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Overview of Wood Preservative Chemicals

Wood preservative products are those that control wood degradation problems due to fungal rot or decay, sapstain, molds, or wood-destroying insects. Both the treatment process and the use of treatedproducts can result in risks to human health and the environment. Treated wood is most commonly used outdoors.

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.
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Reevaluation of Older Wood Preservatives

The three heavy-duty wood preservatives (chromated arsenicals, creosote, and pentachlorophenol) are currently undergoing  registration review, a process EPA conducts for all registered pesticides every 15 years to ensure that products can carry out their intended function without creating unreasonable risks to human health and the environment. 

In 2008, EPA determined that chromated arsenicals, creosote, and pentachlorophenol could remain in use as long as certain mitigation measures identified in the Reregistration Eligibility Decision Documents (REDs) were implemented. These measures included engineering controls such as ventilation and automatic doors for locking and unlocking treatment cylinders.

In 2019, EPA completed its draft risk assessments for chromated arsenicals, creosote, and pentachlorophenol as a part of its registration review. In each case, EPA found that although the measures required by the REDs reduced worker exposure, these products continued to pose health risks of concern to the workers who apply them. Creosote and chromated arsenicals were also found to pose risks to the environment.

In 2021, EPA issued proposed interim decisions for chromated arsenicals, creosote, and pentachlorophenol to address the human health and environmental risks of using these chemicals. EPA determined that pentachlorophenol’s risks outweighed its benefits and proposed cancelation. For creosote and chromated arsenicals, EPA proposed additional mitigation measures to protect the health of workers in wood treatment facilities.

Next, EPA will issue interim decisions finalizing the measures put forward in the proposed interim decision. View EPA’s registration review schedules.

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. It can be used to produce commercial wood poles, posts, shakes, shingles, permanent foundation support beams, pilings, and other wood  products permitted by approved labeling. 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 used for commercial purposes, mainly for treating utility poles.  Only pressure and thermal treatments of PCP are allowed. Read more about PCP.

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Alternative 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 was 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.

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.

Isothiazolinones

Three chemicals in a class called isothiazolinones can be used as wood preservatives.

The most common of these is DCOIT (3(2H)-isothiazolone, 4,5-dichloro-2-octyl), which was first registered in 1996 as a wood preservative for use via pressure treatment, for sapstain protection, and in millwork applications. In 2018, it was also approved for use in utility poles. Further information is available at in docket EPA-HQ-OPP-2014-0403.

OIT (2-n-octyl-4-isothiazolin-3-one), another isothiazolone, is used as a sapstain wood preservative. Information on OIT is available in docket EPA-HQ-OPP-2014-0160.

Finally, a mixture of the isothiazolones MIT (2-methyl-4-isothiazoline-3-one) and CMIT (5-chloro-2-methyl-4-isothiazoline-3-one) is used in pressure treatment of wood. Further information is available in docket EPA-HQ-OPP-2013-0605.

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Newer Wood Preservatives for Residential Uses

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: 

  • Alkaline copper quatenary (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.

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For More Information

Many of the documents about these pesticides, such as registration review workplans or REDs are available in the Chemical Search database.

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