Coinciding with the 50th anniversary ofEarth Day, this report investigates the impact COVID-19 lockdowns have had on global particulate pollution levels.As lockdowns were implemented in response to the deadly coronavirus pandemic, the dramatic changes brought about by these restrictions have been described as the ‘largest scale experiment ever’ into air quality. In many places, the halt of movement and industry has shown a glimpse of a cleaner world, with many reports of exceptional blue skies. However, visual perception alone can be deceiving when observing air pollution. This report compares measurements of the world’s deadliest air pollutant, fine particulate matter (PM2.5), prior to and during the pandemic in 10 major global cities under lockdown: Delhi, London, Los Angeles, Milan, Mumbai, New York City, Rome, São Paulo, Seoul and Wuhan. Findings reveal a drastic drop in PM2.5 pollution for most global locations under lockdown conditions. These findings coincide with the 50th anniversary of Earth Day, whose theme this year is climate action. While ground-level air pollution poses our greatest environmental health hazard today, many sources of PM2.5 pollution - such as fossil fuel combustion - also contribute to climate change.