With the coronavirus pandemic quickly spreading, U.S. health officials have changed their advice on face masks and now recommend people wear cloth masks in public areas where social distancing can be difficult, such as grocery stores. But can these masks be effective?President Donald Trump, in announcing the change in the Centers for Disease Control and Prevention’s guidance on April 3, stressed that the recommendation was voluntary and said he probably wouldn’t follow it. Governors and mayors, however, have started encouraging the precautions to reduce the spread of the virus by people who might not know they are infected.Some cities have gone as far as setting fines for failing to wear a mask. In Laredo, Texas, anyone over the age of five who walks into a store or takes public transit without their mouth and nose covered by a mask or bandana could now be fined up to $1,000. These new measures are designed to “flatten the curve,” or slow the spread of the coronavirus responsible for C...