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Timber preservatives in perspective 

Over the last decade timber preservatives applied in domestic premises have come under increasing attack for reportedly causing all kinds of ills. Occasionally, horror stories published in the tabloid press report on those who have allegedly become seriously ill following, for example, a simple spray treatment of their property (or even their neighbours property) for woodworm. When investigated fully and objectively, usually by the Health & Safety Executive, the basis for the case is almost always unfounded.

It is interesting to note that these 'journalists' have little or no experience of pesticides and seem unable to acknowledge the Legislation, Health and Safety Executive or indeed the Pesticide Incident Register. Most frequently, their 'reports' are based on statements like, "Scientists are now concerned", or "There is growing evidence", or even, "It is well documented"; these are clearly designed to attempt to add some credence to their stories when presented to a gullible or naïve public.

When challenged (but most seem not to want to face their peers directly) they cannot produce the evidence supported and validated by other scientists, doctors, etc. Instead we have heard from one of these individuals when someone managed to challenge him on an article, "Well, it makes a good story, doesn't it"; one even published that, "He worked on construction sites for the next 15 years, and became a card carrying bricklayer". Enough said?



So why the publicity then?

The 'buzz-word' appears to be 'Chemicals'. Some people believe that because a material is a 'chemical' it is dangerous, and timber preservatives are 'chemicals' and must therefore be dangerous. (Heaven preserve us when they realise that there is a chloride atom in common salt, and chlorine gas was used against soldiers in the 1st world war!!)

Everyday 'chemicals' are present all around us in the home; these will include almost anything, paint, bleach, medicines, common salt, sugar -- all are 'chemicals'. However, we do not seriously worry about our health with a number of these because we know that the way we use them, or the dose we take, is unlikely to present a health hazard. How do we know? The label gives the dose or the amount to use, and experience has shown that when any material is used as directed there is no great problem.
timber preservation?
Health and Safety

Why then should timber preservatives be different from any other materials when applied as directed? --- basically, they aren't! When used correctly they are highly unlikely to cause any undue hazard to health or the environment.

Like all medicines, etc., timber preservatives and their active ingredients must be fully evaluated for toxicity, mutagenic, carcinogenic activity and many other factors before they can be cleared for use. This process is undertaken by a combination of various government bodies incorporating medical experts and toxicologists; these include the Advisory Committee on Pesticides and the Health and Safety Executive: a similar process is undertaken for any proposed new medicine. Should the evaluation be successful then the preservative formulation is cleared for use as directed and a specific number is issued for each product by the Health and Safety Executive; this must be clearly shown on the label. It is the result of these stringent procedures by independent government bodies which protect the public and the environment from potentially harmful materials: indeed, these are all embodied under Legislation including the Control of Pesticide Regulations, part of the Food and Environmental Protection Act, and the Control of Substances Hazardous to Health, part of the Health & Safety at Work Act. It is through this legislation that the UK has probably the most stringent controls on the manufacture, sale and use of pesticides.

Whilst it doesn't give the whole of the picture but does give guidance, one of the evaluation procedures for timber preservatives, or indeed almost any 'chemical' including medicines is the LD50 test. The LD50 is defined as the lethal dose to kill 50% of the test population -- usually rats. By the use of such testing comparative toxicities can be made. In the past timber preservatives containing dieldrin (LD50 = 10mg/kg body weight), pentachlorophenol (LD50 = 27mg/kg body weight), and tributyltin oxide ((LD50 = 200mg/kg body weight) were in common use. Some also had 'side effects' in that they caused skin burns, or induced violent sneezing because they irritated the nasal membranes, even at low levels. But in most cases one had to almost come into direct contact with the material.

However, in the last decade significant changes have taken place. More recent preservatives containing newer biocides such as organo-boron esters (LD50 = 1700mg/kg body weight), permethrin (LD50 = 4,570 mg/kg body weight) and boron/glycol mixes (LD50 up to 15,000 mg/kg body weight) are now common in most insitu preservative treatments. For comparison the LD50s' of aspirin, household salt and sugar are 1000 mg/kg, 3,000 mg/kg and 29,000 mg/kg respectively. Thus, there has been a radical change in the toxicity of virtually all timber preservatives which now bring them within the order of toxicity of many common household products.

Furthermore, petroleum solvents, the common carrying agent for the pesticide, have now been eliminated from most formulations: it was these that tended to cause the problems experienced by a few, not the biocide.


The hazards in relation to timber preservatives are given on the labels; this is a requirement of the legislation. Not only are there hazard warning phrases, there is also a Hazard Warning symbol.

In the case of almost all preservatives this symbol is that for 'Irritant' - the St Andrew's Cross on an orange background. This is considerably further down the hazard 'list' than 'Toxic' (skull and croos-bones). Furthermore, no preservative fluid has ever been classified as 'Toxic'! Indeed, the same St Andrew's Cross symbol appears on a number of common household products such as lime descaler, bleach and even fabric conditioner! Furthermore, the symbol is not even required on the 'diluted' concentrate preservatives, the most commonly used materials today.


There is also a further consideration in that the 'risk' of picking up a timber preservative from treated wood is very low indeed. With modern emulsion based formulations carpets can be laid very quickly following treatment which isolates the surface from the inhabitants, and it is unlikely that exposed timbers in, for example, a roof space will be often frequented. Thus, the risk of picking up the materials directly is extremely low. Indeed, a survey undertaken in the USA showed that while people perceived pesticides to be high risk materials, in reality they were of considerably lower risk than smoking, garden machinery and even contraception! Of around 150,000 properties treated each year in the UK, there were only 4 complaints of problems with timber preservatives made to the Health and Safety Executive in 1993. However, over 200 complaints were received in relation to the application of agricultural chemicals.

What about airborne contamination? Highly unlikely following the required period before re-entry. Evaluations of airborne levels of permethrin, for example, necessary as part of the Health & Safety Executive's requirement for evaluation of 8 hour re-entry products has shown the insecticide to be just measurable -- after 24 - 48 hours it becomes below detectible levels. This is not surprising because the material is almost non-volatile.


To treat or not?

Whilst timber preservatives used correctly are of very low hazard, there is always the need to be careful when using any material within the living environment; this includes the use of medicines and other household products. Where their use is deemed unnecessary then they shouldn't be used.

Unfortunately, timber preservatives are still, in many cases, overused by both the general public and professional timber treatments companies. Whilst there is probably justification for their use to control insect attack, they are undoubtedly overused for the control of rot. The fundamental control of all wood rotting fungi is the control of the water which caused them to develop; without water they will eventually die! Any chemical treatment should be considered secondary where rot is concerned.

There are, however, situations where more reliance may have to be put on the use of preservatives for the control of rot perhaps because of cost constraints, or where it is not possible to totally eliminate the source of moisture. In these cases the newer biocides such as the boron/glycol formulations with their relatively low toxicity are ideally suited since they can distribute well into the potentially damp timbers. Remember, only damp wood is at risk to rot.



A few people may be highly sensitive to timber preservatives as indeed are some people sensitive to other materials in the environment; we are probably all sensitive to some materials to a greater or lesser extent. It is therefore incumbent on any applicator of a preservative to determine the possibility of hypersensitive people in the property, and make appropriate arrangements for the conduct of the treatment and the use of materials which are less likely to cause a problem. However, it should be added that these problems are significantly lower than in the past because of the introduction of lower hazard biocides.


Past treatments

Obviously, there are a large number of older properties that have been treated in the past with the older preservatives such as described above. However, even these should present no particular problem to the vast majority of people because they will never come into direct contact with treated surfaces.

It is possible for a property to have been treated more than once therefore negating the further requirement for the use of more preservative and potentially expensive treatment. Chemical analyses of timber samples will determine the presence of most of the common preservatives: this may be useful where preservative treatment is suspected. However, it is the author's experience from over 300 timber analyses from treated properties that multiple treatments with different insecticides are rare.

Timber preservation will always have a place in the preservation of our homes and historic buildings. Like virtually all other material, including basic household products, there will be a very limited hazard factor where they are used. But this should be no more than other common materials encountered, provided they are applied as directed.

Finally, it is often said that if a material is a 'natural' product it can do no harm or it is good for you --- the most toxic materials known to man are natural products and not man-made, and consider that pneumonic plague and death are natural!

A cautionary tale:
Imagine scientists had just discovered a miraculous liquid called water. This substance was beyond belief - you could drink it, grow plants in it, mix it with all kinds of things to make other materials, etc. A godsend, a miraculous material that was a real benefit to all mankind.

But wait a minute! Someone has just discovered that if you stick your head in a bucket of the stuff for 5 minutes it will kill you!! -- and there are 'oceans' and rivers of the stuff out there!!!!

Ban it!!!


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