What mesh size do I need to keep out no-see-ums, not just mosquitoes?

Mosquitoes can be stopped by standard mesh, but biting midges and no-see-ums require ultra-fine mesh, often described as 256+ holes per square inch or roughly 310 holes/cm². Several head nets and the SANSBUG screen tent advertise no-see-um-rated mesh; standard bed canopies often do not block the smallest biting flies.