Cardboard is mostly wood fiber, but it contains far more than just cellulose. From the moment trees are pulped to the point a finished box reaches your doorstep, dozens of chemicals enter the picture as manufacturing aids, adhesives, coatings, inks, and, in the case of recycled board, contaminants carried over from previous uses. Some of these substances are benign; others have drawn regulatory scrutiny, particularly when cardboard touches food.
The Wood Fiber Foundation
At its core, cardboard starts as wood. The three main components of wood fiber are cellulose, hemicellulose, and lignin. Cellulose is the long-chain sugar polymer that gives paper its structure. Hemicellulose is a shorter, branched polymer that helps bind cellulose fibers together. Lignin is the stiff, dark-colored compound that makes wood rigid. In a freshly cut softwood log, cellulose makes up roughly 40 to 45 percent of the dry weight, hemicellulose accounts for about 25 to 30 percent, and lignin fills in most of the rest.
The type of cardboard determines how much of each survives into the final product. White, bleached paperboard for milk cartons and cereal boxes goes through aggressive chemical pulping that strips away most of the lignin. The brown corrugated shipping boxes you get from online orders, on the other hand, retain more lignin, which is why they are brown. That leftover lignin also explains the distinctive smell of a fresh cardboard box and contributes to the volatile compounds the material releases over time.
Chemicals Added During Pulping and Bleaching
Turning wood chips into paper pulp requires strong chemicals. The dominant industrial method, called the kraft process, cooks wood in a solution of sodium hydroxide and sodium sulfide. This dissolves much of the lignin and frees the cellulose fibers. Traces of sulfur compounds remain in the pulp, which is one reason unbleached cardboard can have a faintly sulfurous odor.
When white paperboard is needed, the pulp goes through additional bleaching stages. Modern mills largely use chlorine dioxide, oxygen, or hydrogen peroxide for bleaching. Before the late 1980s, elemental chlorine was common, and it produced trace amounts of polychlorinated dioxins and furans as byproducts. A study of Canadian bleached paperboard milk cartons from that era found dioxin and furan levels ranging from about 1.4 to 55 nanograms of toxic equivalents per kilogram of board. After mid-1989, when mills switched to chlorine-free or chlorine-dioxide bleaching, those same contaminants dropped below detection limits.1Journal of Agricultural and Food Chemistry. Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans in Canadian bleached paperboard milk containers (1988-1989) and their transfer to fluid milk That transition was one of the major environmental wins in the paper industry, and modern bleached cardboard carries essentially no detectable dioxins.
Sizing Agents and Wet-Strength Resins
Raw paper fibers are naturally absorbent, which is a problem if you want a box that holds up in a warehouse or survives a rainstorm on your porch. To fix this, papermakers add sizing agents, chemicals that reduce a sheet’s tendency to absorb liquid water. Two of the most common are alkyl ketene dimer and rosin sizing, both of which coat the fibers and make the surface more water-repellent.2BioResources. Effects of hydrophobic sizing on paper dry and wet-strength properties: A comparative study between AKD sizing of NBSK handsheets and rosin sizing of CTMP handsheets Rosin sizing is the older technology and uses a natural resin derived from pine trees, combined with alum (aluminum sulfate) to bind it to the fiber. Alkyl ketene dimer is a synthetic wax that reacts directly with the cellulose.
For applications where the cardboard absolutely must not fall apart when wet, such as beverage carriers or frozen-food boxes, manufacturers add polymeric wet-strength resins. These are typically synthetic polymers that cross-link with the cellulose fibers, locking them together even when the paper is saturated. They play a big role in the durability and dimensional stability of the finished product.3PubMed Central. Polymeric Wet-Strength Agents in the Paper Industry: An Overview of Mechanisms and Current Challenges The most widely used wet-strength resin is polyamidoamine-epichlorohydrin, often abbreviated PAE. It is effective in small amounts but is a synthetic chemical that stays in the finished board.
Starch Adhesive and Corrugating Chemicals
If you peel apart a corrugated cardboard box, you will notice the wavy fluted layer glued between two flat liner sheets. That bond is made with a starch-based adhesive. The glue is a two-phase mixture: a gelled blend of starch and sodium hydroxide in water forms the liquid carrier, while ungelled starch and borax are suspended in it as a solid phase.4Alexandria Engineering Journal. Additives for enhancing the drying properties of adhesives for corrugated boards The sodium hydroxide gelatinizes part of the starch so it becomes sticky, and the borax acts as a cross-linker that stiffens the bond once it dries. This adhesive is cheap, effective, and mostly food-safe, which is one reason corrugated cardboard is accepted for shipping food products. Some corrugating plants also add small amounts of wax emulsions or water-resistance additives to the adhesive when the box needs to handle moisture.
Inks and Printing Chemicals
Nearly every piece of cardboard you encounter has been printed on, whether with a brand logo, shipping label, or barcode. Printing inks used on cardboard are complex mixtures in their own right. They contain pigments or dyes for color, binders (usually resins) that adhere the pigment to the surface, solvents or water as a carrier, and various additives like driers and wetting agents.
For high-volume packaging, flexographic and offset lithographic inks are most common. Water-based flexo inks have largely replaced solvent-based formulations for food-contact packaging, reducing the volatile organic compound load. UV-curable inks, which harden under ultraviolet light rather than evaporating a solvent, are another growing option. These inks contain photoinitiators, chemicals that absorb UV light and trigger the curing reaction. Some photoinitiators have raised food-safety concerns because they can migrate from the outer printed surface through the board and into food on the other side.
Research on how printing inks interact with multilayer cardboard has shown that the fiber composition matters. When packaging paper contained modified synthetic fibers, for example, it achieved maximum print density with a thinner ink layer, meaning less ink was needed for the same visual result.5Proceedings of the National Academy of Sciences of Belarus, Chemical Series. IR-spectroscopic analysis of interaction with multi-layer paper and cardboard containing synthetic fibers with printing inks This matters because less ink means fewer ink-derived chemicals in the finished product.
Grease Barriers and PFAS
When cardboard needs to resist grease, as in pizza boxes, microwave popcorn bags, or fast-food wrappers, manufacturers have historically applied per- and polyfluoroalkyl substances, commonly known as PFAS. These fluorinated chemicals create an effective barrier that keeps oil and moisture from soaking through the paper. The problem is that PFAS are extraordinarily persistent in the environment and in the human body, earning them the label “forever chemicals.” They have been linked to immune system effects, hormonal disruption, and elevated cholesterol in epidemiological studies, though the exact risk from food-packaging exposure specifically remains an active area of research.
Regulatory pressure has been mounting. Several U.S. states have banned PFAS in food packaging outright, and the FDA announced in 2024 that major manufacturers had voluntarily phased out PFAS-based grease-proofing agents for food-contact paper and paperboard. Alternatives include wax coatings, silicone-based treatments, and bio-based barrier coatings, though none match the performance of PFAS across every application. If you are concerned about PFAS in cardboard food containers, look for products labeled PFAS-free, though independent testing has shown that labels are not always reliable.
What Recycled Cardboard Picks Up
Recycled cardboard deserves its own discussion because the recycling process brings in chemicals that were never part of the original manufacturing. Every time paper and cardboard go through the recycling stream, they carry residues from their previous life: printing inks, adhesives, coatings, and whatever they were in contact with during use. The result is that recycled board is chemically more complex than virgin board.
The most studied contaminant category is mineral oil. Mineral oil hydrocarbons enter recycled cardboard mainly through the printing inks used on newspapers, which are a major feedstock for recycled board. Unprinted recycled board has been found to contain roughly 300 to 1,000 milligrams per kilogram of mineral oil, while newspapers themselves contain around 3,000 milligrams per kilogram.6European Food Research and Technology. Is recycled newspaper suitable for food contact materials? Technical grade mineral oils from printing inks These are not the purified mineral oils used in cosmetics. They are technical-grade oils that contain roughly 15 to 25 percent aromatic compounds, which are the fraction that raises the most health concern.6European Food Research and Technology. Is recycled newspaper suitable for food contact materials? Technical grade mineral oils from printing inks
Researchers distinguish two classes within these mineral oils: mineral oil saturated hydrocarbons (MOSH) and mineral oil aromatic hydrocarbons (MOAH). Testing of foods packaged in recycled cardboard has found measurable levels of both. One study detected 102 milligrams per kilogram of MOSH and about 8 milligrams per kilogram of MOAH in a tea sample packaged in recycled cardboard, making it the most contaminated sample in that analysis. Rice and sugar powder also showed elevated levels.7PubMed. Rapid and sensitive solid phase extraction-large volume injection-gas chromatography for the analysis of mineral oil saturated and aromatic hydrocarbons in cardboard and dried foods Dry, porous foods like rice, pasta, and tea are especially vulnerable because they absorb volatile compounds from the surrounding packaging over time.
Beyond mineral oils, a broad chemical profiling study of paper recycling grades found that phthalates, long-chain aliphatic compounds, and fatty acids were the most commonly detected compounds. Phthalates and bisphenols were also prevalent when more sensitive analytical techniques were used.8PubMed. Chemical profiling of paper recycling grades using GC-MS and LC-MS: An exploration of contaminants and their possible sources Phthalates come from adhesives, inks, and coatings in prior product lives. Bisphenols, most famously bisphenol A (BPA), enter the recycling stream largely through thermal receipt paper, which uses BPA or its substitutes as a color developer. When receipts get tossed into paper recycling bins, their bisphenol content spreads throughout the recycled pulp.
How These Chemicals Migrate into Food
Having chemicals in the cardboard is one thing. Whether they actually reach the food inside is a separate question, and the answer depends on several factors: what the chemical is, how volatile it is, whether the food is dry or moist, how long it sits in the packaging, and the storage temperature.
Volatile and semi-volatile substances like mineral oil hydrocarbons can migrate through the gas phase, meaning they evaporate from the cardboard and condense onto the food without the two ever touching directly. This is why a plastic inner bag alone does not always protect food inside a recycled cardboard box. Migration studies have validated models for the movement of key contaminants, including aliphatic and aromatic mineral oils, photoinitiators from inks, and plasticizers, across temperatures ranging from 20 to 60 degrees Celsius.9PubMed. Project SafeFoodPack Design: case study on indirect migration from paper and boards Higher temperatures accelerate migration substantially, which matters for products stored in warm warehouses or shipped in summer heat.
European regulators have been more aggressive than their American counterparts on this issue. Germany, for instance, introduced draft legislation to set limits on mineral oil migration from recycled paper into food, though a binding EU-wide regulation is still in development. In the meantime, some food manufacturers have switched to virgin cardboard or added functional barrier layers, such as aluminum foil or specialized polymer films, between the recycled outer box and the food product.
Volatile Organic Compounds That Off-Gas Over Time
Even without recycling, cardboard naturally releases volatile organic compounds as it ages. The cellulose and lignin in paper slowly break down through oxidation, releasing acetic acid, aldehydes, alcohols, and esters. Research has shown that hexanal is particularly damaging to the paper itself, degrading the cellulose and reducing mechanical strength more than acetic acid does.10Scientific Reports. Degradation of paper products due to volatile organic compounds This matters most for archival storage and museum contexts, where acid-free boxes are used to protect documents and artifacts from self-destructing.
Recycled cardboard off-gasses more diverse compounds than virgin board. A comparison of different packaging materials found that cellulose-based materials, particularly recycled varieties, emitted elevated levels of long-chain alkanes, substituted phenols, and aromatic compounds on top of the usual aging byproducts.11PubMed Central. Quality Assessment of Box Materials for Long‐Term Archival Storage: VOC Emissions Are Not a Significant Concern These extra emissions likely reflect the processing residues, resin remnants, and additive traces carried over from recycling. For everyday shipping, the levels are low enough that most people would never notice. For long-term storage of sensitive goods like fine art, rare books, or museum specimens, the choice between recycled and virgin acid-free board is a real consideration.
Heavy Metals in Recycled Board
Recycled cardboard can also carry trace amounts of heavy metals, introduced through printing inks, pigments, adhesives, and whatever the paper contacted during its previous use. A study examining food packaging made from recycled papers found detectable levels of copper, zinc, manganese, chromium, cadmium, and lead. Most metals fell below the limits set by the European Commission and the U.S. EPA, but lead and cadmium exceeded the stricter EU standards.12PubMed Central. Food packaging from recycled papers: chemical, physical, optical properties and heavy metal migration
The practical risk here is modest for most consumers. Heavy metals in cardboard are bound within the fiber matrix and do not migrate readily into dry foods. The concern grows when recycled board contacts moist or acidic foods, which can pull metals into solution. This is one reason regulators draw a distinction between cardboard used for outer shipping containers, where direct food contact is minimal, and board used for primary food packaging that sits right against the product.
How Virgin and Recycled Board Compare
The pattern across all these chemical categories is consistent: virgin cardboard made from fresh wood pulp is chemically simpler and cleaner. It contains the base wood polymers, the manufacturing additives deliberately put there (sizing agents, wet-strength resins, starch glue), and any inks or coatings applied to the finished product. Recycled cardboard contains all of those plus the accumulated chemical history of its previous lives, including mineral oils, phthalates, bisphenols, and trace metals that virgin board does not carry.
This does not mean recycled cardboard is unsafe. For shipping boxes, storage containers, and non-food applications, the contamination levels are a non-issue. The questions arise specifically at the food-contact boundary, where the European Food Safety Authority and other bodies have pushed for tighter controls on what recycled board can be used for and what barrier layers are needed between the board and the food. If you are storing dry goods like cereal or pasta in a recycled cardboard box at room temperature for a few weeks, the exposure is small. If you are storing loose tea in direct contact with recycled board in a warm pantry for months, the mineral oil migration could become meaningful.
Chemicals You Will Not Find in Modern Cardboard
A few historically important contaminants are worth mentioning precisely because they have largely disappeared. Elemental chlorine bleaching, as noted earlier, produced trace dioxins and furans in white paperboard through the 1980s but was phased out across most of the developed world by the early 1990s. Formaldehyde-based wet-strength resins, once common, have been substantially replaced by PAE and other alternatives that release far less free formaldehyde during use. Pentachlorophenol, once used as a biocide to prevent mold growth during pulp storage and shipping, was banned or restricted in most countries by the late 20th century.
Asbestos is another chemical that occasionally comes up in alarmed social media posts about cardboard, but it has no connection to modern cardboard manufacturing. The confusion sometimes arises from old industrial insulation boards that were made with asbestos fibers, which are an entirely different product from corrugated cardboard or paperboard. Standard wood-pulp cardboard does not and has never contained asbestos as a component.