Several months into a pandemic that has claimed hundreds of thousands of lives and decimated economies around the world, scientists still lack a complete understanding of how the virus that caused it is transmitted. Lockdowns are already easing in some places, and people are preparing to return to a version of work and social life. But a crucial question stubbornly remains: Can the pathogen behind COVID-19 be “airborne”? According to the U.S. Centers for Disease Control and Prevention and the World Health Organization, the novel coronavirus is primarily spread by droplets from someone who is coughing, sneezing or even talking within a few feet away. But anecdotal reports hint that it could be transmissible through particles suspended in the air. After attending a choir practice in Washington State in early March, dozens of people were diagnosed with or developed symptoms of COVID-19 even though they had not shaken hands or stood close to one another. At least two died. After dining at an air-conditioned restaurant in China in late January, three families at neighboring tables became sickened with the virus—possibly through droplets blown through the air.