Wood Treatments

AUTHOR
United States Environmental Protection Agency

Washington, D.C., 2016

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Overview of Wood Preservative Chemicals

Wood preservative products are those that claim to control wood degradation problems due to fungal rot or decay, sapstain, molds, or wood-destroying insects. Both the treatment process and the use of treated-products can result in exposure to pesticides for both people and the environment. Most of the treatment processes and uses of treated products occur outdoors. There are wood preservatives that support a tolerance for indirect food-contact uses such as wooden crates, pallets, and stakes used to store or grow raw agriculture commodities.

Generally, freshly cut logs or lumber are treated and then manufactured into products such as:

  • Seasoned building materials.
  • Utility poles, fence posts and rails.
  • Structural members.
  • Structures and dwellings.
  • Transportation vehicles (truck beds and support structures).
  • Crop containers.
  • Lawn furniture and decks.
  • Playground equipment.
  • Garden/landscape timbers.
  • Log homes.

Re-registration of Older Wood Preservatives

The health and environmental impacts of older wood preservatives (those registered before November 1, 1984) have been evaluated through EPA’s reregistration program. Re-registration involves a thorough review of the scientific database underlying a pesticide’s registration. The purpose of the review is to:

  • reassess the potential hazards arising from the currently registered uses of the pesticide;
  • determine the need for additional data on health and environmental effects; and
  • determine whether or not the pesticide meets the “no unreasonable adverse effects” criteria of FIFRA.

In 2008, EPA determined that the three heavy-duty wood preservatives were eligible for reregistration provided the mitigation measures and associated label changes identified in the Reregistration Eligibility Decision Documents (REDs) were implemented. These wood preservatives are:

  • chromated arsenicals (contains copper and some combination of chromium and/or arsenic);
  • pentachlorophenol (PCP); and
  • creosote.

 In the risk assessments for the REDs, we identified risks of concern associated with occupational exposure (i.e., treatment plant workers) to all three preservatives and ecological exposure to pentachlorophenol and creosote. Since then, all heavy duty wood preservative product labels have been amended to reflect the mitigation measures specified in the RED.

Chromated Arsenicals

Wood preservatives containing chromated arsenicals include preservatives containing chromium, copper and arsenic. Since the 1940s, wood has been pressure treated with chromated arsenicals to protect wood from rotting due to insect and microbial agent attack and wood-boring marine invertebrates. From the 1970s to the early 2000s, the majority of the wood used in outdoor residential settings was chromated arsenical-treated wood.

Effective December 31, 2003, chromated arsenical manufacturers voluntarily canceled virtually all residential uses of CCA, and wood products treated with CCA are no longer used in most residential settings, including decks and children’s playsets. EPA has classified chromated arsenicals as restricted use products, for use only by certified pesticide applicators. Read more about CCA.

Creosote

Creosote has been used since 1948 as a heavy duty wood preservative. Creosote is obtained from high temperature distillation of coal tar. Pesticide products containing creosote as the active ingredient are used to protect wood against termites, fungi, mites and other pests that can degrade or threaten the integrity of wood products.

Currently, creosote is used for commercial purposes only; it has no registered residential uses. Creosote is a restricted use pesticide that can be used in outdoor settings such as in railroad ties and utility poles. Indoor applications of creosote are prohibited as well as application to wood intended for use in interiors or for use in contact with food, feed, or drinking water. Read more about creosote.

Pentachlorophenol

Pentachlorophenol (PCP) was registered as a pesticide on December 1, 1950. PCP was one of the most widely used biocides in the United States before 1987 when pentachlorophenol uses as an herbicide, defoliant, mossicide and disinfectant were removed from product labels.

Currently, there are no registered residential uses. PCP is a restricted use pesticide that is only used for commercial purposes such as utility poles, railroad ties and wharf pilings. Only pressure and thermal treatments of PCP are allowed.

Registration Review of Older Wood Preservatives 

Another re-evaluation of these older wood preservatives is occurring under EPA’s Registration Review Program. Under FIFRA section 3(g), EPA will review each registered pesticide every 15 years to determine whether it continues to meet the statutory standard of no unreasonable adverse effects on human health or the environment. We will make full use of the information from the previous evaluations as well as any new information in this process. Chromated arsenicals and creosote began their registration review process in 2015.

Newer Wood Preservatives 

Propiconazole

Propiconazole is a triazole fungicide that was first registered in 1981. Propiconazole has been approved by EPA for preserving wood used in millwork, shingles and shakes, siding, plywood, structural lumber and timbers and composites that are used in above ground applications only. Propiconazole by itself does not protect wood against insect damage.

Propiconazole has been approved for surface application or pressure treatment of siding, plywood, millwork, shingles and shakes and above-ground structural lumber and timbers.

Triadimefon

Triadimefon is a triazole fungicide that was first registered as a wood preservative in 2009. Triadimefon has been approved by EPA for preserving wood-based composite products and wood products intended for above ground and in ground contact such as wood decking, patio furniture, millwork, guardrails, utility poles, foundation pilings, and fences. Triadimefon can be applied by dip or pressure treatment.

Acid Copper Chromate (ACC)

ACC is a wood preservative that is only registered for industrial and commercial uses. The compound will be reevaluated under the Chromated Arsenicals registration review case.

Newer Wood Preservatives for Residential Users

More recently, EPA has registered several new wood preservative active ingredients. These wood preservatives have lower toxicity profiles when compared to older wood preservatives. As required under section 3(g) of FIFRA, these newer wood preservatives will be re-evaluated through EPA’s registration review process.

The following chemical wood preservatives are registered for treatment of lumber to be used in the residential lumber and timber market:

  • ACQ.
  • Borates.
  • Copper azole.
  • Copper naphthenate.
  • Copper-HDO (Bis-(Ncyclohexyldiazeniumdioxy-copper)).
  • Polymeric betaine.

Of these chemicals, ACQ currently is the most widely used wood preservative for residential applications.

ACQ

ACQ (alkaline copper quaternary) is a water-based wood preservative that prevents decay from fungi and insects (i.e., it is a fungicide and insecticide). It also has relatively low risks, based on its components of copper oxide and quaternary ammonium compounds.

Water-based preservatives like ACQ leave a dry, paintable surface. ACQ is registered for use on: lumber, timbers, landscape ties, fence posts, building and utility poles, land, freshwater and marine pilings, sea walls, decking, wood shingles, and other wood structures.

Borates

Disodium octaborate tetrahydrate (DOT) is specially formulated for use as a water-based wood preservative and is registered by EPA as well as government agencies throughout Asia, North America and Europe. Typical applications include: furnishings and interior construction, such as framing, sheathing, sill plates, furring strips, trusses, and joists.

Copper Azole

Copper azole is a water-based wood preservative that prevents fungal decay and insect attack; it is a fungicide and insecticide. It is widely used throughout the United States and Canada.

Water-based preservatives like copper azole leave wood with a clean, paintable surface after they dry. Copper azole is registered for treatment of millwork, shingles and shakes, siding, plywood, structural lumber, fence posts, building and utility poles, land and freshwater piling, composites, and other wood products that are used in above-ground, ground contact and fresh water as well as in salt water splash (marine) decking applications.

Copper Napthenate

Copper napthenate was first registered in 1951 and is used to brush, dip, spray, and pressure treat wood that will be used in ground contact, water contact, and above ground such as utility poles, docks, posts, piers, fences, and landscape timbers. Copper napthenate is effective in protecting wood against insect damage.

Copper – HDO (Bis-(Ncyclohexyldiazeniumdioxy- copper)

Copper – HDO was first registered in 2005 and is used to pressure-treat wood that will be used as decking, rails, spindles, framing, sill plates, gazebos, fencing, and posts. It is restricted from use in aquatic areas, construction of beehives, or any application associated with the packaging of food or feed.

Polymeric Betaine

Polymeric betaine was first registered as an active ingredient in the United States in 2006. It is a borate ester that, when applied to wood, breaks down to DDAC (didecyl dimethyl ammonium chloride) and boric acid. Polymeric betaine is applied by pressure treatment to forest products.

Pressure Treatment Preservatives

James P. Wacker and Douglas M. Crawford

Wood preservatives are broadly classified as either oil-based or water-based, based on the chemical composition of the preservative and the carrier used during the pressure treating process (Lebow and Makel 1995). Table 2 summarizes wood preservatives commercially available and applied with pressure-treatment methods for timber bridges. Some key preservative characteristics important to timber bridge applications are chemical composition, surface cleanliness, migration potential, odor, and environmental status.

treated timber
Wood Treatments Chart

a) Based on current regulations of U.S. Environmental Protection Agency. “Restricted use pesticides” does not ban the use of treated wood, but requires certified applicators trained in proper and safe handling techniques. “Unclassified” does not require certified applicators and are commercially available.
b) Waterborne preservatives using ammonium solutions are typically used to achieve good penetration in difficult-to-treat species.
c) Beginning in 2004, the use of CCA wood preservative in the United States will be restricted to industrial and commercial uses (such as bridges) and not permitted for residential applications.