The Centers for Disease Control and Prevention (CDC) and other health authorities do not recommend face shields as a primary substitute for cloth face coverings because they lack a peripheral seal.
In laboratory simulations, face shields blocked only 2% of exhaled cough aerosols, whereas N95 respirators blocked 99% and medical masks blocked 59%. Comparison: Face Shields vs. Masks Study: Face Shields Not a Good Mask Substitute
Studies published in journals like Physics of Fluids highlight several critical reasons why face shields fail as standalone protection: The Centers for Disease Control and Prevention (CDC)