The Clean Air Act Amendments of 1990 were significant updates to the original Clean Air Act, aimed at improving air quality in the United States. These amendments introduced stricter regulations on pollutants, including sulfur dioxide and nitrogen oxides, which contribute to acid rain and smog. They also established a market-based approach for reducing emissions, allowing companies to buy and sell pollution allowances. Additionally, the amendments focused on reducing ozone-depleting substances and addressing toxic air pollutants. The Environmental Protection Agency (EPA) was given more authority to enforce these regulations, ensuring that states and industries comply with the new standards to protect public health and the environment.