The Lumber Industry has many different ways of treating wood so that the wood
will not rot / be eaten by insects / fall apart. Some of the different ways
that wood has been treated over the years:
1) Creosote
2) CCA (Chromated copper arsenate)
3) ACZA (Ammoniacal copper zinc arsenate)
4) Borate Preservatives
5) Copper naphthenate
6) ACQ (Alkaline Copper Quaternary)
7) CC (Ammoniacal copper citrate)
8) CDDC (Copper dimethyldithiocarbamate)
9) CBA / CA (Copper Azole)
10) MCA / MCQ (Micronized Copper Azole / Quat) – what most 'pressure treated' lumber at the big box stores is today
See the USDA Forest Products Laboratory wood preservation research for full descriptions of the chemistry.
There are many fact sheets about CCA on the Internet but you have to critically read each to make sure
that the author's perspective (or monetary interests) doesn't taint the article. That
includes the material on this page – read it with a skeptical eye and follow the sources.
While some of these treatments were thought to be safe for use in residential areas and playgrounds,
it was found that some of the chemicals leach out of the wood and pose a hazard to humans. Children are
especially susceptible because of their size and because of hand–to–mouth behaviour. As a result CCA was
voluntarily phased out for residential use
effective December 31, 2003. That was more than two decades ago now, BUT treated wood has a very long service
life – which means millions of decks, fences, playsets and raised beds built before 2004 are still standing
and still contain arsenic. If your structure predates 2004, assume it is CCA until proven otherwise.
The EPA does NOT require removal of existing CCA structures; the guidance is to manage them sensibly:
1) NEVER burn treated wood of ANY kind – not in a burn pile, not in a fireplace, not in a campfire.
Burning CCA wood puts the arsenic into the smoke and concentrates it in the ash. This is the single
most important rule on this page.
2) Do not chip or reuse CCA wood as mulch or compost, and do not use the ash as a soil amendment.
3) When sawing, sanding or machining: work outdoors, wear eye protection and a dust mask, wear gloves,
wash your hands afterward and clean up the sawdust. Essentially all documented human health impacts
have been to people working the wood – sawing and sanding – not people touching intact structures.
4) Sealing an existing CCA structure every 1 to 2 years with a penetrating oil–based semi–transparent
stain significantly reduces the amount of arsenic released
from the surface. Film–forming paint is NOT recommended – it peels and then needs sanding, see rule 3.
5) Disposal: for households, CCA wood generally goes in regular trash pickup or a lined landfill – NOT
recycling, NOT the burn pile. Rules vary, so
check your state and local waste program.
6) Do not use CCA wood (new–old–stock or salvaged) where it will contact food, drinking water,
beehives or vegetable garden soil. For raised beds use naturally durable species (cedar), modern
copper–based treated lumber, or line the bed.
Good starting references, all still maintained:
EPA – Chromated Arsenicals (CCA) – the primary reference, includes disposal guidance and links to the CPSC outdoor structure guidance
National Pesticide Information Center – What are the risks of CCA–treated wood?
ATSDR Toxicological Profile for Arsenic – Public Health Statement
EPA CLU–IN Contaminant Focus – CCA overview
Beyond Pesticides – CCA fact sheet (2003, advocacy perspective – skeptical eye applies)
Here is some additional information from my original research on this page (early 2000s). Most of
these sites have since gone dark, so the links now point at the Internet Archive's Wayback Machine
copies – the content is preserved even though the organizations moved on:
Leach rate of CCA (USDA Forest Products Laboratory, Lebow 2002):
fpl.fs.fed.us – lebow02a.pdf (archived)
Connecticut DPH on pressure treated wood:
dph.state.ct.us – pressurtr.pdf (archived)
Can Garden Plants Take Up Enough Arsenic to be a Concern?
There is very little need to test soils for leached arsenic. Rather, the key precaution is
that soils beneath and directly adjacent to CCA–treated wood structures not be used for play
or gardening activities.
Is Treated Lumber a Risk in the Home Garden? (Alachua County report):
Alachua County CCA report (archived)
Leaching study (Environmental Health Canada):
environmentalhealth.ca (archived)
Copper, chromium and arsenic leaching from new and weathered wood were found at all pH levels,
with higher metal concentrations in acidic conditions. At pH 5.5, for instance, 92% of the
copper, 12% of the chromium and 32% of the arsenic leached out. This raises concern for the
amount of leaching caused by acid rainfall along eastern areas of Canada and the U.S. It also
means that the rate of leaching is accelerated in acid environments such as bogs, silage and compost.
History of the CCA industry fight (St. Petersburg Times, 2001):
sptimes.com – Treated wood industry (archived)
Creosote
Creosote is a distillate of coal tar produced by high temperature carbonization of bituminous coal.
It remains a restricted–use pesticide in the US – railroad ties and utility poles, not consumer lumber.
Creosote Leaching (USDA Forest Products Laboratory):
fplrn286.pdf (archived)
And why Creosote doesn't get into plants (but is carcinogenic), however
it does retard the plant growth:
Creosote can volatilize into the air, especially during hot weather, and plant foliage in the
vicinity of the ties may be damaged by the vapors. It can also leach into the soil near the ties,
but it will not be absorbed by the roots and will therefore not get into the plants' tissues.
UK guidance on creosote railway sleepers in gardens (UK DTI, agency abolished 2007 – archived):
dti.gov.uk – creosote FAQ (archived)
My raised garden bed consists of creosote–treated railway sleepers. Should I be digging these up?
No. Wood treated with creosote prior to the coming into force date of the Creosote Directive, may be
used in gardens, providing there is no risk of frequent skin contact. Furthermore, the prohibition
on the use of treated wood does not apply where the treated wood was in such use before 30 June 2003,
the date these Regulations came into force.