sealer/treatment for ash staircase

Hi,

First post from a newbie looking for help. We have had an ash staircase installed in our office, I'm wanting to seal this in order to prevent staining from moisture/dirt from shoes.

Ideally I would like to keepkeep the wood looking as natural as possible, without wishing to darken it or cover in varnish etc.

Does anyone have any suggestions as to how this fairly high traffic staircase should be sealed to prevent staining? I'm loathed to put a central runner on and hide the wood.

Many thanks for any help, suggestions or examples that people can provide.