text: The NSR law was written to apply to new sources only if they exceed threshold levels established in the NSR regulations, and to existing major sources only if they are undergoing modifications that result in significant net increases in emissions.5 The expectation was that older plants would eventually be phased out and newer, cleaner, plants would take their place. This "grandfathering" of existing sources of emissions was driven partly by politics, to be sure, but also by economics and common sense.