Is Treated Timber Environmentally Friendly?

A recent project had me looking at various types of treated timber. As you read on, you’ll getter better understanding of why I opted not to use any of the following for a raised garden bed application and purchased composite plastic timber made from recycled waste materials instead.

What is treated timber?

I don’t think there’s anything more beautiful than timber when it comes to construction materials. White ants (termites) share a similar passion, but theirs is more a culinary appreciation. While some wood is more termite resistant than others, it all eventually succumbs or is affected by other elements, such as moisture, heat, mold and fungi unless somehow protected.

Treated timber is simply wood that has had chemical and/or other processes applied in order to make it more resilient to all of the above threats.

While wood used in housing doesn’t have to be treated if a building is constructed properly, where wood touches the ground or is exposed to the elements; you’ll need some sort of treated product. Paint may not be enough as it only takes a small chip to provide a doorway for termites and moisture and paint has its own environmental issues.

Types of timber treatment

Unlike paint, the chemicals used in treated timber sink deep into the wood, offering superior protection. When looking at treated timber products, the following acronyms may not be mentioned. Often treated wood will just come under a brand name, so when shopping around, be sure to ask what the actual chemical treatment is.


This is a type of treated timber most of us would be familiar with. CCA is often applied to pine logs used in car park barriers, playground equipment and houses.

The preservation method is Copper Chrome Arsenate. Copper and arsenate are the insecticide/fungicide components. “Arsenate” is arsenic – a very toxic substance and a known human carcinogen. Chromium is used to fix the copper and arsenate in the wood. Chromium is a heavy metal and also a carcinogen. 

Copper compounds will bind to sediment or soil particles. Copper can negatively impact on micro-organism activity in soil and it can become concentrated in plants, which are then consumed by animals that suffer adverse health effects.

As with other forms of treated timber, the concern is the copper, arsenate and chromium can leach into soil over time.

Creosote / Pigment Emulsified Creosote

I was always under the impression creosote treatment utilised the same material that is the by-product of burning wood – the stuff that clogs up your chimney known by the same name. I always wondered why it smelled different. Because it is.

Creosote has been in use for over 170 years. The original creosote for preserving timber was made from coal tar. Wood creosote is used in other applications, or if used in treated timber, it is mixed with coal tar creosote. Other varieties of creosote made from petroleum and oil are called oil-tar creosote. 

Fossil fuel based creosote products are known to be carcinogenic and can contaminate water supplies.

I chose creosote posts as fencing strainers on my property thinking they were treated with the wood form and probably more environmentally friendly than CCA posts. I’m not so sure now.


Alkaline Copper Quartenary. This is what I was going to purchase for my raised garden bed, but it has higher copper leaching than CCA and therefore higher aquatic toxicity in that respect.


Copper dimethyl-dithiocarbamate. Contains copper and sulfur compounds. Seems to be a relatively uncommon treatment and I don’t know much about it. It appears to have less copper leaching than CCA, but I found quite a few references to CDDC being toxic to animals in laboratory testing.


Ammoniacal Copper Citrate. Contains copper and citric acid. As with ACQ, it leaches more copper than CCA.


Liquid Organic Solvent Preservative. It may include tributyltin (TBT), a persistent organic pollutant and permethrin; a common synthetic chemical widely used as an insecticide. As to its toxicity, let’s put it this way – LOSP treated timber is not recommended for children’s toys or anything touching food, such as cutting boards.

Treated timber for marine applications use other chemical compounds, which appear to be even more nasty.

Wood is natural. So are copper, arsenic and chromium. Natural doesn’t harmless. All the types of treated wood mentioned above come with a variety of safety warnings to be observed when working with it – such as wearing masks, long clothing, washing thoroughly after working with it, etc. etc. etc. There is no form of treated timber I know of that can be safely burned either. It doesn’t exactly inspire confidence as to its general health or environmental street cred.

It’s not just the wood

The chemical treatments applied to timber don’t appear magically – they must be applied somewhere. Sites that treat timber have been in the headlines a number of times in regard to contamination issues. Before the treatment, there’s also the source of the chemicals and elements themselves. For example, copper mines are notorious for making the land around them barren or saturating vegetation in the surrounding with copper and other toxins, making it harmful to grazing animals.
While the above all seems rather frightening, we need to bear in mind timber is treated to repel living things – by its very nature it is meant to be toxic. It certainly has its place, but the term environmentally friendly treated timber is an oxymoron.
Recycled Composite Plastic Timber