Actually no piece of scuba equipment is "harmless".
Said that, the kind of danger associated with releasing air from a cylinder is a bit difficult to identify for me.
If you just open slightly the valve and leave the air flowing gently, I do not see any serious risk.
The air, by itself, is just dry air, you can breath it. It is clean. So I can see some risk only if one opens too much the valve, creating a fast air jet, which can cause the cylinder to fall as a reaction to the thrust of air jet, hitting your feet. Another risk, in a dirty environment, is that the air jet can launch some small particles in your eyes.
And finally, after emptying the cylinder too quickly, the valve becomes very could and is not safe to be touched with your naked skin. But this is a danger which is not directly coming from air...
Probably the most relevant danger associated with releasing air from cylinder is that this produces a lot of "white noise", so if you stay around for the whole day, you risk some noise over-exposure...