Table 1.

Eligibility criteria following the PECOS scheme, adapted from (University of York 2009)⁹

Criterion	Inclusion	Exclusion
Participants	Hairdressers, patients, cosmetic products	None
Exposure	Exposure to (an) eligible chemical(s)	N/A
Comparator	Clients, consumers, normal population (no, or less, exposure)	N/A
Outcome	Skin toxicity event (contact allergy, irritancy)	N/A
	Systemic toxicity (CMR, ED, respiratory)	N/A
Study design	Experimental studies, for example,	Qualitative studies
	Chemical analyses
	in vivo toxicological studies
	in vitro toxicological studies
	Observational studies, for example,
	Case–control studies
	Prospective and retrospective cohort studies
	(Repeated) cross-sectional studies
	Case reports, clinical series

CMR, carcinogenicity/mutagenicity/reproductive toxicity; ED, endocrine disruption; N/A, not applicable.