Is it Safe to Compost Paper and Cardboard?

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

It is clear that 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 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 addresses 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 a lot of applications we want 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 sun and it’s 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 the exposure of BPA to “the general population are 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 a number of “food products including fresh turkey, canned green beans, and canned infant formula“.

BPA exposure is a concern in other areas of our life – 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 glue is used. One is to make the actual carboard and the second is used to form the boxes.

The glue used to make carboard 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 carboard 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 newspaper 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 in 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 prevents 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 composts very slowly because of a high lignin content. Personally, 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 level low. Use paper as sheet mulching to kill weeds the first year, but don’t add them to gardens in any significant amounts.