W. von Rotberg1, M. Gagelmann2, H. Piening3, R.W. Sieke4, S. Michaelis5, N. Wilke1 and K.Roux6
Abridged translation. The original paper was written in German and contains additional information.
Up till the early Eighties preventative treatments of construction and ancillary materials even in interiors and of wooden and textile objects as well as those made of, e.g., paper and leather as a precaution against insect and fungal attack used to be carried out using pentachlorphenol (PCP) as active ingredient. PCP evaporation from such large material surfaces treated in such a manner can reach levels of concentration even today which are still hazardous to human health (Gagelmann and Fonfara, 1992).
It is possible to record even today very high levels of interior contamination from pentachlorphenol (PCP) particularly in buildings of historical significance with many wooden interior fittings. There are as yet no appropriate methods to decontaminate protected historic buildings or objects. All processes in use hitherto, such as the steam-tight panelling, the removal or encasement and surface coating of contaminated construction elements cannot be justified from a curatorial point of view. It was therefore the aim of the investigation to test the application of a new humidity-controlled thermal process with which it is possible to evaporate out the volatile wood preserver ingredients contained in the surfaces to be treated so as to arrive at a distinct improvement of interior air quality.
The humidity-controlled thermal treatment processed developed and patented by Thermo Lignum has been successfully used for several years for the purpose of disinfestation from insect pests in items made of organic materials, notably works of art, museum exhibits, antiques, libraries and archives. Another application is the treatment of mould on objects and dry rot in buildings.
In an earlier pilot project it was possible to demonstrate the beneficial effect of the Thermo Lignum method when an small reconstructed cottage was treated against wood-boring infestation in its structural timbers. The building was heated to a core temperature of 55 °C in the same way as described above. This particular infestation treatment was completed in under 24 hours due to its size and the fact that a holding phase of one to two hours is sufficient to achieve a 100% kill rate of all forms of infestation. This was the first time this pioneering treatment involving combined heating and humidity control had been carried out on a whole building (Zeuner, 1997).
After preliminary tests on a laboratory scale the applicability in principle of a humidity-controlled thermal process for the detoxification of contaminated interiors could be confirmed. The process consists of the heating of a closed room whilst simultaneously controlling its humidification. The room to be treated is sealed off tightly and is slowly heated to 60°C by inducing hot air from a closed-circuit heating system developed and patented by Thermo Lignum GmbH,Germany, which ensures an even air distribution throughout the room. (The maximum heating capacity/hour of the modular system is 8,000 m3 thus making large-scale decontamination possible). Structural damage due to drying out is pre-empted by keeping the relative humidity (50%) in the induction air constant by means of a computer-assisted control unit. Room temperature increases gradually guided by the wood and masonry core temperatures (Nicholson and von Rotberg, 1996). After reaching the target temperature there follows a holding phase lasting several days up to several weeks during which time the contaminants are mobilised.
At the same time the mobilised contaminants are broken down by means of oxidation in the sealed reaction compartment of a separate parallel air cycle. The reactive oxidation product is generated for this purpose in an ozone generator in which oxygen molecules are converted into radicals which form ozone structures in high levels of concentration with half lives between 70 msec and 70 sec.
To verify the success of the decontamination treatment air measurements were taken after a 14 to 16 hour long closing of the room and at temperature levels typical for residential occupation. Air sampling (2-3 m3; 2 m3/hour) was done on fibre glass filters (dust phase) and polyurethane foams (gaseous phase) (Leitfaden 1994; VDI 4300, 1994). Following specific extraction with toluene analysis gas chromatography mass spectrometry was carried out.
In the first phase of the pilot decontamination a severely PCP contaminated room of an approximate volume of 72.5 m3 (room I) with a room contamination load ratio of approx. 1.2 m-1 (contaminated wood surface/ room volume) was subjected to a ten day long decontamination treatment. The emission into the ambient air emanated from structural timbers (half-timbering) and decorative wood (ceiling beams, wall panelling, etc.) with surface contamination readings ranging from 360 to 4000 mg PCP/kg. Depending on the point of measurement (window areas with lesser and rear wall area with higher timber content) the PCP concentration in the ambient air ranged from 1112 to 1186 ng/m3 (22.7 °C) with 792-759 ng/m3 apportioned to the dust phases. Similarly, for Lindane the ambient air measurements ranged from 220-223 ng/m3 (gaseous and dust phases).
Measurements after the decontamination treatment (23.8 °C) resulted in significantly lower PCP contamination of the ambient air of 182-367 ng/m3 (71-143 ng PCP/m3 in the dust phase). A similar reduction of the Lindane contamination of the ambient air could also be recorded (143-150 ng/m3).
As a consequence of these results the scope of the decontamination was enlarged to include four other rooms. Room I was included again in the second phase (approx. 205 m3) of the pilot decontamination and the holding phase was extended to 14 days. A compilation of the test results is given in Table 1.
Development of PCP and Lindane contamination of the ambient air after the second decontamination phase in two selected test rooms (Room I) treated in the first decontamination phase and an additional test room II with an interior room contamination load ratio of 0.67 -1 with reading points at rear wall, room centre and window wall. The PCP measurement results of the relevant gaseous phase are emphasised in bold letters.
|Test Room/Reading point||Days after Decontamination||Temperature °C||Dust phase/Gas phase||Lindane ng/m3||PCP ng/m3|
|I/Rear wall||2||24.6||Dust phase||372|
|I/Rear wall||9||19.9||Dust phase||372|
|I/ Centre||9||20.1||Dust phase||501|
The investigation two days after the completion of the second decontamination phase still showed an increased PCP contamination of the ambient air in room I. The reasons for this may be sought in the diffusion rate still being accelerated at this point in time, possibly due to a building core temperature still above normal and/or a mobilised secondary contamination from the extended decontamination scope, and these need to be further discussed. Contrary to the PCP measurements the Lindane concentration in the ambient air followed the expected trend towards increasingly lower results. However, readings after 9, 22 and 26 days show a rapid reduction for the gaseous phase to concentration levels below 20 ng/m3 (background level).
The increased dust contamination in the air does not come unexpected since, as a consequence of the humidity-controlled thermal decontamination, surface consolidant coatings, such as fats, oils, soot, etc. are (visibly) removed. A reduction of the dust contamination of the ambient air can be achieved by conventional cleaning and consolidation methods for all surfaces.
The decontamination time (holding phase) may be shortened, depending on the air accessibility, by raising the heating temperature and relative humidity. The possibility of an oxidative breakdown of mobilised interior contaminants even outside the reaction compartment is currently being investigated. The applicability of the process to other active ingredients of low volatility (e.g. permethrine) which have been applied as a pest control measure has also been investigated successfully.
Gagelmann, M. and Fonfara, J.J.: "Sick building syndrome" und Innenraumbelastungen durch Holzschutzmittel, polychlorierte Biphenyle, Asbest und kuenstliche Mineralfasern. ("Sick building syndrome" and interior contamination from wood preservatives, polychlorinated biphenyls, asbestos and synthetic mineral fibres). Klin. Lab. 38 (1992) 447-455.
Nicholson, M. and von Rotberg, W.: Controlled environment heat treatment as a safe and efficient method of pest control. In: Proceedings of the 2nd International conference on Insect Pests in the Urban environment (K.B. Wiley, publisher), Herriott-Watt University, Edinburgh, Scotland, 7 to 10 July 1996., 263-265.
Leitfaden für die erste Ermittlung der Belastungssituation für holzschutzmittelbehandelte staatliche Gebäude, Oberste Baubehörde im Bayrischen Staatsministerium des Innern (Stand 29.11.94). (Guidelines for a first assessment of the contamination levels in government buildings treated with wood preservatives, update 29 Nov. 94. Superior Building Directorate in the Bavarian Interior Ministry).
VDI-Richtlinie 4300: Messen von Innenraumluftverunreinigungen. Messtrategie für Pentachlorphenol (PCP) und y-hexachlorcyclohexan (LIndan) in der Innenraumluft. Kommission Reinhaltung der Luft im VDI und DIN. Arbeitsgruppe PCP/Lindan. VDI- Handbuch Reinhaltung der Luft, Band 5 (Entwurf: November 1994) (The measurement of interior air pollution. A measuring strategy for pentachlorphenol (PCP) and y-hexachlorcyclohexane (Lindane) in the interior air. Commission for the preservation of air purity in the VDI and DIN. Working group PCP/Lindane. VDI Handbook Preservation of air purity, volume 5 (Draft: Nov. 1994))
Zeuner, D. (Ed.) : Whittaker's Cottages move into the space age. Weald & Downland Open Air Museum Magazine, Vol. 7/8 No 17., March 1997.