Final answer:
The two general types of self-contained breathing apparatus are open-circuit and closed-circuit systems, used for providing breathable air in inhospitable environments.
Step-by-step explanation:
The two general types of self-contained breathing apparatus are open-circuit and closed-circuit systems. Open-circuit systems, like those used commonly by scuba divers, involve exhaling the used air into the water, thus not reusing it. Closed-circuit systems, sometimes referred to as rebreathers, recycle the exhaled air, remove the carbon dioxide, and replenish the oxygen before it is breathed again, which is ideal for extended or covert diving operations. Both systems are essential for providing a reliable supply of breathing gas in environments where the ambient atmosphere is unsuitable for breathing.