Creosote: An Environmentally Damaging Wood Preservative

Creosote is one of the best-known environmentally damaging preservatives still used on heavy-duty wood such as fence posts, utility poles, pole barns, and farm outbuildings. It is a complex mixture of hundreds of chemicals, including polycyclic aromatic hydrocarbons (PAHs), phenols, and heterocyclic compounds, many of which are toxic, mutagenic, or carcinogenic.

Originally developed during the Industrial Revolution to extend the life of railroad ties and marine timbers, creosote is derived from the distillation of coal tar, a by-product of coal gasification. The coal tar is heated to high temperatures and the resulting vapors are condensed to produce the thick, oily black liquid recognized as creosote. Depending on the feedstock and process, creosote can contain especially high levels of PAHs that are known to be carcinogenic and highly toxic to aquatic life.

Creosote-treated wood is dark brown to black and has a strong, tar-like odor. It repels moisture and inhibits fungi and insects, which explains its long history in outdoor construction. However, these same chemicals can volatilize into the air, wash off into nearby streams and ditches, or leach into surrounding soil.

Health risks from creosote include:

• Skin irritation, burns, and photosensitivity from direct contact.
• Respiratory irritation from inhaling vapors or dust.
• Liver, kidney, and immune-system effects with prolonged exposure.
• Increased risk of skin and lung cancer with long-term occupational exposure.

Ecologically, creosote can persist for decades in saturated soils and sediments, damaging invertebrate communities, contaminating fish, and reducing overall biodiversity. For these reasons, many jurisdictions restrict creosote to industrial applications and advise against its use near homes, gardens, or water bodies.

Where possible, landowners should avoid creosote-treated timbers around livestock areas, children’s play spaces, vegetable gardens, and streams or wetlands, and look to safer materials and naturally durable woods instead.

Pentachlorophenol (PCP): A Highly Persistent Wood Preservative

Pentachlorophenol (PCP) is another historically common, environmentally damaging preservative used to treat railroad ties, utility poles, fence posts, and farm structures. PCP is a chlorinated hydrocarbon—essentially a phenol molecule with five chlorine atoms attached—that is highly persistent and extremely toxic to humans and wildlife.

First synthesized in the 1930s as a broad-spectrum pesticide, PCP was adopted as a wood preservative because it penetrates deeply, repels moisture, and is lethal to fungi, insects, and many microorganisms. Technical-grade PCP often contains impurities such as dioxins and furans, which are among the most toxic compounds ever studied.

PCP is manufactured by chlorinating phenol in the presence of an acid catalyst, then purifying the resulting solid. Even with purification, the process generates trace levels of dioxins that may end up in the final product or waste streams.

In treated lumber, PCP has been used on:

• Wood decks and fence posts in agricultural and industrial settings
• Pole barns, sheds, and rural outbuildings
• Utility poles and cross-arms

PCP is usually applied by pressure treatment, soaking, or brushing concentrated solution onto timbers. Over time, it can volatilize into the air or leach into soil and groundwater.

Health and environmental concerns include:

• Skin and eye irritation; respiratory distress when vapors or dust are inhaled
• Damage to liver, kidneys, and nervous system from longer-term exposure
• Increased cancer risk due to PCP itself and associated dioxin contaminants
• Severe toxicity to fish, amphibians, and aquatic invertebrates when PCP reaches water bodies

Because of these impacts, PCP is heavily restricted or banned for residential applications in many countries. Property owners and contractors should avoid using PCP-treated materials around homes, wells, ponds, or food production areas, and explore low-toxicity alternatives instead.

Copper-Based Preservatives: Cleaner Image, Real Risks

Copper-based wood preservatives such as copper sulfate, copper azole (CA), and alkaline copper quaternary (ACQ) are widely used for decks, fence posts, landscaping timbers, and structural posts. They were promoted as a safer replacement for chromated copper arsenate (CCA) in residential use, yet they still pose significant environmental concerns when used at scale.

Copper-treated lumber typically has a greenish or brownish tint. At the chemical level, the active ingredient is copper, combined with co-biocides and carriers that help the preservative bind to wood. Copper sulfate is relatively simple to produce (dissolving copper in sulfuric acid), while CA and ACQ require more complex organic chemistry and specialized manufacturing.

These preservatives are most often applied by pressure treatment, forcing the solution deep into the wood fibers. They are effective at repelling moisture and controlling fungi and insects, which makes them attractive for ground-contact posts, deck framing, and agricultural structures.

However, copper-based preservatives come with their own set of issues:

• Copper can leach into surrounding soils, especially in acidic or saline conditions.
• Runoff from treated lumber can elevate copper levels in nearby streams and ponds.
• High copper concentrations are toxic to fish, invertebrates, and many beneficial soil microbes.
• Handling treated lumber can cause skin irritation and respiratory complaints in sensitive individuals.

As with all treated wood, careful design, installation, and disposal are essential. Avoid using copper-treated lumber where leachate can drain straight into ponds, creeks, or sensitive wetlands, and never burn treated wood, as it can release toxic fumes and residues.

Comparison of Common Wood Preservatives

The Process Used to Coat Wood Fence Posts with Preservatives

The way wood is treated has a big influence on how much preservative ends up in the environment. For oil-based products like creosote and PCP, posts may be fully soaked in a tank until the solution saturates the outer layers of the wood. Excess chemical can drip off the lumber onto soil or into collection basins that must be managed as hazardous waste.

For water-based, copper-containing preservatives, most commercial material is produced as pressure-treated lumber. Posts are loaded into a large steel cylinder, a vacuum removes air from the wood, and then the preservative solution is pumped in and forced deep into the fibers under high pressure. Once removed from the cylinder, the wood is allowed to “fix” or cure, but in practice, fresh posts may still weep preservative and should never be installed directly next to streams, wells, or vegetable beds.

Regardless of the chemistry, treated wood can leach chemicals over its service life—especially where cut ends are in the soil, where water flows over the surface, or where the structure is in constant contact with wet ground.

Reducing the Environmental Footprint of Decks and Fence Posts

To minimize environmental damage from wood preservatives, homeowners, farmers, and contractors can take several practical steps:

• Choose naturally durable woods for posts and ground-contact elements, such as black locust, cedar, or other rot-resistant species where locally available.
• Use non-wood materials like galvanized steel, concrete piers, or modern composite products for deck framing and fence posts in sensitive areas.
• Design structures to keep wood out of constant soil contact—for example, by using metal post bases or concrete footings for deck posts.
• Capture and properly dispose of sawdust, offcuts, and old treated lumber at approved facilities; never burn treated wood.
• Locate treated posts away from streams, drainage swales, wells, and ponds to reduce the risk of chemical runoff.
• Consider integrating trees and shrubs into property planning—see agroforestry and reforestation—to improve soils and buffer waterways from potential contaminants.

For long-term resilience, pairing careful material choices with good site design often yields better results than simply reaching for the strongest chemical preservative on the shelf. In many cases, a combination of naturally durable wood, smart drainage, and structural design can provide decades of service with far less environmental impact.

When in doubt, ask suppliers to identify exactly which preservative is used on their treated lumber, provide safety data, and suggest alternatives that align with your goals for healthier soils, cleaner water, and a safer home landscape.

Environmentally Damaging Preservatives