Impact of impurities on the effective charge state distribution in the limiter plasmas of Wendelstein 7-X

•We studied the influence of suspected main plasma impurities like carbon and oxygen on plasma parameters such as ne and Zeff profiles•The effective charge state distribution was identified as an essential input for the data processing step of Langmuir probes installed on W7-X•This is now further underlined by quantifying the influence of the next candidate impurity (oxygen) on Zeff.•Plausible oxygen release rates as obtained from parameter scans must indeed be considered as significant, at least judging on the basis of the present status of the impurity transport model.