The Clean Air Act was emended in 1990 and again in 2003 when Bush added the extension called the Clear Skies Act. The element under discussion is that Bush expanded the 1990 provision for purchasing pollution credits to offset pollution and in doing so allowed industries to over-pollute to the extent they were willing to pay for the opportunity to do so by purchasing unused permitted pollution quotas (or credits) from other industries. The most significant element of this expansion is that Bush tied it to market factors asserting that the pollution credit market would eventually push credit purchase prices so high that industries would rather under-pollute than pay the cost of purchasing credits.
The theory goes that the costs of compensating for pollution and over-pollution through credit purchases would eventually turn from credits to technological upgrades and advancement to reduce pollution thus making pollution credits less or unneeded. It's hard to find current statistics on the Internet but Bachmann (2002) indicated projections for reductions of between 67% and 73% in three key pollutants regulated by the CAA/EPA. The reports seem contradictory with some saying reductions have been implemented through credits while others assert Bush is continuing to undermine pollution controlling measures.
The Clean Air Act has absolutely helped our environment and the economy has benefited from advanced technology. The verdict on the Bush 2003 Clear Skies expansion to CAA seems harder to reach since data seems harder to find. If the Bachmann projections were accurate, then it may be supposed Bush's market pressure theory was correct and credits led through a circuitous route to pollutant reductions.
"Instead of the government telling utilities where and how to cut
pollution, we will tell them when and how much to cut. We will give
them a firm deadline and let them find the most innovative ways to
meet it." (George W. Bush)