A powered air purifying respirator (PAPR), comprising: a power source; an ambient gas module including a first housing and a first inlet configured to receive an ambient gas; a gas filter in fluid communication with the first inlet and configured to filter the ambient gas; a blower configured to boost at least one of a flow rate or a pressure of the ambient gas entering and/or exiting the gas filter; a non-ambient gas module including a second housing and a second inlet that is independent from the first inlet and configured to receive a non-ambient gas; a first outlet configured to selectively provide a breathing gas to a user from at least one of the ambient gas or the non-ambient gas; and a switching device configured to select between the first inlet and the second inlet as a source of the breathing gas, wherein the gas filter, the ambient gas module, the non-ambient gas module, and the power source are configured to be joined together in an integral unit, and wherein the respirator is modular and configured to be operable in a plurality of different assemblies including an ambient gas-only assembly in which the non-ambient gas module is entirely removed from the integral unit; characterized in that the respirator further comprises a control module configured to control an ambient gas module setting and a non-ambient gas module setting, the control module including a breathing hose adapter in fluid communication with the first outlet, and a mounting interface configured to join directly to the blower in the ambient gas-only assembly and directly to the non-ambient gas module in a combination ambient and non-ambient gas assembly, wherein the housing of the non-ambient gas module is configured to mount the non-ambient gas module between the blower and the control module in the combination ambient and non-ambient gas assembly. a power source; an ambient gas module including a first housing and a first inlet configured to receive an ambient gas; a gas filter in fluid communication with the first inlet and configured to filter the ambient gas; a blower configured to boost at least one of a flow rate or a pressure of the ambient gas entering and/or exiting the gas filter; a non-ambient gas module including a second housing and a second inlet that is independent from the first inlet and configured to receive a non-ambient gas; a first outlet configured to selectively provide a breathing gas to a user from at least one of the ambient gas or the non-ambient gas; and a switching device configured to select between the first inlet and the second inlet as a source of the breathing gas, wherein the gas filter, the ambient gas module, the non-ambient gas module, and the power source are configured to be joined together in an integral unit, and wherein the respirator is modular and configured to be operable in a plurality of different assemblies including an ambient gas-only assembly in which the non-ambient gas module is entirely removed from the integral unit; characterized in that the respirator further comprises a control module configured to control an ambient gas module setting and a non-ambient gas module setting, the control module including a breathing hose adapter in fluid communication with the first outlet, and a mounting interface configured to join directly to the blower in the ambient gas-only assembly and directly to the non-ambient gas module in a combination ambient and non-ambient gas assembly, wherein the housing of the non-ambient gas module is configured to mount the non-ambient gas module between the blower and the control module in the combination ambient and non-ambient gas assembly.