The districts hereinafter designated as combining X districts are established to be combined with other districts in certain areas which are uniquely susceptible to air contamination because of terrain, contour, elevation, winds or other meteorological and physical conditions. The regulations are intended to prevent concentration of air contaminants which may cause injury, detriment, nuisance or annoyance to the health or welfare of persons or damage to property. The regulations of any district with which an X district is combined shall remain the same, except as to matters specified in Section 17.28.020, which shall apply in lieu of the corresponding regulations otherwise effective. (Prior gen. code § 8-43.0)