This bill revises the determination of when changes to a stationary source, or the operation of such source, are classified as a modification for purposes of New Source Review permitting requirements. Specifically, the bill provides that a modification of a stationary source results in an increase in air pollution only when the maximum achievable hourly emissions rate from the source is higher than (1) the rate of the source as it was originally designed, and (2) the rate that was actually achieved during the 10 years before the change.