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. 2020 Jan 15;10:292. doi: 10.1038/s41598-019-56578-6

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as 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 images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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The year-boxplot of PM₁₀ (a), PM_2.5 (b), NO₂ (c), and SO₂ (d) based on 24-hr concentrations and CO (e) and O₃ (f) based on 8-hr concentrations in Tehran from 2012 to 2017. Black solid lines and black long-dash lines represent the World Health Organization air quality guideline and Iranian standard levels, respectively.

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