Boron, our Health and the Environment

Inorganic borates such as borax and boric acid have a multiplicity of industrial applications, including in the production of glass fibre, heat resistant glass, enamels and ceramics. Of considerable interest to the environmental building movement is the use of boron compounds as preservatives for timber, and as fire retardant preservatives in cellulose and wool insulations. Boron is one of the essential micronutrients needed for healthy plant growth, so inorganic borates are used as necessary for agricultural fertilisers.

Properties and Toxicology
Most boron compounds are white solids that are water-soluble. There have been no cases of industrial intoxication on exposure to inorganic borates. European Union toxicology tests on borates have found them to be of similar toxicity to common sodium chloride salt which most of us are quite happy to sprinkle on our food. Studies on human volunteers showed that boric acid is excreted rapidly in the urine when administered intravenously as a 500mg dose, disappearing completely from the body within 4 days. Symptoms of intoxication only appear if massive quantities are absorbed internally, temporarily overloading the kidneys. Symptoms include vomiting, diarrhoea, rash, shock and coma. It must be re-emphasised that these symptoms have never been encountered in any industrial application.
Boron compounds are poorly absorbed through intact skin, but should be washed out if contact with an eye takes place. However, borax and boric acid are in common use in many pharmaceutical preparations, including as buffering agents in eye lotions (up to 5% concentration).
Although a level of 5mg/m3 has been set for borax dust in air, there appears to be no sound medical reason for setting that particular level. Studies have shown that borax dust can be an acute respiratory irritant, but no abnormalities have been seen on chest x-rays of exposed workers. Later tests on the same group of workers showed no greater deterioration in lung function when compared with an unexposed control group.
Various studies have found boron to be neither carcinogenic nor mutagenic. Fairly high levels of boron compounds can be tolerated before any adverse reproductive effects are recorded in animals (the things they do with animal testing!). A level of 6.2mg/day of boron has been set to protect the average man from any chance of testicular atrophy – a somewhat anomalous value, bearing in mind that the normal human daily food intake is estimated to contain between 3.5 and 41mg of boron.

The Environment
As stated previously, boron is an essential micronutrient for healthy plant growth, and is therefore present in all our diets. The daily dietary intake of boron given above is well within the capacity of our kidneys for excretion.
Boron is widely distributed in the form of water-soluble borates in low concentrations in both the soil and the sea. The effects of borate in industrial effluents are observable in plants. Although essential as a micronutrient, it is toxic to plants at high concentrations. Care should be taken, therefore, to prevent contamination of watercourses, and local authorities should be informed before the disposal of any significant quantities. To avoid possible plant damage, slightly contaminated products should be considered for some other purpose, or recycled rather than landfilled.

Preservation of building Timbers
The boron diffusion process, unlike most other timber treatments, is carried out on green timber, which makes the process unsuitable for treating ready sawn imports. This process was first used in Australia in the early 1940s. Boron compounds are effective against decay and insect attack, but have a relatively low mammalian toxicity.
The boron diffusion process involves a concentrated solution of the preservative being applied to the exterior surfaces of the fresh sawn timber, usually by dipping. The timber is then close-stacked while still wet, covered to restrict drying, and left to allow the borate to diffuse to the centre of the timber, a process which takes several weeks. After this, the timber is dried in the normal way.
The diffusion process utilises water present in the unseasoned timber as the medium for movement of the dissolved preservative. The higher the moisture content, the more readily diffusion takes place. Diffusion moves the borate from the high concentration zone at the timber surface, the effect being to equalise the concentration throughout. The highly soluble disodium octaborate tetrahydrate used is a colourless substance which is highly toxic to wood-rotting fungi and wood-boring insects.
There are many advantages with this treatment:
No expensive equipment needed.
Complete penetration possible provided initial moisture content suitable.
Treated timber can be reworked without exposing untreated surfaces.
Effective against fungi, insects but low toxicity to mammals.
Colourless, but a dye can be added if desired.
No pre-drying necessary – should lead to greater use of local timber as imports will be too dry.
Treated timber can be air dried or kilned – no risk of organic solvent emission during drying.
UK supplies of sawn softwood have a high proportion of Sitka spruce which is difficult to treat by pressure methods.
A few disadvantages:
Can only be used on green timber – moisture content over 50%.
Some preservative redistribution can occur on kiln drying.
The boron preservative is not chemically fixed within the timber, thus susceptible to leaching on prolonged rain exposure.
Cannot be used where leaching could be severe, i.e. in contact with permanently wet ground. For example, not suitable for fence posts or below damp-proof course on buildings.
Boron treated timber can be used for most building structural work, including timber which is exposed to a temporary leaching hazard during construction (e.g. roofing battens, wall plates etc). Leaching losses on external joinery will be less if timber is painted before prolonged exposure. Treated timber can be glued as normal, provided the wood is dried to below 18% moisture.

Acknowledgements
this article is adapted from the following documents:
Preservation of building timbers by boron diffusion, published by the Building Research Establishment (BRE), 01923 664000
Safe use at work of Borax, Boric acid and other Inorganic Borates, published by Borax Consolidated Ltd, 071 834 9070