Visible Emissions

Air pollutants in sufficient amount to be observable to the human eye or  the observation of an emission of opacity or optical density above the threshold of vision.

ref:

https://www.ecfr.gov/current/title-40/chapter-I/subchapter-B/part-49/subpart-C

https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-63/subpart-A/section-63.2

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