As the weather gets colder and people head indoors, the risk of catching Covid-19 is rising. WSJ explains why air ventilation and filtration are one of our biggest defenses against the coronavirus this winter. Illustration: Nick Collingwood/WSJ More from the Wall Street Journal: Visit WSJ.com: http://www.wsj.com Visit the WSJ Video Center: https://wsj.com/video On Facebook: https://www.facebook.com/pg/wsj/videos/ On Twitter: https://twitter.com/WSJ On Snapchat: https://on.wsj.com/2ratjSM #WSJ #Coronavirus #Science