(a) After January 1, 1971, no person shall cause, permit or allow the emission of air contaminants, which will exceed the limits specified in table 1^*, from any ferrous jobbing foundry cupola or ferrous jobbing foundry open hearth in operation on or prior to the effective date of this Part, this prohibition to be applicable, also, to modifications made after the effective date of this Part, to such a cupola or open hearth.

(b) The commissioner may modify the limits specified in table 1 [Appendix 4, infra], if in view of the properties of the emissions, isolated conditions, stack height and other factors, it can be clearly demonstrated that discharge of the contaminant or contaminants will not result in measurable or observable effects on receptors, nor add to an existing or predictable atmosphere burden of such contaminant or contaminants so that adverse effects would reasonably be expected therefrom.

(c) The provisions of this section shall not be construed to allow or permit any person to emit air contaminants in quantities which alone or in combination with other sources would contravene any established air quality standards. In such cases the commissioner may specify the degree of treatment required.