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Survey of Dioxin Emission from PCP-treated Wood

Summary and conclusions

• The survey
• Project results
  • Disposable pallet boards
  • Construction wood
• Main conclusions

Pentachlorophenol-treated wood has in the two Substance Flow Analyses from 2000 and 2003 been identified as a possibly substantial source of the Danish dioxin emission [Hansen, 2000] and [Hansen and Hansen, 2003].

The emission of dioxin to air from this source was estimated to 0.5 - 26 g I-TEQ/year.

This interval is however uncertain, as no detailed investigations on the subject have been available so far. The estimate was therefore based on literature studies of the physical and chemical properties of dioxins and similar substances. The size of the interval reflects the uncertainty of the estimate which has been examined in this project through an assessment of potential sources and measurements of the most substantial sources.

The objective of the project has therefore been to get a confirmation of whether PCP-treated wood should be regarded as a substantial source of dioxin in Denmark that may contribute to dioxin emissions in Denmark e.g. by evaporation of dioxin from the wood, and if possible to reduce the estimated intervals of dioxin emission from PCP-treated wood. The survey is furthermore a part of Denmark's contribution to fulfil the obligations in article 5 in the Stockholm Convention on Persistent Organic Pollutants (POPs).

The survey

The project has been carried out by COWI A/S in cooperation with the National Environmental Research Institute (NERI). NERI has undertaken the chemical analyses for PCP and dioxin, while COWI A/S has been responsible for the market analysis, selection of samples and evaluation of the analysis results.

Within this project it has been possible to analyse 10 samples of disposable pallet wood and five samples of PCP-treated construction wood.

Because of the limited number of analyses the survey can only give an indication of the general level of dioxin content in disposable pallet board wood and PCP-treated construction wood. The evaporation from the wood is assessed on the basis of the dioxin concentration in the wood, but as the evaporation is difficult to measure, the estimate of evaporation is subject to some uncertainty. The estimates of dioxin concentration and evaporation are therefore given as 90 % confidence intervals, assuming a normal distribution.

Project results

The following sources of dioxin emission to air from PCP-treated wood have been considered:

• Import of wood treated with PCP used for anti-sap-stain control.
• The construction wood treated with PCP preservation in Denmark in the period 1950 to 1977 and still in use.

Since 1996 it has been prohibited to sell products containing 0.1 % PCP or higher concentrations. Sale, import and export of goods with a PCP content of 5 ppm or higher are also prohibited. As an exception, countries with ocean coast (France, Ireland, Portugal, Spain and the United Kingdom) can choose to use products with more than 0.1% PCP until the end of year 2008, and a statement of sales from 1999 shows that only France, Spain, England and Portugal still use the substances Na-PCP and PCP.

Na-PCP is used for anti-sap-stain control in France, Spain and Portugal on disposable pallet boards and other types of wood. Disposable pallet boards from the three countries are regarded to be the primary source of import of PCP-treated wood to Denmark. The use of PCP anti-sap-stain control has also been common to some types of tropical wood produced outside the EU, but new legislation is also being implemented in these countries.

The other substantial source of dioxin emission from PCP-treated wood is wood that in the period 1950 - 1977 was treated with wood preservation containing PCP. Experience shows that almost all types of wood preservation used in this period contained pentachlorophenol.

Disposable pallet boards

The 10 samples have been selected from disposable pallet boards imported to Denmark with goods from Spain, Portugal and France.

In spite of the fact that it has not been possible to state the country of origin of the pallet boards, but only of the goods on the pallet boards, the analyses of PCP content show that PCP is found in 6 out of 10 samples. This result indicates that the use of PCP is still common in Spain, France and Portugal. Table 1 shows the results of the PCP-screening:

Table 1 Measured PCP concentrations in wood from disposable pallet boards.

The sample with the highest PCP concentration (no. 2) has subsequently been analysed for dioxin, and the analysis showed a dioxin concentration of 4.6 ng I-TEQ/kg wood which is a low concentration compared to PCP-treated construction wood.

Although the present import of dioxin and PCP with PCP preserved wood thus probably is small the emission of dioxin to air must be expected basically to be determined by the quantity of dioxin still present in PCP-preserved wood imported during the 1980'ties and the 1990'ties. The emission has consequently been recalculated to 0.03 – 5 g I-TEQ/year, which is only slightly less than the emission figure of 0.03 – 6 g I-TEQ/year previously assumed.

The wood in question is used for construction purposes and will by time slowly will be disposed of resulting in that the emission of dioxin also slowly by time should be expected to be reduced. It is emphasised that the above calculations must be taken as extremely uncertain and should be regarded as an indication only of the relevant order of magnitude for the dioxin flows in question.

Construction wood

The selection of samples for the analyses on PCP-preserved construction wood was made on the basis of a mapping of the historical facts for a number of houses from the period. The year of construction, type of wood protection and the number of applications have among other things been examined. The samples for dioxin analyses were afterwards selected based on the available information.

Table 2 Measured concentrations and expected intervals of the concentration in 2003.

1 The expected concentration is calculated based on knowledge on number of primings and preservations, the content of PCP in primers (typical 5%) and preservatives (typical 1 %) and the amount of wood treated by each application. Furthermore an annual evaporation of 0.5 % of the actual dioxin content in the wood has been assumed.

2 The measured concentrations have been assuming to follow a normal distribution. The indicated minimum and maximum concentrations represent the interval within which 90 % of all concentrations are likely to be registered.

Dioxin was detected in all five analysed samples, and table 2 shows the registered concentrations which corresponds rather well to the estimated intervals expressing the expected concentrations of dioxin in 2003 in the five samples. The intervals are based on the assumption of an annual evaporation rate of 0.5 % of the actuel content of dioxin.

The total amount of dioxin in PCP-treated construction wood still in use in Denmark by 2003 is estimated at 42 - 974 g I-TEQ for 2003. The annual emission from the amount of wood is based on an assumed annual evaporation rate of 0.5 % of the actual content of dioxin, estimated at 0.2 - 4.9 g I-TEQ/year for 2003.

Main conclusions

• The content of dioxin in the single sample from disposable pallet board analysed for dioxin was low. The concentration of PCP in the imported pallet boards was also low compared to the concentrations that could be found in imported wood in the beginning and middle of the 1980'ties. This knowledge indicates that the present import of disposable pallet boards is probably not a serious source of dioxin emission in Denmark.
• Although the present import of dioxin and PCP with PCP preserved wood probably is small the emission of dioxin to air from imported PCP preserved wood must be expected basically to be determined by the quantity of dioxin still present in PCP-preserved wood imported during the 1980'ties and the 1990'ties. The emission has consequently been recalculated to 0.03 – 5 g I-TEQ/year, which is only slightly less than the emission figure of 0.03 – 6 g I-TEQ/year previously assumed.
• The remaining amount of dioxin in Danish PCP-treated construction wood is estimated at 42 - 974 g I-TEQ.
• The dioxin analyses made on the construction wood samples have neither confirmed nor rejected the assumption of an annual evaporation rate of approximately 0.5 % of the actual content of dioxin in the PCP-treated wood. This evaporation rate fits reasonable well with the other data available on the samples inclusive of the measured concentrations, the number and timing of applications of primer and preservatives etc. As the measured concentrations are in the low end of the interval of the expected concentrations it is deemed unlikely that the evaporation rate should be lower than assumed, while a higher evaporation rate may be easily explained by assuming a higher initial concentration of dioxin in the wood.
• Accepting an evaporation rate of 0.5 % of the actual content of dioxin in the PCP-treated wood as the best estimate the annual dioxin emission from PCP-treated construction wood in Denmark in 2003 is estimated to 0.2-4.9 g I-TEQ.
• The total emission from PCP-treated wood can on the basis of the experiences from the project be summed up to 0.23 - 9.9 g I-TEQ/year, which is lower than the earlier used estimate of 0.5 - 26 g I-TEQ/year [Hansen and Hansen, 2003]. In spite of that the interval has been reduced PCP-preserved wood must still be regarded as a noteworthy source to the total Danish dioxin emission.

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Version 1.0 Juli 2004, © Danish Environmental Protection Agency