine. Dioxin by Raymond Demers). Evidence for absorption of dioxins in soil is sparse and inconclusive.
Dioxin does not seem to be a big problem with composted paper, provided you are not composting huge amounts.
Bisphenol A (BPA) in Paper
Bisphenol A (BPA) is used in a variety of consumer products, including thermal receipt papers. You might not compost these, but they do end up in recycled products and are therefore a potential concern.
A study looked at 15 types of paper products, including thermal receipts, flyers, magazines, newspapers, food contact papers, food cartons, printing papers, and paper towels, and found that the exposure of BPA to “the general population is minor compared with exposure through diet.” Bisphenol A (BPA) is used for lining metal cans, in plastic wraps and polycarbonate plastics, and is found in several “food products, including fresh turkey, canned green beans, and canned infant formula.”
BPA exposure is a concern in other areas of our lives, but exposure through composted paper is not an issue.
Glue in Cardboard
There is concern about the glue used to make cardboard boxes. There are two places where glue is used. One is to make the actual cardboard, and the second is used to form the boxes.
The glue used to make cardboard is almost exclusively made from starch, which is derived from natural carbohydrates found in roots, tubers, and seeds of higher plants such as maize, potatoes, wheat, rice, and tapioca. They easily degrade in the composting process.
Glue is also used to make boxes, and it’s less clear which glue is used. However, the amount of this glue is minimal.
Glue on cardboard is not a real problem.
Chemically Treated Cardboard
Cardboard boxes can be treated with a variety of chemicals, including fire retardants, waxes, and anti-static compounds. Most of these are relatively safe for handling, or they would not be approved for use. In 2016, the U.S. Food and Drug Administration banned three PFAS chemicals used in many pizza boxes and other food packaging that were added to cardboard to combat grease.
It is unlikely that food grown in compost from such paper would have higher concentrations than what you get on your hands when handling the material, but there is little information about composting such products.
Ink and Heavy Metals
There are two classes of inks used to print paper and cardboard: vegetable dyes and colored inks.
Most newspapers and nonglossy paper use vegetable dyes. These are perfectly safe in the garden.
Colored inks are used on some glossy paper and some cardboard, like cereal boxes. The problem with these inks is not the ink itself, but the fact that they may contain heavy metals. Heavy metals do not decompose in a compost pile or soil. Plants do absorb them from the soil, and both plants and animals accumulate them in tissues, which means our bodies have more and more each year. Even quite small amounts of heavy metals are a health concern.
One thing to remember is that native, organic soil also contains heavy metals. A Canadian study found average lead values ranged from 13 to 750 mg/kg, but this can be higher in older neighborhoods and industrial areas. Compare that to 2.6 mg/kg found in recycled cardboard. You would need to add a lot of composted cardboard to make a significant change to most soils.
Also, consider that bringing any type of organic material into your garden also adds heavy metals. This includes manure, compost, and mulch. Plants accumulate heavy metals, and bringing them onto your property increases the metals in your soil. But … remember, one of the important questions above is, do plants absorb more? An interesting study showed that adding compost made from biosolids (sewage sludge) to lead-contaminated soil reduced the amount of lead absorbed by plants. Organic matter has a high CEC and holds on to heavy metals and preventing roots from getting to them. Even though the soil lead amount was not reduced and was probably increased a bit, the plants contained less lead.
Adding composted paper containing heavy metals is not a great idea, but small amounts won’t impact the heavy metals in the food you produce.
But It’s Compostable!
A lot of products these days are claimed to be compostable, even some types of plastic. Check this link to understand what this really means.
Just because something can be composted does NOT mean the resulting compost is free of harmful chemicals.
Should You Compost Paper or Cardboard?
Understand that paper and cardboard compost very slowly because of a high lignin content. I found that even shredded paper is still mostly intact when the rest of the compost is done. I don’t think it is a good addition to a compost pile.
Is it safe? White, non-glossy paper, like newspaper or office paper, is quite safe. Dioxin, dyes, chlorine, and BPA are not a big concern.
Any composted paper is safe in a non-food ornamental bed.
Printed glossy paper and cardboard contain low amounts of heavy metals, which could be a concern. Unless you use a lot, you probably get a higher dose of heavy metals from driving to work (smog, exhaust, tire dust, etc.) than from eating produce from your garden. But heavy metals accumulate in your body, so it is prudent to try and keep the levels low. Use paper as sheet mulching to kill weeds the first year, but don’t add it to gardens in any significant amounts.
Hello! Thank you for writing this up. I’m making a “keyhole garden” (example here: https://texascooppower.com/keyhole-gardening/).
This uses layers of cardboard and compostable material to retain moisture in places of heat and drought, with a couple of inches of dirt on top.
Would the metals leach out through the bottom?
Would this kind of garden be toxic?
Are there any kits available to check vegetables for chemicals?
I’d love to hear your thoughts. Thank you!
Keyhole gardens don not have a layer of cardboard on top, under a couple of inches of soil. If the material is safe to use – it won’t be toxic.
There is no kit for measuring chemicals in vegetables. Everything in a vegetable is a chemical.
https://www.gardenmyths.com/natural-pesticides/
Shredded or cut into strips, how about cardboard for mulching (in low public areas)? I have a LOT of it my husband has hoarded…would love not to burn it.
I add 6lb of shredded, corrugated cardboard to my compost bay every week from March to October, mixed with grass clippings to help prevent the latter forming a slimy mass, turning once into the next bay when full.
By December, when I spread the first two lots (I usually fill three bays a year), composting & worm activity has completely broken down the cardboard.
The third bay which is filled by late October still has visible fragments of cardboard when spread in late March but this soon breaks down.
I’ve no concerns regarding the minuscule amounts of chemicals which may be present.
Well, worms digest things. Could possibly make a difference? As could, possibly, fungi, some of which have been known to digest some heavy metals (my friend’s doctoral thesis from 20 years ago).
Yes worms digest things – so do microbes in a compost pile.
“fungi digest heavy metals” – that depends on how you define “digest”. Assuming you mean to change the molecules as in digesting protein into amino acids – then fungi DO NOT digest heavy metals. They probably absorb them into their cells, but they are still there and when they die, the molecules are released. Heavy metals are elements that don’t change.
I was wondering that over time fungi would decompose and deactivate the dioxin and BPA. I also understand that they are also used to remediate heavy metal waste. What has research shown along those lines?
I burn all my cardboard & paper & add it to the compost bin in the ash. But not sure whether burning it changes the unwanted chemicals?
I won’t get rid of the heavy metals. It does make it alkaline, and burning is not good for the environment.
Thanks for the interesting
facts
I think a common source of paper or other fibre that ends up being composted in the home garden is coffee filters and tea bags that go into the kitchen compost pail every day, that’s a lot of fibre. But not all filters or bags are easily composted. I was drinking one brand of tea and they claimed their bags were made to be biodegradable. But even 2 years later I still find partial square bags in my compost and soil. I switched to another brand of tea and their round bags compost quite readily. ( the bag shape let’s me tell which brand is not composting)
tea bags are mentioned here: https://www.gardenmyths.com/compostable-plastic/
What about adding shredded paper to worm farms?
why is that different than composting?
Thanks! Really helpful.