One of the key changes made in the 1990 clean air act amendment was the

One of the main changes made to the 1990 Clean Air Act amendment was an emphasis on finding cost-effective ways to reduce air pollution.

Explanation:

Clean Air Act is an American federal law that aims to control air quality and reduce pollutants that can damage that quality. This law proposes a series of guidelines that must be followed throughout the national territory, and which limits the amount of pollutants that large industries can throw on the atmosphere, as well as stipulating guidelines to control the consequences of pollution (such as acid rain, for example).

This law has undergone some modifications over the years, one of which was made in 1990, where emphasis was placed on the need to find economical ways to reduce air pollution.