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A tale of two cities -
Safety testing in Britain and Germany

Safety tests on permethrin use in homes in Britain and Germany have produced strikingly different results and recommendations. In the UK, water-based permethrin timber treatment formulations have been given eight-hour re-entry time approval, whilst in Germany permethrin has been declared a persistent human health risk for up to two years after application. Jeff Howell visited both institutes to investigate these differences.

In Britain, the Building Research Establishment (BRE) tests the application safety of timber treatment chemicals, while in Germany testing is carried out at the Fraunhofer Institute of Toxicology and Aerosol Research.
The main contrast between the institutes is the totally different scientific approach. Furthermore, the British tests appear to have been carried out at the behest of the timber treatment industry, whilst those in Germany were government funded in the interests of public health.
The UK regulations, including the Control of Pesticides Regulations, have required the evacuation and isolation of premises sprayed against woodworm or dry rot. Recommended evacuation times have generally been at least 48 hours, which has been a major impediment to treating occupied houses. Telling people their homes need spraying against woodworm is one thing-telling them to stay away for two days is quite another. So the aim of the timber treatment industry has always been the achievement of eight-hour re-entry time approval for licensed products to allow occupants to return the same day, and minimise inconvenience.

The British system
Since the 1970s, the BRE has measured aerial concentration of pesticides after spraying to determine safe re-entry times. In the early days, the common timber treatment chemicals were dieldrin (now banned) and lindane (now voluntarily withdrawn), both dispersed in white spirit. With these products, aerial sampling initially showed a high concentration of airborne solvent and pesticide, and after 48 hours the levels dropped below the threshold limit values (TLVs) of the time.
    A BRE spokesman noted that each country tests differently-the British have always used aerial concentration sampling, and still believe it is a significant guide to safety.
    By the 1990s the timber treaters had started to introduce water-based emulsion sprays, usually with permethrin as the active ingredient. Because of its relatively high LD 50, permethrin has been promoted as a 'safe' insecticide, and the timber treatment industry has relied on this heavily in marketing-so much so that the achievement of an eight-hour re-entry time could be seen as fundamental to the success of the product. As part of this promotional effort the BRE aerial concentration test was duly carried out on the new water-based formulations. The test involves lining the inside of a 9.3 cubic metre (m3) test chamber with treated plywood panels, applying known ventilation rates, and continually sampling the air to check for pesticide concentrations. Given the water-based formulations of the emulsions and micro-emulsions under test, and the 0.5 air changes per hour within the test chamber, it would seem unsurprising that after eight hours the level of permethrin in the air was measured as less than 20 mg/m3 [micrograms per cubic metre], well below the TLV of 125 mg/m3. On the basis of these tests, various water-based permethrin formulations were given eight-hour re-entry time clearance, the first approval being granted in 1995.

Above: The German tests replicate normal living conditions, with room furnishings, Photos: Jeff Howell
Left: Inside the test chamber showing gas sampler, With permission, BRE

German comparison
Meanwhile, in Germany, the Federal Health Agency, alerted by growing reports of suspected permethrin poisoning cases (around 250 in 1997), commissioned the Fraunhofer Institute to investigate(2).
Professor Levsen of the institute points out that inhalation is probably an insignificant route, so their model was designed to test for oral exposure. Rather than using empty boxes, the Institute adopted a model based upon actual living conditions-ten test rooms furnished with carpets, sofas, tables and chairs, and, critically, they provided each test room with a known quantity of house dust.
The dust was circulated within the rooms by fans, with a ventilation rate of 0.8 air changes per hour. The floors were then sprayed with the various permethrin products, and as well as sampling the airborne concentrations, the German team collected and analysed the house dust on tables, chairs and other furniture.
As with the British tests, they found that aerial concentrations died away quite quickly, but wipe tests on the furniture showed that permethrin was being picked up from the floor by the circulating house dust and deposited on the other surfaces. Notably, significant levels of permethrin were still being detected on the coffee tables and dining tables in the test rooms up to two years after initial application. It is fair to assume that in a real home this pick-up and deposition of permethrin would also be occurring on kitchen worktops and other food preparation surfaces.
As a result of the work at the Fraunhofer Institute, it is now recommended in Germany that permethrin is applied in homes as cautiously as possible, and only on structural timbers known to be subject to active insect attack. Permethrin is not recommended for non-structural timbers such as floorboards. In addition, homes treated with permethrin are now required to be decontaminated by vacuuming and scrubbing with household cleaners before re-occupation.

A lack of harmony
The institutes in different European states apply different testing regimes and come up with different answers. One unexpected observation was that neither party was aware of the others' work. If European harmonisation of pesticide testing and approval is to be achieved, then more EC member state consultation is long overdue.

1. Orsler, R.J., Holland, G.E. and Van Eetvelde, G.M.F., Occupant re-entry times following insecticidal remedial treatment of timber in dwellings, International Research Group on Wood Preservation Document, 1995, No. IRG/WP/95-50055.
2. Berger-Preiss, E., Preiss, A., Sielaff, K., Raabe, M., Ilgen, B., Levsen, K., The Behaviour of Pyrethroids Indoors: A Model Study, Indoor Air, 1997, Vol 7, pp 248 - 261.

Jeff Howell is a construction writer and broadcaster

[This article first appeared in Pesticides News No. 40, June 1998, page 3]

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