These settlers live on a planet, which surface temperature and pressure are compatible with human life. However humans cannot breath its atmosphere. The gas in the atmosphere is not necessarily toxic or extremely dangerous (skin exposure is ok), but it will eventually kill a person (or maybe the atmosphere simply lacks oxygen).
So my people will need a "mask" to wander around. I want it to be as small and simple as possible, and would prefer if it leaves the mouth "free". I was actually thinking of some kind of nose respirator, but I guess it would make their voice funny.
How can I deliver breathable air to people with the minimum amount of hardware around their face?