True or False: The concentration of air pollution has decreased over time, though some metropolitan areas still contain unhealthy levels of pollutants.

The statement is true because, over recent decades, many countries have implemented stricter environmental regulations and technologies to reduce air pollution. As a result, average concentrations of common pollutants like sulfur dioxide and lead have significantly decreased in many places due to these efforts. However, despite this overall decline, certain metropolitan areas still struggle with unhealthy levels of air pollution due to factors such as heavy traffic, industrial emissions, and geographical conditions that can trap pollutants, like urban heat islands and lower air flow.

While pollution levels may vary based on factors such as time of year or geographical features, the broader trend indicates a decrease in air pollution concentrations over time. Therefore, recognizing both the progress that has been made and the persistent challenges in specific locations supports why the statement is accurate. This nuanced understanding highlights the need for ongoing monitoring and action in areas that still experience high pollution levels.