EPA data shows that from 1990 through 2021, sulfur dioxide emissions per unit of coal-generated energy decreased by 94%, and nitrogen oxides emissions decreased by 88%. As a result, the U.S. Department of Energy reported in 2010: "While coal used to be a dirty fuel to burn, technology advances have helped to greatly improve air quality, especially in the last 20 years. Scientists have developed ways to capture the pollutants trapped in coal before they escape into the atmosphere. Today, technology can filter out 99 percent of the tiny particles and remove more than 95 percent of the acid rain pollutants in coal, and also help control mercury." Nevertheless, certain activists and politicians continue to attack coal because burning cleanly it releases CO2, an organic greenhouse gas that is generally colorless, odorless, non-toxic, and non-carcinogenic.