(a) Except as provided in subdivision (b) of this section, no person will cause or allow emissions to the outdoor atmosphere of any material that has an average six-minute opacity of 20 percent or greater, except uncombined water, from a portland cement kiln, portland cement kiln with in-line raw mill, clinker cooler, or any other confined processes at a portland cement plant.

(b) No person will cause or allow emissions to the outdoor atmosphere of any material that has an average six-minute opacity of 10 percent or greater, except uncombined water, from a clinker cooler, raw mill system, finish mill system, raw mill dryer, raw material storage, clinker storage, finished product storage, conveyor transfer points, bagging and bulk loading and unloading systems which commenced construction or modification after August 17, 1971.

(c) Any person who owns or operates an area, parking lot, clinker gallery, railcar loading shed, conveyor tunnel, access road, stockpile, building opening or refuse disposal area, at a portland cement plant that has the potential to emit visible emissions for one continuous hour or longer, must apply corrective measures to eliminate such potential.