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. 2019 Nov 21;18:101. doi: 10.1186/s12940-019-0544-9

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© The Author(s). 2019

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. 1 — Average concentrations of criteria pollutants by 2010 Census tracts in the continental U.S., 1999–2015. PM_2.5, fine particulate matter (particles < 2.5 μm in aerodynamic diameter) in μg/m³; PM_2.5–10, coarse fraction particulate matter (particles 2.5–10 μm in aerodynamic diameter) in μg/m³; SO₂, sulfur dioxide in ppb; NO₂, nitrogen dioxide in ppb; O₃, ozone in ppb, mean for May–September of daily max of eight-hour moving average; CO, carbon monoxide in ppm