sentence1: Based on these timelines, it is estimated, in principal, that the NOX controls needed to comply with a multipollutant strategy can be met provided  (1) an adequate supply of materials and labor is available, and (2) the control technology implementation process begins at least about 35 months prior to the date controls must be in place.
sentence2: The supply of materials and labor is often in shortage, which makes it hard for the NOX controls to comply with that strategy.