Selecting healthy and environmentally sound stains
Alex Pennock wrote this article, with contributions from Marian Keeler and Willem Maas.
Stains color and protect wood surfaces in your home, accentuating the wood’s natural beauty. These finishes are composed of pigment or dye, solvent and binder. Pigments are similar to those in paints and stick to the wood’s surface; dyes penetrate the wood’s fibers and change their color. Solvents, or carriers, keep the colorants and binders in a liquid form until the stain is applied; then they evaporate, leaving the other ingredients to color and protect the wood. Binders, or resins, hold the colorant in place and protect the wood by forming a coating on the surface or penetrating the wood.
QUICK LINKS TO MATERIALS REVIEWED IN THIS ARTICLE
Environmental impacts of stain manufacturing and use vary. Petroleum-derived solvents are particularly damaging: extraction and shipping of petroleum, followed by its processing and manufacturing into solvents and chemicals, requires large amounts of energy. Acrylic and urethane stains contain the highest amounts of petroleum-derived solvents, followed by natural oil stains and then water-based stains, which contain the least.
As a stain dries and its solvents evaporate, it releases volatile organic compounds (VOCs), which contribute to air pollution and can cause health problems. When considering the VOC content of any product, keep in mind that regulatory standards for VOC content are intended to reduce emissions of VOCs that cause smog, not to improve indoor air quality. The U.S. Environmental Protection Agency (EPA) has found that indoor concentrations of VOCs are regularly up to ten times as high as outdoor concentrations, and as much as a thousand times as high when you are applying stains. (See About VOCs for more information.)
Beyond VOCs, many stains are made with toxic substances that come from nonrenewable resources or are energy-intensive or polluting to produce, so even low-VOC stains can have an impact on the environment. For example, water-based stains may use toxic glycol ether as a solvent, and any stain could use toxic or heavily manufactured pigments such as copper-based Paris green, which is poisonous, rather than renewable or abundant pigments such as tannins derived from wood or minerals like iron oxide.
"Natural" does not necessarily mean "safe" when it comes to pigments, so ask questions about any stain not labeled nontoxic. Additionally, manufacturers are not required to disclose all the chemicals used in their products; some ingredients are deemed proprietary information or are used in such small quantities that they do not have to be reported. If you have any doubts about a product, call the manufacturer and ask for a list of ingredients.
STAIN PROS AND CONS IN DEPTH
For a summary of the environmental pros and cons of the materials discussed here, see Green Home Guide’s Buyer's Guide to Stains.
Natural Oil Stains (linseed, tung, and soya oil binders)
Natural oil stains are linseed and tung oils that penetrate wood and either dye the wood grain or leave pigment near the surface as they penetrate. These oils come from flax seeds and tung tree nuts, respectively, so they are renewable resources, but they must be thinned with petroleum-derived solvents in order to penetrate wood.
Because of their durability and good wood penetration, natural oil stains do not require a sealer. These stains last longer and are more durable than unsealed water-based stains because there is no film on the wood to chip or fail. And while the petroleum-derived and synthetic solvents used in natural oil stains lead to high VOC emissions during application, these stains have lower VOC content than the clear oil-based finishes like lacquer and varnish used to seal other stains.
Natural oil stains are environmentally preferable to acrylic or urethane stains because manufacturing synthetic binders is more damaging than processing natural oils.
Acrylic/Urethane Stains (acrylic or urethane binders)
These synthetic stains are compositionally similar to oil paints, forming a colored coating of pigment and binder on the wood’s surface. They are easy to apply, drying slowly and forming a smooth coat, but they contain the most harmful chemicals of the stains discussed here. And because they dry so slowly, they expose you to high VOC levels for longer periods of time.
Acrylic and urethane stains are similar to but less durable than lacquers and varnishes; they contain the same binders, but pigments displace some binder, so less binder is left behind as a coating on the surface. Acrylic and urethane stains can be made more durable by adding a layer of water-based sealer. These stains have lower VOC content than natural oil-based stains, but producing synthetic binders is worse for the environment than producing natural oils.
Acrylic and urethane stains are most often used on exterior surfaces. For similar results with lower environmental and health impacts, use separate water-based stains and sealers.
Water-based Stains (acrylic or urethane binders)
Water-based stains use water in the place of most petroleum-derived solvents, so they are preferable in terms of health and environmental impacts and provide only slightly less protection to wood.
While applying water-based stains you are exposed to fewer VOCs and solvents for a shorter period of time since these stains dry faster than natural oil, acrylic, or urethane stains, and once they are dry their air quality impact is low. However, water-based stains typically contain small amounts of solvents call glycol ethers, which can be absorbed through the skin and damage your liver and kidneys, so check each product's material safety data sheet (MSDS) for glycol solvents like ethylene glycol. Ethylene glycol is identified as a Federal Hazardous Air Pollutant and in California as a Toxic Air Contaminant. Stains that contain no glycol ethers are preferable; propylene glycol is generally considered a less toxic type.
For wood floor refinishing, we recommend using a clear water-based sealer on top of a water-based stain to maintain the stain’s color and increase its durability; without this extra layer, water-based stains are not durable enough for floors.
The Green Guide's Wood Finishes Buying Guide
Consult this guide for a comparison of specific products, including natural oil-based and water-based stains and finishes.
Green Seal’s "Choose Green: Wood Finishes and Stains"
A report by Green Seal, an independent, nonprofit certification organization, describing health and environmental concerns associated with wood stains and finishes. Green Seal's list of certified paint and coating products (still in development) is available here.
"Understanding Stains," from Popular Woodworking magazine
This article, written by an experienced wood finishes expert, describes the composition of different types of stains and offers tips on how to apply them.
Green Home Guide’s "Selecting Healthy and Environmentally Sound Finishes"
For an in-depth discussion of clear finishes (including water-based sealers that can be applied over water-based stains, as described above), check out this article in our series on coatings in the home.
The EPA's "An Introduction to Indoor Air Quality – VOCs"
This site provides more information on the sources and health effects of VOCs and indoor air pollution.