You usually mount mirrors vertically so dust does not settle on them that much (also it is safer to keep laser light propagating horizontally at only one height). Additionally, overhead ventilation is often installed which provides a vertical Laminar flow of dust-free air. If you have to clean the mirrors eventually, you'd use compressed (clean) air or special mirror cleaning wipes. Thorlabs (lab supply company) has a detailed guide on optics cleaning[1]

[1] https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=90...