Dead skin clumps attached to fur

i was brushing my dog and he had a lot of little fur clumps like these. What causes them and should I be concerned?

Hello!  So, the superficial (surface-most) layer of skin cells, or Keratinocytes, are usually well- attached to their neighbours on every side, creating a smooth skin surface.  However, not always - and this is when flaking or scaling of the skin (sometimes called dandruff) might happen.   Flaking tends to be dry; scaling tends to be greasy. Factors which cause skin cells to cling to their neighbours less well are many and varied, from a lack of humidity (central heating - a common factor at this time of year) to exposure to parasites (fleas or mites, for example), to fungal or yeast infections, to allergies, to a possible shortage of the nutrients that bind skin-cells together, to underlying hormonal conditions (sometimes hormone-related diseases), to infection with bacteria, to physical disruption eg scratching.  As is typical when things are triggered by multiple causes, there is a lot of wisdom about ('just give them this,') each of which is only successful with certain causes- and furthermore, scaling can be caused by a combination of them coming together.   Your vet may help by looking for signs of underlying diseases such as hormonal problems, and / or offering appropriate shampoos to help.  Shampoos can be as different as different tablets and tend to be prescribed as appropriate to the situation, so we wouldn't advise simply trialling medical-looking shampoos which is a common reaction, but can make things worse.  I hope that something here helps.