Powdering Bricks

[carcinus]

I have a semi detached house built in 1887 which has cavity brick walls. In the loft, which is very large, a number of the red bricks (1-2%) on the external wall suffer from some sort of erosion, whereby they are eroding away by very fine brick dust. There are also a smaller number on the internal wall. I'm not aware of any dampness in the loft. Its not a new problem and has been there since I moved in back in the 80's, but some of the bricks have lost at least an inch in depth.
Do you know what causes this and the best way to treat it? ie do I PVA them?

[welsh brickie]

on the exterior wall use a silicone solution to seal the bricks.On the interior you can use pva.