Chemicals used to extend the life and usefulness of wood products
are extremely important to wood users. Durability of wood in homes,
service buildings, animal holding pens and fence posts is a major
concern to almost every homeowner and farmer.
Wood-preserving chemicals have made possible a wide variety of
otherwise unsuitable applications. Without preservatives, frequent
maintenance and replacement would make wood too costly for electrical
power poles, bridges, railroad ties, home decks, porches and other
uses subjected to weathering and decay. Thus, preservatives help
American companies compete in a variety of market applications.
Chromated copper arsenate (CCA) is the most common preservative
in use today. In the treating process CCA chemicals are applied
under pressure and the chemicals are driven deep into the wood cells.
Done properly, pressure-treated wood can last up to 40 years, compared
to teh 3 to 7 year life span of similar untreated posts.
CCA works by making the wood poisonous to the fungi and insects
that use it as a food source. The coppor component of CCA is the
main toxin for fungi, and arsenic poisons the few species that are
copper-tolerant. Insect suchs as termites are poisoned by the arsenic
that is digested out of the treated wood they feed upon.
Chromium is the agent responsible for binding the arsenic and cooper
to the molecules that makes up woods. This process renders the toxic
principals water-insoluble, and they cannot leach out of the wood.
EPA and U.S. Department of Agriculture studies have shown that CCA-treated
lumber is safely used in the presence of plants and animals. Plants
do not absorb the chemicals and animals that chew on teh wood cannot
digest out the arsenic. However, these chemicals can be released
as toxic gases if the wood is burned. There is also the possibility
of skin and lung iritation from contact with surface residues or
sawdust created during construction.
Although studies have indicated CCA-treated wood is safe for direct
contact with food, the Food and Drug Administration has not approved
its use for structures or containers such as beehives, feed bins
or silos, where the feed or food would come into direct contact
with the wood.
The United States Environmental Protection Agency (EPA) has reclassified
certain wood-preserving chemicals--creosote, pentachloropophenol
and inorganic arsenicals--as restricted use pesticide materials.
The designation means they are toxic or poisonous and potentially
hazardous to humans and animals. Consequently, the public should
be aware of the precautions for using wood treated with these chemicals.
Frequent or prolonged skin contact with pentachloropehnol or creosote
treated wood should be avoided. When handling pesticide treated
wood, wear tightly woven coveralls and use gloves impervious to
these chemicals (vinyl gloves). Handlers of inorganic arsenical
treated wood do not need to wear such gloves because of the negligible
rate of arsenic absorption through human skin.
Sawdust from treated wood tends to become airborne with resultant
human exposure by inhalation, swallowing and widespread dermal contact.
To reduce the potential for exposure, the EPA recommends that individuals
sawing pesticide treated wood in their occupation should wear disposable
coveralls, made of a material such as nitrile or polyethylene, or
similar protecctive clothing. Homeowners with occasional exposure
should wear tightly woven long-sleeved cotton overalls, if they
do not have disposable coveralls.
Individuals who are sawing pesticide treated wood should wear a
dust mask capable of trapping 80% of particulates greater than 5
microns in size. In addition, sawing operations should be performed
outdoors to prevent indoor accumulations of airborne sawdust and
goggles should be worn to protect eyes from flying particles.
Skin should be washed after contact with pesticides, especially
before eating or drinking. Protective clothing should be laundered
before reuse if oily preservative or sawdust has accumulated on
them. Work clothes should be washed separately from other household
Treated wood should not be burned in open fires, in stoves or in
fireplaces because toxic cehmicals may be produced in the smoke
and ashes. However, larger quantities of treated wood from commercial
and industrial uses, such as construction sites, may be burned in
commerical or industrial incinerators if done in accordance with
state and federal regulations.
Small quantities of treated wood can be disposed of by burial or
by ordinary trash collection and disposal methods. Burial of treated
wood does not pose unreasonable risks to human health and the environment.
Pentachlorophenol treated wood should not be used in residential,
industrial or commercial building interiors, except in laminated
beams or building components that contact the ground and are subject
to decay or insect infestation. When used for such purposes, pentachlorophenol
treated wood must be covered with two coats of an appropriate sealer
such as urethane, shellac, latex epoxy enamel or varnish.
The EPA has advised that creosote treated wood should not be used
in residential building interiors. Creosote treated wood in interiors
of industrial buildings should be used only for wood block flooring
and for industrial building components that contact the soil and
are subject ot decay or insect infestation. Such industrial application
is acceptable, provided that two coats of an appropriate sealer
are applied. For creosote treated wood block floaring, coal tar
pitch and coal tar pitch emulsion are effective sealers. Urethane,
epoxy and shellac are appropriate sealers for creosote treated wood
when used other than as wood block flooring It is believed that
limited interior uses of appropriately sealed pentachlorophenol
and creosote treated wood will not cause unreasonable adverse effects
to humans or the environment.
EPA has not advised against the interior use of arsenical treated
wood because the arsenic air levels measured in homes constructed
with this type of treated wood are not significantly different from
background arsenic air levels when dust has been vacuumed from the
wood surface. Therefore, wood that is pressure-treated with water-borned
arsenical preservations may be used inside residences wtihout a
sealer, providing that all machined sawdust is vacuumed from the
Several types of domestic animals can be exposed to the pesticides
in treated wood by breathing air containing pesticide vapors or
dust particles, by oral contact or by dermal contact. The principal
problem is chemical residues being concentrated in the edible tissues
of animals that humans may consume, rather than acute toxicity in
the animals following direct exposure.
Regulations prohibit use of pentachlorophenol and creosote treated
wood in interiors of farm buildings where the wood may come into
contact with domestic animals or livestock may bite, rub, scrape
or lick the wood. The intent is to prevent any exposure to the animals
or to humans who may consume food products of animal origin.
In interiors of farm buildings where domestic animals or livestock
are unlikely to bite or lick the wood, pentachlorophenol or creosote
treated wood may be used for building components that contact the
soil and that are subject to decay or insect infestation, and where
two coats of an appropriate sealer are applied. For creosote treated
wood, use urethane, epoxy or shellac; for pentachlorophenol treated
wood, use urethane, shellac, latex epoxy enamel or varnish.
Studies have shown that arsenic is readily excreted from cattle
and does not accumulate in the food chain, and that dermal absorption
of arsenic from treated wood is negligible. If domestic animals
do lick the wood, exposure is expected to be minimal. Therefore,
EPA does not prohibit the use of arsenical treated wood in areas
where it may have direct contact with domestic animals or livestock.
The Food and Drug Administration (FDA) has the responsibility for
setting food additive regulations. FDA has stated that preservative
treated wood cannot be used under circumstances where the pesticide
may become a component of food or feed, unless a food additive regulation
prescribes safe use.
FDA also has expressed concern about the use of treated wood under
conditions and in sites where chlorinated phenols from pentachlorophenol
could migrate to animal feed and become concentrated in the edible
tissue of animals. Examples of such sites should be structures or
containers used for storing silage or other animal feeds.
No food additive regulation permits food or animal feed to contain
pentachlorophenol, creosote or organic arsenicals at any concentration.
Any food or animal feed containing any residues from contact with
treated wood would be considered adulterated under the Food, Drug
and Cosmetic Act.
The regulations also apply to food plants. Where treated wood
timbers are used as retainers for raised vegetable gardens, vegetables
may contact the treated wood surface. Covering the treated wood
surface with polyethylene prevents potential food contamination
from the treated wood.
EPA cautions against using treated wood in parts of beehvies that
are likely to come into contact with honey and could result in pesticide
residues in the honey.
EPA has concluded that arsenic treated wood should not be used
where it will come in contact with drinking water, until studies
are conducted to show that the contribution of arsenic to drinking
water via this source cannot be expected to exceed 5 parts per billion
in the water. Areas where treated wood should not be used in drinking
water systems include interiors of reservoir roofs, support beams
and truss structures above water levels and may involve either direct
or indirect contact with drinking water. In these cases, migration
of preservative chemicals at significant rates is possible either
by direct contact and leaching, when direct contact is involved
and by condenstaion and other mechanisms when treated wood is in
indirect contact with drinking water.
Because creosote and other coal tar based products impart an objectionable
taste and odor to water, EPA recommends that creosote treated wood
not be used where it will come in contact with drinkiing water.
Studies have indicated that water in contact with pentachlorophenol
treated wood contained pentachlorophenol levels in excess of recognized
safe levels. EPA has recommended that pentachlorophenol treated
wood should not be used where it will come in contact wiht drinking
water. An exception exists for incidental contact such as dock and
Pentachlorophenol and creosote treated wood should not be used
where it may come in direct contact with drinking water for domestic
animals and livestock. An exception involves incidental contact,
such as dock and bridge pilings. However, arsenical treated wood
may be used for construction of containers for water that will be
consumed by domestic animals or livestock.
Research studies have found that dermal abosrption of inorganic
arsenic from pressure treat wood is negligible. Consequently, this
wood can be used as handrails, sundecks, park benches, stadium seats,
boardwalks and playground equipment.
However, the skin absorption of pentachlorophenol and creosote
is relatively high. EPA recommends that pentachlorophenol and creosote
treated wood not be used where it might be in frequent or prolonged
contact with the skin, unless two coats of an effective sealer have
been applied to the wood (urethane, epoxy or shellac for creosote
treated wood and urethane, shellac, latex epoxy enamel or varnish
for pentachlorophenol treated wood). Chairs and other outdoor furniture
are typical examples of such uses.
from PennState and Delaware Cooperative Extensions, 2001