This article, published in the Proceedings of the National Academy of Sciences, provides a comprehensive review of the evidence supporting the use of face masks in preventing the spread of COVID-19. The authors develop an analytical framework to examine mask usage, synthesizing relevant literature on population impact, transmission characteristics, source control, wearer protection, sociological considerations, and implementation considerations. Key findings include: 1. **Population Impact**: Studies show that mask use can significantly reduce the spread of COVID-19, with high compliance leading to substantial benefits. For example, mask mandates in US states were associated with lower daily growth rates and prevented thousands to hundreds of thousands of cases. 2. **Transmission Characteristics**: COVID-19 is highly contagious, with many asymptomatic or presymptomatic carriers. Masks can reduce transmission by filtering respiratory particles, which are often smaller and more easily suspended in the air than droplets. 3. **Source Control**: Masks worn by infectious individuals can reduce the number of people infected, as they capture and filter respiratory particles. Laboratory studies and simulations show that masks can effectively block particles of various sizes, including those that are smaller and more difficult to filter. 4. **Wearer Protection (PPE)**: Masks can also protect the wearer from infection, although this is more challenging due to the smaller particle size. Various materials and designs of masks have been tested, with some showing high filtration efficiency, especially when combined with proper fit. 5. **Sociological Considerations**: Concerns about risk compensation behavior, where people may neglect other preventive measures due to overconfidence in mask protection, have been addressed. Data suggest that mask wearing is positively correlated with other preventive behaviors like hand hygiene and physical distancing. 6. **Implementation Considerations**: The article emphasizes the importance of maintaining medical supply chains and ensuring widespread access to masks, particularly in regions with shortages. Overall, the evidence supports the use of masks as an effective tool in combating COVID-19, especially when combined with other preventive measures. The authors recommend public officials and governments strongly encourage the use of masks in public settings, including appropriate regulation.This article, published in the Proceedings of the National Academy of Sciences, provides a comprehensive review of the evidence supporting the use of face masks in preventing the spread of COVID-19. The authors develop an analytical framework to examine mask usage, synthesizing relevant literature on population impact, transmission characteristics, source control, wearer protection, sociological considerations, and implementation considerations. Key findings include: 1. **Population Impact**: Studies show that mask use can significantly reduce the spread of COVID-19, with high compliance leading to substantial benefits. For example, mask mandates in US states were associated with lower daily growth rates and prevented thousands to hundreds of thousands of cases. 2. **Transmission Characteristics**: COVID-19 is highly contagious, with many asymptomatic or presymptomatic carriers. Masks can reduce transmission by filtering respiratory particles, which are often smaller and more easily suspended in the air than droplets. 3. **Source Control**: Masks worn by infectious individuals can reduce the number of people infected, as they capture and filter respiratory particles. Laboratory studies and simulations show that masks can effectively block particles of various sizes, including those that are smaller and more difficult to filter. 4. **Wearer Protection (PPE)**: Masks can also protect the wearer from infection, although this is more challenging due to the smaller particle size. Various materials and designs of masks have been tested, with some showing high filtration efficiency, especially when combined with proper fit. 5. **Sociological Considerations**: Concerns about risk compensation behavior, where people may neglect other preventive measures due to overconfidence in mask protection, have been addressed. Data suggest that mask wearing is positively correlated with other preventive behaviors like hand hygiene and physical distancing. 6. **Implementation Considerations**: The article emphasizes the importance of maintaining medical supply chains and ensuring widespread access to masks, particularly in regions with shortages. Overall, the evidence supports the use of masks as an effective tool in combating COVID-19, especially when combined with other preventive measures. The authors recommend public officials and governments strongly encourage the use of masks in public settings, including appropriate regulation.