2. Methods
We conducted a systematic literature search in MEDLINE, Cochrane Library and the World Health Organization COVID‐19 Database up until 30th November 2021 on toxic effects of low level carbon dioxide including mask effects on carbon dioxide breathing. Medical surgical masks on the one hand and N95 masks (FFP2 masks) on the other were of interest here. Search terms were: “carbon dioxide”, “breathing” and “toxicity” as well as “carbon dioxide” and “mask”, including “surgical” and “N95”. We searched PubMed and Google Scholar for additional articles of interest. Two independent researchers identified and screened the eligible studies. The selected papers were checked by all authors for final eligibility. To expand the amount of published data further we reviewed citations from included articles to identify additional research. Only English- and German-language peer reviewed records were considered that explicitly described the toxicity of carbon dioxide at low concentrations as well as studies quantifying carbon dioxide when wearing masks under everyday conditions. Letters to the editor and case reports were not considered. Of the eligible papers, one with methodological weaknesses and one retracted paper were ultimately excluded.
3. Results
The search yielded 1651 papers, of which 43 publications were finally considered for evaluation according to the above criteria. In addition to 25 mask experiments in humans, we found 2 modeling and 2 test suite measurements of CO2 when using a mask. Four reviews describe the toxicity of inhaled low level CO2. From the referenced literature, two of the human and eight of the animal experiments examined the toxicity of carbon dioxide at low concentrations. The literature found demonstrates and quantifies in detail the effect of the face masks in terms of carbon dioxide rebreathing. It also describes in detail the effects of low concentration carbon dioxide toxicity. Figure 1 shows the flow chart of our scoping review.