16. The opacity of grey or black emissions into the atmosphere from a source of contamination must not exceed 20% for each emission point.
During the operation of a source of contamination, emission opacity may exceed 20%, without exceeding 40%, for one or more periods totalling 4 minutes per hour.
In addition, on starting a stationary internal combustion engine, emission opacity may exceed 20% for a maximum period of 4 minutes.
On igniting a combustion furnace or blowing tubes, emission opacity may exceed 20%, without exceeding 60%, for a maximum period of 4 minutes.
For the purposes of this section, emission opacity is measured using the Micro-Ringelmann chart in Schedule D, in accordance with the instructions in that Schedule.