A U.S. military issue gas mask can block certain radioactive particles, particularly alpha-emitting dust, but it does not provide protection against radiation itself. The effectiveness of the mask relies on having the appropriate filter cartridge, which can protect against various chemical and biological agents. However, it is important to note that gas masks are designed primarily for inhalation protection and require a proper fit to ensure a tight seal, making them ineffective if facial hair is present. While gas masks can filter out micro-particles, they allow simpler gases to pass through, which limits their use in certain hazardous environments, such as firefighting. Discussions also highlight the historical context of gas masks, originally designed for chemical warfare, and their continued use in specific scenarios like tear gas exposure.