Final answer:
The Environmental Protection Agency (EPA) is responsible for implementing the US Clean Air Act Amendments of 1990, which has played a crucial role in improving air quality and protecting public health in the United States.
Step-by-step explanation:
The agency of government charged with implementing the US Clean Air Act Amendments of 1990 is the Environmental Protection Agency (EPA). The EPA is responsible for setting National Ambient Air Quality Standards and ensuring that these standards are met, thus protecting public health and the environment. The success of the EPA's command-and-control regulation has been recognized in significantly improving the air quality and thereby contributing to public welfare across the United States.