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Wood Guide
Detailed information about how wood, “wood,” and what it’s stained or finished with, affects your health— plus the rating system I use, so you can keep it all organized.
Wood Ratings
I use these rating scales to stay consistent and unbiased as I rank products containing wood in the Interior Medicine shop from healthiest to least healthy. You can also use it as a guide when choosing wood furniture and flooring on your own. Scroll down to see all the information behind these scales.
Wood or “Wood” Type
Wood Stain
Wood Finish
Why three rating scales for one material (wood)?
Having three (optional) scales captures a more complete picture of wood’s level of material health by considering three things: the wood itself, and if it is treated with a stain and/or a finish. For example, an ultra-healthy piece of 100% solid hardwood may be stained with synthetic stain and finished in polyurethane, making it unhealthy. On the other hand, a composite wood product containing minimal formaldehyde may be finished in a plant-based oil, making it potentially healthier than the first example.
Wood Information
Click on bolded statements for links to information and research.
What is engineered wood?
Engineered wood consists of thin pieces of solid wood attached with glue to either side of a non-solid wood core. The core can be either plywood or ground wood fibers mixed with resin. The outer solid wood surface is called a veneer, and the layers are bound together with adhesives. The adhesives holding the layers together, and the glue-based binders in the plywood or ground wood fibers, are where the health issues potentially lie, since these glues typically contain urea-formaldehyde, a known carcinogen. Some newer engineered wood uses bio-based glues or NAF (No Added Formaldehyde) resins that are much healthier (some don’t, using polyurethane or other harmful substitutions). Engineered wood is often used in furniture construction, like in making couches, because it is stronger than solid wood and can hold up to many years of wear and tear.
Is engineered wood toxic?
It depends entirely on what the glue/resin/binder is that holds the wood together. If it is held together with a soy-based resin with no added formaldehyde, then it is a healthy option. If it is held together with a glue that contains formaldehyde, or a glue that contains other harmful chemicals like isocyanates, then it is harmful to health.
What is composite wood?
Composite wood is very similar to engineered wood, but the materials in the core aren’t always wood-based. Instead, it is often a blend of plastics and wood.
Wood and “Wood” Types
What is plywood?
Plywood is made of many layers of super-thin pieces of solid wood. Each layer’s grain is stacked perpendicular to the one above and below it, and all the layers are attached with an adhesive binder (plywood can have up to 3.5% binder by weight). Alternating the wood grain at perpendicular angles makes plywood even stronger than wood, and is why you see it used for home construction. After the layers are glued together, heat and pressure are applied to make a smooth plywood panel.
What is MDF?
MDF means medium-density fiberboard and it is made with wood pulp mixed with a resin binder (with up to 10% by weight) to make a wood-like product. This is what IKEA uses in their cupboards, for example, then covers them with a layer of laminate.
What is particleboard?
Particleboard is a type of composite wood made with wood chips mixed with a binder (up to 12% by weight) to make a wood-like product. This is what IKEA uses in its KALLAX furniture, for example.
What are other types of engineered and composite wood?
Oriented Strand Board (OSB), Hardboard, Laminated Veneer Lumber, Glulam, and other wood-type products are some of the other types of engineered wood (and there are a lot!) These are a little different than plywood, MDF, and particleboard for two main reasons: one, they’re not regulated by the EPA to ensure low levels of carcinogenic urea-formaldehyde, so may have higher levels of harmful off-gassing. Second, they’re less common in furniture, which of course, Interior Medicine focuses on, so that’s all I have to say about them!
What is a bio-based binder?
Some engineered and composite woods are now bound together using a soy-based resin, or a mushroom root (mycelium) based one.
What is NAF engineered wood (and ULEF?)
NAF stands for “No Added Formaldehyde.” This means the binders used to glue the wood together don’t contain urea-formaldehyde. This phrase is used instead of just “No Formaldehyde,” because all trees (and wood) actually emit super tiny amounts of formaldehyde naturally — an oak tree emits 0.009 parts per million (ppm) into the air. (For reference, it takes 1.0 ppm of formaldehyde in the air to start to irritate the nose and throat.)
ULEF means Ultra-Low Emitting Formaldehyde, and may be achieved by adding a chemical that sequesters, or captures, formaldehyde from escaping. These are called Formaldehyde Scavengers. It may also mean there is a blend of formaldehyde and non-formaldehyde based binders.
Composite wood that is “CARB or TSCA compliant” means that it has low formaldehyde levels, but not ultra-low. Since 2019, all plywood, MDF, and particleboard manufactured or imported into the US must be compliant.
What is flooring that looks like wood made of?
It’s usually either laminate or LVP, luxury vinyl plank.
Laminate flooring can have some wood in its core, or be 100% plastic, depending on the brand. The top layer is a piece of fiberboard with a photo of wood laminated to the top, then covered in a wear-resistant coating. This coating can be scratch-resistant, or water-resistant (which often means it contains PFAS).
LVP is 0% wood and is made with vinyl, widely regarded as the least sustainable and least healthy choice of plastic flooring available. Some brands have phthalates and harmful recycled content added to it. Some brands are coated in PFAS to make them more wear and water resistant.
Is bamboo solid wood?
Sometimes! Bamboo is technically a grass, and can be solid, or more like engineered wood, when used in home products.
Solid bamboo is sometimes used as flooring.
More often, like you see in butcher blocks or cutting boards, bamboo is processed in a vertical or horizontal fashion, a lot like plywood— thin strips of bamboo are glued and compressed together to make a strong, solid board. Or, bamboo veneers are glued to cores made with a variety of materials, depending on the brand.
Are wood stains toxic?
Wood stains are often, but not always, toxic to health. It depends on the kind you use.
A completely natural wood stain like coffee is safe.
Water-based wood stains are relatively non-toxic, especially ones with certifications like GreenSeal-11 ensuring the most harmful additives are avoided. Ones without VOCs, antimicrobials, chemical drying agents, or heavy metals are a better choice.
Oil-based wood stains with standard chemicals that release high VOCs are quite harmful to health.
Wood Stains
What is a 100% natural wood stain?
While I don’t know of any that come in a purchasable can, people do use coffee grounds, tea, walnut husks, and other natural materials to stain wood. Since these are generally less permanent, they require a finish on top to seal them in.
What wood stains do you recommend?
I rate wood stains in the context of wooden furniture, since Interior Medicine is currently focused on interior decoration, design, and tools like air purifiers. I leave specific recommendations on building materials and building material related products like wood stains to the experts in that area. I have used Rubio Monocoat and love it (their Oil Plus2C is perfectly healthy without its accelerator, and I am an affiliate; the link is my affiliate link).
What is the difference between wood stain and wood finish?
Wood stains add color to natural wood, while a finish protects it.
Wood Finishes
Why are oil-based finishes, but water-based stains, better?
Natural, oil-based finishes like linseed oil, hemp oil, or tung oil are minimally processed, and very low in VOCs. Water-based finishes often contain high-VOC, toxic solvents to make them work better.
On the other hand, natural, oil-based stains typically have higher VOCs, whereas their water-based counterparts, have less.
What are plant-based oil finishes?
Linseed, hemp, and tung oil are types of plant-based oil finishes. Linseed oil comes from the flax plant. Hemp oil comes from the cannabis plant. Tung oil comes from the Tung tree (Vernicia fordii), native to China, Myanmar, and North Vietnam.
What are solvent-based finishes?
Varnish, acrylic, synthetic lacquer, and urethane-based finishes all require a much higher solvent concentration than water-based finishes, so are quite high in VOCs that irritate your respiratory tract. They also often contain carcinogenic chemicals.
Is shellac toxic?
It depends which kind you’re thinking about!
Natural shellac comes from the cocoon-like secretions of the female lac bug, and has been used for thousands of years.
Synthetic shellac contains solvents, and has a harmful vapor.
What is FSC certified wood?
FSC, or the Forest Stewardship Council certification, is focused on responsible management of forests. For wood, it means that the trees it comes from meet forest management standards, indigenous people’s rights are respected when harvesting, and old growth forests are not disturbed. It’s a fantastic certification in terms of our environment and community, but FSC-certified wood can still be treated or finished with many other chemicals that harm your health.
Certifications
What does zero VOC mean?
There is no formal set of rules for what qualifies as zero VOC, and different companies and standards define it differently. Typically, zero-VOC or no-VOC means there are less than 5 grams of VOCs per liter and the smell is undetectable (though not all VOCs have a smell!) Ideally, a company that states their products are no- or low- VOC also has certifications from Green Seal, GreenGuard, or SCS Indoor Advantage verifying their claims are true.
What does low VOC mean?
There is no formal set of rules for what qualifies as low VOC, and different companies and standards define it differently. Typically, low-VOC means there are less than 50 grams of VOCs per liter of paint. Ideally, a company that states their products are no- or low- VOC also has certifications from Green Seal, GreenGuard, or SCS Indoor Advantage verifying their claims are true.
What is GreenGuard GOLD certified?
GreenGuard certification is useful for finding wood products that do not off-gas VOCs excessively. To meet the standard GreenGuard level, VOCs must be less than 500 ug/m3. To meet the elevated GreenGuard GOLD level, VOCs must be less than 220 ug/m3.
A GreenGuard certification does not mean it is healthy, and it does not mean there are no VOCs off-gassing from the product. Sometimes I see companies say “It’s GreenGuard so it’s healthy!” which is simply not true. There are harmful chemicals in stains, glues, and finishes that don’t off-gas as VOCs.
That said, it is still a useful certification when choosing between a standard wood product and one with GreenGuard certification. It is still a step in the right direction.
What is SCS Indoor Advantage certification?
Similar to GreenGuard, this is a certification verifying the product is truly no- or low-VOC. It is a respected and independent laboratory.
What is Green Seal 11 (GS-11) certification?
Green Seal’s GS-11 certification means that a wood stain or finish doesn’t have the worst or most harmful additives in it. This is because VOCs are not the only dangerous part of stains, adhesives, and finishes— there are many chemicals that do not smell that are issues for health. This is what the GS-11 covers.
GS-11 limit the levels of VOCs, as well as carcinogens, reproductive toxins, triclosan, phthalates, heavy metals, and others. You can see the full list of what they limit here.
What are 3rd party certifications and why do they matter so much?
Third-party certification means that an independent group that didn’t make the product, and has no financial ties to the product, has reviewed, tested, and verified the claims that the company making the product has made.
Third party certifications protect you. Just like greenwashing makes products appear better for the Earth than they really are, some companies intentionally make their products seem healthier for people than they really are, too.