Abstract: A protective respirator that deactivates pathogens in a kill zone in front of a mouth and nose of a user by emitting Far UV-C radiation (e.g., having a wavelength centered around 222 nanometers). In some embodiments, a controller uses a mathematical model to determine a required intensity or emission time to provide a threshold probability of killing a microbe (e.g., a virus such as SARS-CoV-2). The required intensity or time may be determined based on atmospheric conditions and/or physiological conditions of the user. The Far UV-C radiation may be emitted in a direction through the kill zone that does not intersect with the skin or eyes of the user (e.g., away from the user or across the front of the user's face). Alternatively, the controller may estimate the fluence of Far UV-C radiation at the skin or eyes of the user over time and adjust the Far UV-C radiation emitted.