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. 2017 Apr 17;16:41. doi: 10.1186/s12940-017-0248-y

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© The Author(s). 2017

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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Fig. 3 — Quality assessment for human studies. Risk of bias ratings for seven human experimental trials and one cross-sectional study. Criteria ratings served as basis for the assignment of individual studies to one out of three study quality categories (1^st tier, 2^nd tier, 3^rd tier). Black frames indicate key risk of bias criteria