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Survey of Dioxin Emission from PCP-treated Wood

6 Interpretation of results

• 6.1 Interpretation of results - construction wood
• 6.2 Revised estimate of emission of dioxin for construction wood
• 6.3 Interpretation of results - disposable pallet board

6.1 Interpretation of results - construction wood

The results presented in section 5.3 clearly documents that dioxin is still present in construction wood treated with PCP-preservatives before 1977. However, the results do not in itself address the issues of evaporation of dioxin from PCP-preserved wood and likely evaporation rates.

In order to address these issues it is necessary to compare the results with the present knowledge regarding dioxin in construction wood preserved with PCP in Denmark during 1950 - 1977. The fundamental knowledge and assumptions can be summarised as follows:

• The content of dioxin in technical PCP during the 1970'es and the beginning of the 1980'es was in the range of 0.16 - 7 mg I-TEQ/kg PCP (reference is made to section 3.1). It is assumed, that these figures are representative also for technical PCP used during the 1950'ties and the 1960'ties.
• The evaporation rate for dioxin in wood is estimated to around 0.5 % of the actual content per year in accordance with the knowledge on evaporation rates presented in section 2. It should be noted that in the original study (reference is made to [Hansen, 2000]), the evaporate rate was calculated as 0.5 % of the original content per year meaning that the evaporation per unit of wood was taken to be constant over the years no matter the amount of dioxin left in the wood. To assume that the evaporation rate is related to the actual content of dioxin instead of the original content seems more logic besides that it represents a better approximation to the knowledge on evaporation rates presented in section 2.
• The PCP-content in the wood preservatives varied between 1 % (preservation) and 5 % (primer) [COWIconsult, 1985].

Assuming a mean-consumption of the wood preservatives of 7 m2/litre, a density of 0.85 kg/l, a thickness of the samples of approx. 2 cm and a mean density for wood of 520 kg/m³, the total dioxin content applied to the wood can be calculated as follows:

Preservation: (0.85kg preservative/l preservative)(0.01kg PCP/kg preservative)(0.16 - 7 mg I-TEQ/kg PCP)/(7m2*0.02m*520 kg wood/m³)= 18.7- 817.3 ng I-TEQ/kg wood

Priming: (0.85kg preservative/l preservative)(0.05kg PCP/kg preservative)(0.16 - 7 mg I-TEQ/kg PCP)/(7m2*0.02m*520 kg wood/m³)= 93.4 - 4086.5 ng I-TEQ/kg wood.

Table 6.1: Estimate of expected dioxin concentration in the selected construction wood samples assuming 0.5 % evaporation per year of actual content.

Based on these assumptions the concentration of dioxin to be expected in the samples has been calculated in table 6.1. For each sample a calculation based on the low limit of dioxin content in PCP as well as a calculation based on the high limit has been presented. The calculations further take into consideration the number of applications of wood preservatives (for each sample and the year of the individual applications.

In table 6.2 is the expected dioxin concentration (2003-figures) for the five samples as calculated in table 5.1 compared with the measured 2003 concentrations as stated in table 4.3.

Table 6.2: Expected dioxin concentration in wood in 2003 compared to measured values.

1. 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.

All measurements are within the interval that could be expected. However, it is obvious that generally the measurements belong to the low end of the interval and generally far below the average of the expected values.

The fact that the measurements generally are low could be explained by assuming:

• Either that the dioxin content in PCP generally was in the low end of the range 0.16 - 7 mg I-TEQ/kg PCP identified in [Hansen, 2000]. This hypothesis fits reasonable well with the fact that the sample with the highest content is sample no. 1 from a house constructed and initially treated in 1960 at a time when the dioxin issue were hardly calling for any attention. The other four samples and in particular sample 2 and 5 are from houses constructed and initially treated at a time when dioxin was beginning to become an issue - studies on toxicology etc. of dioxin was published in 1971 [MST 1977] indicating that dioxin was an issue at that time - and manufacturers of PCP and wood preservatives were aware of the need to minimize the content of dioxin in PCP and wood preservatives.
• Or that the rate of evaporation of dioxin from wood is underestimated.

It must, however, be recognised that the measurements also may be influenced by the following factors:

• The exact quantity of primer or preservative used in each case. The calculation of expected concentration is based on an assumed consumption of 1 litre of primer/preservative per 7 m2 of wood surface. In reality this consumption rate may have been higher as well as lower.
• The actual content of dioxin in the primer/preservative. The primers/preservatives used represent different brands and the dioxin content may well have differed between the brands.

Based on the discussion above it is obvious that the outcome of the measurements is influenced by a number of factors. Thus the results do neither confirm nor reject the assumed evaporation rate of 0.5 %/year of the actual content of dioxin in the wood. However, the fact that the results are in the low end of the interval that could be expected and for one of the measurements almost equal to the lower limit makes it unlikely that the evaporation rate should be lower than assumed. A higher evaporation rate may on the other hand be easily explained by assuming a higher initial concentration of dioxin in the wood.

The emission rate of 0.5 % will therefore be used as a best estimate together with the measured values to assess the annual evaporation from PCP-treated construction wood.

6.2 Revised estimate of emission of dioxin for construction wood

The measured concentrations have been analysed statistically, and the minimum and maximum concentration levels corresponding to a 90 % confidence interval have been identified in table 6.2. Based on these concentration levels and the estimated amount of PCP-treated wood left in Denmark by 2003 the amount of dioxin in the wood and the annual evaporation can be estimated. The calculation is presented in table 6.3.

Table 6.3: Amount of dioxin in construction wood, 2003 and evaporation in 2002.

1. Reference is made to section 2.

2. Reference is made to table 6.2.

3. Calculated on the basis of an annual evaporation rate of 0.5 % of the actual dioxin content in the wood.

As stated in table 6.3 the evaporation of dioxin from PCP-treated wood still in use in Denmark in 2003 can be estimated to be in the range of 0.2 - 4.9 g I-TEQ/year. This figure can be compared with the estimate presented for year 2000 of 0.5 - 20 g I-TEQ/year [Hansen, 2000] and repeated for 2002 [Hansen and Hansen, 2003].

The revised estimate presented for 2003 must be regarded as a considerable improvement compared to the previous estimates, as the estimate is now based on actual measurements documenting that significant quantities of dioxin can be found in PCP. Among the assumptions still causing uncertainty may be mentioned the following:

• The exact rate of evaporation of dioxin from PCP-treated wood. The estimate is based on an assumed rate of 0.5 % of actual content per year. As discussed in section 6.1 this assumption could be in the low end, but is so far accepted as the best estimate based on the present knowledge.
• The life of PCP-preserved construction wood. In the calculations in this report is assumed a maximum life of 40 years. As documented by sample no. 1 from 1960 the actual life of PCP-preserved wood may in some cases exceed this maximum.

The reduced estimate for 2003 compared to the estimate from 2000 is apart from being based on actual measurements also explained by the following:

• The amount of PCP-treated wood still in use in Denmark was in 2000 assumed to correspond to an initial PCP quantity of approx. 680 tons, while the estimate for 2003 is assumed to correspond to an initial PCP-quantity of 430 tons. This change which is due to natural replacement of construction wood is in itself causing a 37 % lowering of the estimate for dioxin evaporation from PCP-treated construction wood in Denmark.
• The assumed rate of evaporation has been changed from 0.5 % of the original content to 0.5 % of the actual content. This change in itself will lower the estimate for dioxin evaporation by around 10-15 %.

It is possible that some dioxin having migrated to the surface of the wood may be washed off together with the precipitation to different recipients. Evaporation to air is however expected to be the predominant type of emission.

It is noted that the estimate of the amount of PCP-treated wood still in use in Denmark is based on an assumption of that the wood treated in all cases have a thickness of approximately 2 cm. The calculation of the amount of PCP-treated wood still in use thus corresponds to the characteristics of the samples of construction wood analysed in study (reference is made to section 4.1) meaning that the estimate of amount of dioxin still present in construction wood should be regarded as reliable.

However, the estimated amount of amount of PCP-treated wood still in use in Denmark may not be reliable, as the actual dimensions of the construction wood treated with PCP may differ from the dimensions of the samples, especially with respect to the thickness of the wood.

6.3 Interpretation of results - disposable pallet board

It is quite surprising that it has been possible to find traces of PCP in 6 out of 10 randomly chosen samples, chosen only on the basis of the country of origin of the imported goods. Although 10 samples are a relatively small number of samples the analyses indicate that the use of PCP anti-sap-stain control is still common in Portugal, France and Spain.

The registered PCP concentration levels are however of a limited scale compared to the previously used estimates of imported PCP-preserved wood. In the publication [COWIconsult, 1985] is given an estimate of the PCP concentration range of 50 - 100 g PCP/m³ wood for imported anti-sap-stain preserved wood (corresponding to approximately 0.1 - 0.2 g PCP/kg wood, when a mean density of wood of 520 kg/m³ is used). The PCP screening of the samples have shown PCP concentrations in the range of 20-164 μg PCP/kg wood (90 % confidence level and assuming that data follows a normal distribution). The measured values are therefore considerably lower than the estimated value of 0.1 - 0.2 g PCP/kg wood.

There might be several different explanations of the difference between the estimate and the measured values:

• The anti-sap-stain control is carried out when the timber is freshly sawn, and before the wood is being worked up to pallet boards or other products. As the predominant part of the PCP is applied to the upper 1.5 mm surface wood [COWIconsult, 1985], the wood products will have a much lower concentration if it does not contain the surface wood. Therefore the analysed samples do therefore not necessarily include the lowest and the highest values of the preserved wood.
• PCP anti-sap-stain control can at present perhaps be made with lower PCP concentrations than earlier.
• It is possible that some of the PCP has evaporated from the samples of pallet-board wood, before the samples were taken as the exact age of the pallet wood is not known and the sap-stain protection is not assumed to be effective for more than 6 - 11 months [Kirk-Othmer 1993].

It is difficult to say which of these explanations that are credible, and most likely a combination of them explains the lower values. On the basis of the results it was decided to carry out a single dioxin analysis of the sample with the highest concentration of PCP in order to examine the concentration of dioxin in the sample.

As table 4.2 shows, the sample with the highest PCP concentration only contained approx. 4.6 ng I-TEQ/kg wood. The estimate of 0.1 - 0.2 g PCP/kg imported preserved wood stated above combined with the assumed content of dioxin in PCP of 0.11-4.2 mg I-TEQ/kg PCP [Eduljee 1999 quoted by Hansen and Hansen 2003] corresponds to a concentration of 11- 840 ng I-TEQ/kg wood.

Assuming that the registered value of 4.6 ng I-TEQ/kg wood is representative of all PCP-preserved wood imported in 2003 the quantity of dioxin imported may now be estimated as

(1-10 tons PCP/year)*(4.6 ng I-TEQ/kg wood)/(0.1-0.2 g PCP/kg wood)=
0.046 - 0.230 g I-TEQ/year

In this expression is assumed an import of PCP to Denmark of the same size (1-10 tons PCP/year) as assumed in the Substance Flow Analysis from 2003. This import of PCP has been translated to wood by assuming the same concentration of PCP in the wood that was assumed by [Hansen and Hansen 2003].

It may be noted that the import calculated in Substance Flow Analysis from 2003 for year 2000 came to 0.11 - 42 g I-TEQ/year [Hansen and Hansen 2003].

The resulting emission of dioxin to air caused by evaporation of dioxin from anti-sap-stained preserved wood imported from the beginning of the 1980'ties and until 2003 was in the Substance Flow Analysis from 2003 [Hansen and Hansen 2003] estimated as

10% ^[1]((0.6+0.05)/2 – (105+21)/2 g I-TEQ/year) = 0.03 – 6 g I-TEQ/year

As an elaboration of this expression it may be noted that the import of dioxin with PCP-preserved wood at the beginning of the 1980'ties had been estimated as 0.6 -105 g I-TEQ/year of which all was used for construction purposes while around 50% of the import in year 2000 of 0.11 - 42 g I-TEQ/year was assumed to be with disposable pallet anticipated to be directed by and large directly to incineration. Futhermore, the expression assumes a liniar development of the amount of dioxin introduced in the Danish society besides that the expression does not consider the evaporation of dioxin to the air that already has taken place. The us the expression in reality estimates the evaporation based on the average consumption over the period of around 1980 to around 2000 assuming an average emission of 0.5% of the consumption per year..

The emission of dioxin to air from anti-sap-stained preserved wood assuming a present import of only 0.046 - 0.230 g I-TEQ/year of which 50% is directed to incineration can correspondingly be recalculated as

10%((0.6+0.023)/2 – (105+0.115)/2 g I-TEQ/year) = 0.03 – 5 g I-TEQ/year

As the measured dioxin concentration should be taken as an example mainly, as only one measurement on a disposable pallet has been carried out, it may indicate that the present import of dioxin with imported PCP-preserved wood is lesser than previously assumed.

However, as illustrated by the calculatios above the immediate consequence regarding emission of dioxin to air is small, as the emission 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.

As the wood in question also is used for construction purposes and by time slowly will be disposed of also the emission of dioxin slowly by time should be expected to be reduced. However, it will be difficult to estimate the development as the precise uses of the wood are not well known and may cover a multitude of applications.

As already emphasised by [Hansen and Hansen 2003] 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.

Footnotes

[1] 10% should actual be read as 20 times 0.5% meaning an evaporation of 0,5% of the content per year accumulated over 20 years.

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Version 1.0 Juli 2004, © Danish Environmental Protection Agency