Book excerpt: The science behind wearing a mask to stop the spread of Coronavirus, from a top expert in the field. In America, the debate over whether or not masks should be worn to prevent the spread of COVID-19 has become enmeshed with political affiliation, views on religious and personal freedoms, and conflicting media reports on the benefits and dangers of facial coverings. But now, several months into this pandemic, what does science say? What have we learned from international case studies? Dr. Hashimoto, the chief medical officer who oversees the Workplace Health and Wellness division at Mass General Brigham, a Harvard Medical School affiliated healthcare system, presents the current research, making the case that wearing masks in public is a key part of saving lives and bringing this pandemic to a halt. Citing specific examples of situations where infected individuals wore masks versus ones who didn't and how that changed the outcome, as well as population-based studies in individual states and by country, and the undeniable effect that universal masking had on Mass Brigham Hospital's staff of 75,000, Dr. Hashimoto offers a clear and compelling argument for the benefits of masking. In addition, he explains the complementary roles of social distancing, washing hands, coronavirus testing, and face shields, and a thorough exploration of what kinds of masks are most effective at stopping the spread of viruses and how they should be fitted and worn. He addresses safety concerns and medical misconceptions about mask wearing, why the CDC didn't recommend universal mask wearing at the beginning of the pandemic, and how employers can promote mask wearing in their workplaces. Don't wear a mask just because someone told you to. Find out the real reasons for masking and understand the science for yourself.