9.1
Scope of this Chapter
This chapter examines the environmental impacts of the most common wood preserving chemicals available on the market. Products covered include creosote, copper, chrome, arsenic, zinc boron and fluorine compounds, pentachlorophenol, dieldrin, lindane, tributyl tin oxide and permethrin. Due to the extremely large number of patented formulations, we have looked at preservatives in terms of the main chemicals used, the solvent (water, organic or other) and the mode of application. These can easily be related to specific products which, under the Control of Pesticides Act 1986, must have an ingredients list printed on the container.
9.2 Introduction
9.2.1
What are preservatives?
Wood preservatives comprise a mixture of solvent (organic or water) and active ingredients such as pentachlorophenol (PCP), arsenic, chrome and tributyl tin oxide (TBTO).7,8 Preservatives are also available as sticks, pastes and ‘smokes’.
Timber preservatives are, by their very nature, highly persistent and ALWAYS toxic—some extremely so.34,46 Most preservatives have been cited as causes of ill health, based on well documented toxicological effects due to occupational exposure of wood treatment operatives.7
Exposure to the users of treated buildings is usually through the inhalation of dust particles which have the compound attached to them. Woodworkers will also be exposed to sawdust from preservative treated wood, and skin contact through carrying treated timber.7,8 The groups most at risk are workers involved with the pretreatment and remedial treatment of timber.
As well as being irritants and nerve poisons, some of these compounds target organs such as the liver and can accumulate there from a number of sources, such as non-organic agricultural produce, as well as from building sources.7
Such treatments may not always be necessary at all, and there may also be a non-toxic or less toxic treatment technique available.46 (see Alternatives section, page 114).
9.2.2
Types of Preservative
Tar-oil preservatives (BS 144:1973 & BS 3051:1972)
Creosote, produced by distillation of coal tar, is the most common preservative in this category and is highly toxic to fungi, insects and marine borers. Heavy-oil creosote is used for high hazard situations such as telegraph poles, marine and freshwater pilings and railway sleepers. Medium and light oil creosote are typically used on fencing and farm buildings. All forms of creosote have a characteristic smell which remains for long periods after application, and treated timber cannot usually be painted.2
Water-borne Preservatives
Solutions of a single salt or mixtures of salts in water, of which there are two categories—fixed salt treatments, and water-soluble treatments.
The salts in fixed salt treatments react with the timber to become insoluble.
They are usually applied by high pressure process, after which the wood is usually redried, a process which may result in dimensional changes in the timber.
The majority of water soluble treatments are boron compounds in water. Boron compounds are fungicidal and insecticidal, but can be leached from the timber, so their use is restricted to dry areas where leaching is not expected, such as internal structural timbers.2
Mixtures containing combinations of copper, chrome, arsenic zinc or fluoride are also popular.
Organic/Solvent Borne Preservatives (BS 5707)
Consist of fungicides and/or insecticides dissolved in an organic solvent. Most are highly resistant to leaching, and are suitable for low to medium hazard situations.2 Common organic preservatives are pentachlorophenol (PCP), organic zinc copper and tin compounds, lindane (y-HCH) and permethrin.
The organic solvents used as carriers may be volatile, or relatively non-volatile petroleum fractions.3 Most are applied by low-pressure processes or by immersion, and cause no dimensional movement of the timber. After evaporation of the solvent, the timber is no more flammable than untreated wood.
Organic preservatives may be modified to include water repellents, tints for recognition, or other additives. Most are compatible with paints and glues.2
Pastes
Emulsions applied as thick pastes to the surface of timber, where deep penetration or precisely placed concentrations of preservative are required.34
Insecticidal Smokes
Used for the control of insects such as deathwatch beetle, where there is a need to kill emerging adults. Normally carried out as an annual treatment.34
Solid Plugs
Fused rods of soluble boron compounds, inserted into predrilled holes, which are dissolved by moisture and diffuse through the wood. These have a fungicidal action and are popular for use in window frames and similar products.34
Surface Coatings
Organic solvent based protective paints, which protect the wood by preventing moisture ingress. These sometimes contain fungicides.34
9.2.3 The Main Issues
Many preservatives, particularly those developed some time ago, are highly hazardous to health and ‘must be handled and used with the utmost care’.7
There is potential exposure to wood preservatives at every stage of construction from site preparation to occupation.7 Maintenance work, where preservatives are applied in-situ, is likely to present the greatest hazard.
Dry rot, wet rot and many wood boring insects will only occur in damp timber.
Solve your damp problem and you have gone a long way to solving your pest problem.46
The development of a specialist wood preserving industry over the last 40 years has had the effect of allowing professionals such as surveyors, architects and general building, to ignore the problem of timber decay by providing an instant spray-on `solution'.61
9.2.4
Why Preserve & When?
(a)
Hazards to Timber
Fungal Attack
Normally occurs when the moisture content of the timber exceeds about 20%.
Staining fungi disfigure timber and can disrupt surface coatings of paint or stain, but do not cause significant deterioration in strength.3 The two main types are blue staining fungi, which penetrate the sapwood with an intense blue-grey stain, and moulds, which only cause superficial staining.2 These can be useful in indicating that a timber is damp.
Wood decaying fungi attack the cellulose cell walls of the wood, causing structural weakening and eventually complete destruction. The three types are wet rot, dry rot, and soft rot.2
Dry Rot
Also called brown rot, dry rot is caused by the fungus Serpula lacrymans, which attacks mainly softwood. The name is misleading as dry rot requires wet conditions with a moisture content of between 20% and 30%.62 This is one of the most difficult rots to control as it spreads rapidly to attack sound wood.62
Wet Rot
Several fungi are responsible for wet rot, which can cause shrinkage62 and in extreme cases threaten the structural integrity of timbers.2
Timbers most at risk form wet rot are those with moisture content between 30%
and 50%—usually those exposed to persistent moisture or condensation, external joinery and timbers in ground contact.2
Soft Rot
Less damaging and less detectable than the other two types of rot, soft rot usually attacks timber in ground contact. The most damaging soft rot fungus is Chaetomium globosum, which can grow at high temperatures and pH and with restricted oxygen.62
Insects
Wood boring insects are less of a threat in European countries than in the tropics and subtropics.
The most common European wood boring insects are woodworm—the larvae of the common furniture beetle, death watch beetle, house longhorn beetle, weevils and other less common insects.66 These tunnel through the wood for up to 5 years before they pupate, and emerge a number of weeks later as adults.46
Many wood borers cause only superficial damage which does not affect the strength of the timber. The few insects that are capable of causing serious structural damage only do so when infestations are severe.46
(b)
The Argument for Preservatives
Preservative treatment involves impregnating wood with chemicals which are toxic to fungi or insects, in order to control or eradicate these organisms.
Effectiveness depends on achieving penetration and retention of the chemical in the timber, and the type of preservative.2
Preservative treatment is argued to have a number of technical and environmental benefits.
The component service life is extended, reducing the frequency of replacement,1 and treatment can extend the use of lower cost, plantation grown softwoods to
‘high-hazard’ situations,1 thus reducing demand on scarcer high durability woods.
Due to the growing demand for timber, the use of sapwood and low durability timbers is increasing, which are particularly susceptible to biological attack, sometimes requiring extensive repair within the first few years of service. Proper application of timber preservatives before the timber is put into service is one way of avoiding the wastage of valuable timber and the consequent costs of repair and replacement.30
The British Wood Preserving and Damp Proofing Association (BWPDPA) point to research in the USA which credits wood preservatives with savings of about 12% of the total timber harvested each year in that country, through extending the lifespan of timber products, thus assisting in the conservation of forests.26
(c)
The Argument Against Preservatives
When considering preservative treatment of timber, it should be remembered that timber is probably the healthiest of building materials, and it is paradoxical to
‘poison’ it, especially in situations where other methods such as good building design and non-toxic preservatives would suffice to protect it.9
It should also be remembered that virtually no preservative treatment was carried out 60 years ago. The London Hazards Centre (Tel. 0171 837 5605) book
‘Toxic Treatments’ suggests that the timber preserving industry has been very successful in promoting a wood rot ‘paranoia’, also blaming a ‘bonanza of rot and decay brought about by the post-war drive to cut costs and boost profits in the building, construction and timber trade’.22 This is compounded by the apparent cost advantage of preservative treated softwood over an inherently durable timber3 and building society demands for guarantees of timber treatment in order to preserve their investments against the slightest risk of decay.29
In reality, there are strong arguments which support the theory that the chemical treatment of timber is less effective, more expensive and more dangerous than alternative traditional methods46 which are dealt with in the ‘Alternatives’ section, p. 114.
Buildings maintained in good condition with a damp proof course to prevent rising damp, and kept at an adequate temperature will not usually develop problems of insect or fungal attack.
Pressures towards chemical treatments are spurred on by the large profits being made by firms specialising in timber treatment and by the results of poor building construction, rather than any inherent need for heavy use of pesticides.63
Insect attack is often related to geography, but within any particular area, it is unrelated to any identifiable cause. All timber has some degree of resistance to insect attack due to the presence of natural oils and resins, and the more durable species should require no treatment.33
In new buildings, preservation of wood against insects is usually less justified than against fungi, except where there is an unusual risk, such as in the Tropics against termites and in parts of South-East England, where building regulations require softwood roof timbers to be protected against the House Longhorn Beetle.3 Even in high risk locations, alternatives to chemical treatment are available (see Alternatives section, p. 114).
For internal joinery, and other ‘permanently dry’ situations, preservative treatment is not required against fungi, even for sapwood.3 Good construction
should not put even perishable timbers at risk.3,7 If the specification, design and detailing of a building is such that the moisture content of the timber is below 20%, fungal attack is extremely unlikely and chemical control unneccessary.33
(d) If In Doubt
The use of preservatives should be avoided unless recommended in the relevant Codes and Standards.4 In the UK, these are BS 5589 and BS 5268: Part 5.4
Preservation should also be avoided if the required life of the component is shorter than the expected life of the timber.2 The exception is if the reuse of the element is guaranteed.
(e)
Do Chemicals Work?
There are serious doubts about the effectiveness and necessity of most timber treatments.34
Up to 80% of PCP may evaporate from treated wood within 12 months,24 and a BRE report found that after only 40 months of normal aging, PCP levels in dip- treated, painted redwood sapwood were only partially effective in hindering fungal growth.22 The loss of Lindane from wood was found to be more rapid than PCP.22 The loss of organic preservatives is increased by painting, due to the preservative dissolving into the paint, then evaporating into the atmosphere.
If timber becomes wet and cannot dry out, then the use of preservatives will at best do little more than delay the onset of fungal and/or insect attack and may be completely ineffective.56,60 There is a lack of understanding of why chemical treatments can fail, and success is often achieved by chance.61 Guarantees for remedial treatment of dry and wet rot generally state that the guarantee is only valid as long as the structure is kept free of moisture.56 When one considers that wet rot, dry rot and woodworm as a rule only become a significant problem in damp areas, this calls into question the value of chemical timber treatment.
(f)
Surveying and Vested Interests
Wood preserving companies place around 100 tons of wood preservatives in an estimated 100,000–150,000 buildings each year,29,66 mainly on the recommendations of surveyors attached to the wood preserving companies.29 There is great variation in the quality of inspections—A Building Trades Journal (BTJ) investigation of firms specialising in timber and rising damp treatment summed up its findings in the headline ‘Staggering incompetence revealed by decay investigation’.23 Firms were reported to have quoted for treatment for non- existent infestations and rot, yet missed the areas genuinely requiring treatment.22
Pest control and timber treatment companies are only too willing to provide simple solutions based on chemical pesticides,46 yet none of the firms in the BTJ study reported the causes of the dampness, or suggested treatment other than chemical treatments.22
The Pesticides Trust recommend that anyone concerned about possible damp in their property should use an independent surveyor who has CTIS training—and avoid those who are attached to a damp proofing or timber treatment company.4
Best BuysÐAt a Glance
First Choice: No preservative
Second Choice: Water based boron or zinc, copper, and/or fluoride compounds
Try to avoid: Chrome/Arsenic compounds, permethrin
Avoid: Lindane, pentachlorophenol, tributyl tin oxide, Creosote and (dieldrin)
9.3 Best Buys
The environmental ‘Best Buy’ is well detailed, properly seasoned wood with a protective finish, in buildings designed with adequate ventilation and avoidance of moisture sources. In such a situation, the use of preservative chemicals is usually unnecessary.
Overall preservation of wood is hardly ever necessary, but timbers in ‘high risk’
situations such as window sills may require localised treatment.
If preservatives are required, the best option is boron compounds, which appear to have a low toxicity to humans and the environment, followed by water based zinc/copper/fluoride compounds.
Solvent borne organic preservatives should avoided, as both the solvent and preservative tend to be volatile, leading to an inhalation hazard. Lindane, pentachlorophenol and tributyl tin oxide are particularly toxic and should be avoided.
Formulations containing chrome and/or arsenic (eg: Copper chrome arsenate) and permethrin are slightly less hazardous, the former due to its stability in the wood once dry, and the latter due to a slightly lower toxicity than the alternative synthetic preservatives.
9.3.1 When to Treat:
Pretreatment appears to be the least hazardous form of application, as it is carried out at specialist plant. On site treatment should be avoided, with the exception of localised treatment with low hazard preservatives such as boron rods.
Note: Many of the active ingredients listed are only available in combination with other active ingredients —eg Copper chrome arsenate. However, due to the huge range of formulations on the market, the impacts of each ingredient have been listed separately.
NB: Wood stains often contain preservatives, for which the same `best buys' apply.
9.3.2
Application Methods:
In order of decreasing risk:
Smokes Spraying
Dipping timber or joinery in tanks Pressure impregnation
Brushing
Spreading/mastic-gunning pastes/mayonnaises Drilling and injecting jellies
Drilling and inserting rods (Source:22)
Water based paste or solutions are recommended as the safest method of application after borax/boron rods.7
Organic solvents can be a “safe and specific” method of application7 although many solvents are potential hazards in themselves, and tend to be volatile.7
Pressure treatment in pretreatment plants is recommended by the AECB as the most controlled way of using preservatives. The timber is impregnated properly and any waste is collected and re-used.29
Dipping should be relatively safe, although bad management of the job can lead to high exposure. Studies of US timber treatment workers confirm that dipping operations can lead to some of the highest levels of PCP absorption.22
If these methods are not suitable, spraying is probably the next best option but only if it is carried out with due regard to the potential hazards.7 Spraying creates a high concentration of mist or vapour ideal for both inhalation and absorption through the skin.22 It is also less precise, leading to contamination of non-target areas.
Smoke bomb applications are indiscriminate, uncontrolled and best avoided, and their efficiency has been questioned.7
On site use should be kept to an absolute minimum as it is very difficult to control and can therefore be extremely hazardous. Unpredictable weather conditions can result in seepage into watercourses, and lack of care resulting in spillage can cause problems of ground and groundwater contamination.29
EFFECTIVENESS
In 23 year exposure tests, using L-joints to simulate window and door external joinary, BRE ranked the performance of preservatives in the following order;
Creosote PCP
Copper & Zinc Naphthenate 1% TnBTO.
The addition of water repellents did not cause any consistent and long lasting improvement in long term performance.32
Untreated timbers were ranked in the following order (decreasing durability);
Hemlock Beech Whitewood Pine Sapwood
This order was the same for timbers treated with the same preservative.32
9.4 Impact Analysis
9.4.1
Pretreatment, Remedial Treatment, Professional or DIY?
(a) Pretreatment
BRE recommend that where timber treatment is unavoidable, specification should favour pre-treatment, as this is carried out in specialized industrial plants by trained specialists,1,4 subject to “rigorous health and safety checks”1 by HSE inspectors and emissions controlled by integrated pollution control (IPC) guidelines,36 whereas on-site treatment is often carried out by non-specialist personnel.4 Pre- treatment plants are also more likely to have good trade union organisation.22
Pre-treatment in a specialised plant is also claimed to reduce the potential for solvent emissions into the building after completion,4 and to allow more efficient and effective use of the preservative chemicals.1
On the other hand, pretreatment presents additional risks due to the huge amounts of chemicals present on site and the high pressures used to drive preservatives into wood. Although under normal operating conditions the airborne pollution risk is less than for remedial work, handling wet timber can lead to chronic or acute poisoning.22
(b)
Remedial Treatment
This presents the greatest risk to operatives and building users. Before remedial treatment, curtains, carpets and soft furnishings should be removed. Care must be taken not to contaminate water supply or food.7
The treated area should be inspected to ensure that there are no exposed areas with unwanted contamination. The area should be left ventilated but unoccupied for at least 24 hours to allow airborne pesticide levels to drop.7
Some of the carriers used in wood preservatives are volatile and have an odour of their own—often mistaken for the preservative by the occupier, leading to a loss of confidence in the contractor. It may be best to delay reoccupation until the area is odour free, not least because some carriers are irritants in themselves.7
Timber treatment contractors have no legal requirement to undergo any training or hold any certificates.46 It is therefore possible for anyone to set themselves up as “specialists” in the field of remedial treatment, without training or prior experience.22 It is worth checking if a contractor is a member of the British Wood Preserving and Damp Proofing Association (BWPDPA), which tries to maintain high professional standards in the industry, and eliminate ‘cowboys’.46
Health & Safety
It is only jobs involving building work and lasting over 6 weeks that have to be notified to the HSE. Spraying of preservatives is not notifiable, even under the Control of Pesticides regulations. There is therefore no mechanism to inform enforcers that a home is about to be treated, and the prospects for law enforcement are therefore poor for small scale remedial work.22
Once a property or workplace has been treated, any problems should be reported to the Environmental Health department of the local council.22
(c) DIY
For householders, the greatest risk is from DIY work without full protection.8 PEGS (see page 111) claim that the majority of callers with preservative-related complaints were exposed during DIY work.