Table 1. Summary of the key features of the methodological approaches in the studies considered [4, 5].

Feature of the methodological approach	Symanzik et al. 2019 [ 4 ]	Symanzik et al. 2021 [ 5 ]
Country	Germany	Germany
State	Lower Saxony and North Rhine-Westphalia	Lower Saxony, North Rhine-Westphalia, and Schleswig-Holstein
Number of hair salons visited	12	18
Price segment of the visited hairdressing salons	Low, medium, and high	Low, medium, and high
Reagent for nickel spot testing	Dimethylglyoxime (CAS 95-45-4)	Dimethylglyoxime (CAS 95-45-4)
Test solution for nickel spot testing	Chemo Nickel Test, Chemotechnique Diagnostics, Vellinge, Sweden	Chemo Nickel Test, Chemotechnique Diagnostics, Vellinge, Sweden
Reagent for cobalt spot testing	N/T	Nitroso-R salt (disodium;3-hydroxy-4-nitrosonaphthalene-2,7-disulfonate) (CAS 525-05-03).
Test solution for cobalt spot testing	N/T	Chemo Cobalt Test, Chemotechnique Diagnostics, Vellinge, Sweden
Standardization of the test areas	Yes	Yes
Number of tools tested in total	229	475
Tested tools	Scissors, sectioning clips, hair rollers, tail combs, whisks, tweezers, and hand showers	Scissors, sectioning clips, hair clips, hair rollers, tail combs, tweezers, straight razors, whisks, hand showers, and crochet hooks

N/T = not tested.