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As stated in Recital 8 of the Directive 2005/69/EC (OJ L323, 9.12.2005, p.51), there are at present no harmonized test methods for measuring PAHs in the extender oils, or for measuring PAHs in tyres that contain such oils. Until suitable harmonized methods are available, the only named method that is permitted for measuring the PAH content of extender oils is the IP346 analysis method. This method is permitted providing that certain additional conditions are met. These additional conditions are necessary because the IP346 method does not measure the PAH content directly. In fact IP346 measures the total content of polycyclic aromatic compounds (PCA) rather than the PAH content. The PCAs are a group of substances to which PAHs belong, but in which PAHs are present in only very small amounts. The legal limit for PAHs in extender oils, which is 1 part per million (ppm) of BaP and 10ppm total PAH content, is considered to be met if the total PCA content is <3%. In other words, the PCA content of 3% is taken as a proxy measurement for a PAH content of 10ppm. The proxy measurements will be valid only if the ratio between the PAH and PCA content in the extender oil is known and does not change over time. The additional conditions therefore require an initial calibration of the technique (measurement of the PAH/PCA ratio) and recalibration at intervals of six months, or after "major operational change", in order to ensure that the measurements remain valid over time. The term "major operational change" should therefore be taken to mean any change in materials or processes that could invalidate the results of the proxy measurement. The principle cause of invalid results would be a change in the PAH/PCA ratio in the extender oil. However, it should be remembered that not only is IP346 a proxy method for measuring PAH, but that the quantity that it does measure, namely PCA content, is meaaured in a rather indirect way, namely by a change in the refractive index of a solution, and that PCAs are not the only substances that affect the refractive index of a solution. The potential for obtaining invalid results is therefore quite high and the method should therefore be used with considerable caution. It would therefore be advisable to recalibrate in case of doubt. The provision to control the calibration of the PAH/PCA ratio every six months is to safeguard the validity of the IP346 results against unintentional or unknown changes. This would apply for the case where the manufacturing process and materials used remain the same, and where there is no reason be expect a change in the PAH/PCA ratio. However, it is possible to imagine, for example, that a tyre manufacturer receives a reformulated extender oil from his supplier without being made aware of the change that has been made, and the results from the IP346 could be invalidated as a consequence. A six month recalibration interval was considered sufficient to cover such occurrences. Conclusion: The provision to control the calibration of the PAH/PCA ratio after each "major operational change" is to safeguard the validity of the IP346 results. A major operational change is therefore a deliberate change to materials or processes that might be expected to significantly influence the PAH/PCA ratio, or otherwise affect the validity of the measurement. Examples of such a change would be where the source of supply for the extender oil is changed, or where the method of using the oil is changed. Judgment of whether a particular change is sufficiently important to trigger the need for recalibration will necessarily be made case-by-case and will require expert opinion.