You're misinterpreting that sentence. They are talking about data missing from _prior_ work there. The title of the article is literally "Chronic irradiation w

You're misinterpreting that sentence. They are talking about data missing from _prior_ work there. The title of the article is literally "Chronic irradiation with 222-nm UVC light induces neither DNA damage nor epidermal lesions" and a few sentences below the discussion of prior work they write "chronic irradiation with 222-nm UVC light was revealed not to induce mutagenic or cytotoxic effects in the epidermis".