The document discusses the results of a study on the impact of COVID-19 lockdowns on air pollution. Researchers analyzed data from dozens of countries and found that lockdowns led to an average decline of nearly 30% in nitrogen dioxide levels over cities. However, they also observed that this improvement was temporary and air pollution rebounded once lockdowns were lifted as vehicle traffic increased again. Overall, the study highlights how human activities are a major contributor to air pollution but also that systemic changes are needed for long-term air quality improvements.