Table 1 Description of quality aspects considered in critical appraisal and criteria for high risk of bias

Selection bias

Information about samples and what they represent must be sufficient to determine whether samples from different sampling occasions are comparable

Large differences in sample populations or food source between time points. Populations affected by contaminated land or drinking water

Accuracy of dating

In most cases it is known when samples were collected, but samples from environmental archives such as sediment or ice cores are more complicated to date accurately. Dating by means of e.g. isotope techniques that give an absolute age to each individual sample may be regarded as high quality dating. Dating by means of historical markers (e.g. peak concentrations of other contaminants) providing relative ages may be regarded as acceptable dating. Post-depositional perturbations caused by for instance thawing-freezing cycles in snow or bioturbation in sediments should be discussed and assessed

No dating or probable perturbations of deposited material or migration of PFASs

Sample integrity

Samples may deteriorate with time (especially critical to specimen banks with long storage times)

Sample pre-treatment, sample preservation, prevention of contamination or storage methods not suitable for the sample type

Analytical quality

Analytical procedures should be appropriate and consistent throughout the study period. Was there a risk that analytical parameters may have biased the temporal trends?

Internal standards not used, procedural blanks or field blanks not run, limit of detection (LOD) or limit of quantification (LOQ) not reported, significant changes in analytical techniques during the study period

Study design

Was the sampling dedicated to PFAS analyses or were the samples originally taken for other purposes? When was the study period in relation to known phase-outs (i.e. intervention time points)?

Sampling methods not suitable for PFAS analysis or sampling strategy (study period and sampling frequency) cannot be related to known interventions