In article ,
"James" writes:
I did have some concerns about it restriciting air flow, and I'm sure it may
have a very minor effect if looked at from a scientific view. It made me
wonder why air bricks do not come with mesh already, perhaps fitted on the
inside when the house was built?

The finer the mesh, the more likely it is to get clogged over
time. When I fit airbricks, I use the plastic ones with clip
on grills. These have finer openings than in conventional
brick ones, but you can take them off and clean out the grills
and the airway through into the house. (Was also useful for
feeding copper pipes under the floor.)

--
Andrew Gabriel