(1) No person may cause or authorize emissions to be discharged into the outdoor atmosphere from any source installed on or before November 23, 1968, that exhibit an opacity of 40% or greater averaged over six consecutive minutes. The provisions of this section do not apply to transfer of molten metals or emissions from transfer ladles.
(2) No person may cause or authorize emissions to be discharged into the outdoor atmosphere from any source installed after November 23, 1968, that exhibit an opacity of 20% or greater averaged over 6 consecutive minutes.
(3) During the building of new fires, cleaning of grates, or soot blowing, the provisions of (1) and (2) shall apply, except that a maximum average opacity of 60% is permissible for not more than one four-minute period in any 60 consecutive minutes. Such a four-minute period means any four consecutive minutes.
(4) This rule does not apply to emissions from:
(a) wood-waste burners;
(c) motor vehicles;
(d) those new stationary sources listed in ARM 17.8.340 for which a visible emission standard has been promulgated;
(e) residential solid-fuel combustion devices such as fireplaces and wood or coal stoves; or
(f) recovery furnaces at kraft pulp mills.