Persistent organic pollutants (POPs) have been of environmental and health concern for more than half a century and have their own intergovernmental regulation through the Stockholm Convention, from 2001. One major concern is the nursing child's exposure to POPs, a concern that has led to a very large number of scientific studies on POPs in mothers' milk. The present review is a report on the assessment on worldwide spatial distributions of POPs and of their temporal trends. The data presented herein is a compilation based on scientific publications between 1995 and 2011. It is evident that the concentrations in mothers' milk depend on the use of pesticides and industrial chemicals defined as POPs. Polychlorinated biphenyls (PCBs) and "dioxins" are higher in the more industrialized areas, Europe and Northern America, whereas pesticides are higher in Africa and Asia and polybrominated diphenyl ethers (PBDEs) are reported in higher concentrations in the USA. POPs are consequently distributed to women in all parts of the world and are thus delivered to the nursing child. The review points out several major problems in the reporting of data, which are crucial to enable high quality comparisons. Even though the data set is large, the comparability is hampered by differences in reporting. In conclusion, much more detailed instructions are needed for reporting POPs in mothers' milk. Temporal trend data for POPs in mothers' milk is scarce and is of interest when studying longer time series. The only two countries with long temporal trend studies are Japan and Sweden. In most cases, the trends show decreasing concentrations of POPs in mothers' milk. However, hexabromocyclododecane is showing increasing temporal concentration trends in both Japan and Sweden.

Within the Swedish National Monitoring Programme for Contaminants in Freshwater Biota, perch, pike and Arctic char are being analysed from 32 lakes distributed all over Sweden. One of the main objectives of the monitoring has been to detect temporal trends in different areas and for different contaminants. However, the measured concentrations in biota can be affected by biological factors such as age, length, trophic position (TP), fat or dry weight content. A change in one or several of these variables over time, or even among specimens taken within the same year, is likely to result in an increased variation of the measured concentrations and hence in a lower chance of detecting a trend. The purpose of this study was to evaluate if an adjustment of contaminant time trends by primarily stable isotopes, but also other confounding factors can:  A) reduce the individual and between-year variation and hence B) improve the likelihood of detecting temporal trends in time series for metals and organic contaminants. The study revealed an improvement in the likelihood of detecting a contaminant trend after adjustment of the data for almost all lakes and for all tested substances. For some lakes and substances, this improvement was substantial. Age seems to be an important confounding factor for metals and per- and polyfluoroalkyl substances. For lipophilic substances, fat content also appears to be an important factor in some lakes together with adjustments for carbon and nitrogen isotope ratios (d¹³C and d¹⁵N), which all reduced the variation in contaminant data. This suggests that there is a benefit of adjusting data on a regular basis in contaminant monitoring programmes in the future.