​​​​Assessment protocol

data processing

Since the threshold value is defined with the protection goal to prevent adverse effects on human health via consumption of fishery products, and human fish consumption is mainly focused on muscle fillet of fish, the status evaluation is calculated based on PFOS concentrations in fish muscle. The data may require transformation into the relevant unit and base for the threshold value which is µg/kg wet weight.

Ideally, the data should be expressed in the same matrix, which for the purposes of the indicator evaluation ought to be muscle fillet concentrations in fish, representing a trophic level of 4 (European Commission 2014). However, the majority of the PFOS data reported are analysed in liver tissue in different fish species at varying trophic levels. The PFOS concentration values that are originally measured in fish liver are then recalculated to concentrations in muscle.

In the present indicator report, conversion from PFOS concentrations in liver to PFOS concentrations in muscle was done by the use of conversion factors generated in a study, based on Swedish national monitoring, comparing muscle and liver concentrations (Faxneld et al. 2014). The conversion was performed with the use of the general conversion factors for 'all species' (liver:muscle ratio: 17.9).


Assessment protocol table 1. Mean liver:muscle ratios for PFOS with 95% confidence intervals within parentheses. The column indicated "all species" includes herring, perch (marine and limnic), eelpout, pike, arctic char and cod. Data taken from (Faxneld et al. 2014)

 All speciesHerringPerchEelpout
PFOS17.9 (16-20)19.0 (17-21)18.2 (16-20)11.1 (6.0-16)

The use of liver values would lead to an overestimation of PFOS concentrations in relation to the threshold value since PFOS is reported by several studies to accumulate in protein rich tissue, with liver being one of the tissues where the highest concentrations are found (Goeritz et al. 2013; Shi et al. 2012). However, it is of great importance to be aware of the uncertainties introduced to the results in the conversion procedure.

Additionally, no correction for trophic level has been made. The monitored species are at a lower trophic level than the general trophic level estimated for commercial fish and suggested by the EC Guidance Document No. 32 (European Commission 2013), implying a risk of underestimation of the concentrations, since PFOS biomagnifies in the food web. The information on trophic level, is lacking for the reported results and a proper trophic magnification factor (TMF) has not been agreed upon yet. It is therefore presently not possible to translate the results to the recommended trophic level 4 for the status evaluation. The results should therefore be considered tentative at this time.

More studies on relations between liver, muscle and whole body concentrations of PFOS in relevant Baltic Sea fish species are needed in order to improve the comparisons to the threshold value.


Statistical evaluation

The assessment protocol is structured in three main parts, 1) changes in log concentrations over time are modelled, 2) check for compliance against threshold value and evidence for temporal change of contaminant concentration per station and 3) a spatial aggregation of status per assessment unit.

It should be noted that the assessment protocol makes the assumption that monitoring data stems from the same monitoring stations during consecutive years. The stations used by the protocol are defined in the ICES Station Dictionary. Stations with similar station name are grouped together, but it is also possible to define a group of stations with different names to be defined as the same station in the Station Dictionary. Usually a station is defined in the Station Dictionary with coordinates and a valid box around these coordinates, but coordinates outside of the box will only give a warning when reporting the data, and are not used in the actual data extraction.


Time series of contaminant concentrations are assessed in three stages:

  1. The concentrations are log transformed and changes in the log concentrations over time are modelled using linear mixed models. The type of temporal change that is considered depends on the number of years of data:

      1. 1-2 years: no model is fitted because there are insufficient data

      2. 3-4 years: concentrations are assumed to be stable over time and the mean log concentration is estimated

      3. 5-6 years: a linear trend in log concentration is fitted

      4. 7+ years: more complex (smooth) patterns of change over time are modelled

  2. The fitted models are used to assess status against available threshold value and evidence of temporal change in contaminant levels in the last twenty years

  3. The fitted models are also used for spatial aggregation to assess status against available threshold value and evidence of temporal change in contaminant levels on a scale 4 level HELCOM assessment unit. 

These stages are described in more detail in the link below. There is also information on how the methodology is adapted when there are 'less-than' measurements (treated as 'initial' data), i.e. some concentrations are reported as below the detection limit, and missing uncertainties, i.e. the analytical variability associated with some of the concentration measurements was not reported.

Assessment methodology for contaminants in biota and water

All initial data is handled in a highly precautionary manner to further ensure that the risk of false positives is minimalised. For all initial data the 95% confidence limit on the mean concentration, based on the uncertainty seen in longer time series throughout the HELCOM area, is used.  Applying a precautionary approach, the 90% quantile (psi value, Ψ ) of the uncertainty estimates in the longer time series from the entire HELCOM region are used. The same approach is used for time series with three or more years of data, but which are dominated by less-than values (i.e. no parametric model can be fitted). The mean concentration in the last monitoring year (meanLY) is obtained by: restricting the time series to the period 2011-2016 (the last six monitoring years), calculating the median log concentration in each year (treating 'less-than' values as if they were above the limit of detection), calculating the mean of the median log concentrations, and then back-transforming (by exponentiating) to the concentration scale. The upper one-sided 95% confidence limit (clLY) is then given by: exp(meanLY + qnorm(0.95) * Ψ / sqrt(n)), where n is the number of years with data in the period 2011-2016 (HELCOM 2018).


Assessment units

PFOS is considered as a global environmental chemical, widely spread in biological samples and even present in samples from as remote places such as the Arctic region. The PFOS core indicator is therefore relevant for the whole Baltic Sea and can theoretically be applied in all regions.

The core indicator evaluates the status with regard to concentration of PFOS using HELCOM assessment unit scale 4 (division of the Baltic Sea into 17 sub-basins and further division into coastal and offshore areas and division of the coastal areas by WFD water types or water bodies). This division is applied in order to take into account the different routes by which PFOS enters the Baltic Sea - via air and via run-off from land, including also potential point sources.

The assessment units are defined in the HELCOM Monitoring and Assessment Strategy Annex 4