To overwhelm the DVD error correction / parity system, an roughly round unburnt spot should be rather large, say about 5mmÂ² (or even more than that).
But even a tiny unburnt spot will appear in a scan as a PIF peak, and often as a PIF cluster. With a large cluster, some lesser readers may have to slow down or even re-read to retrieve the data.
In my experience, with most dust-induced unburnt spots, there is nothing to be concerned about in terms of immediate readability, which is not affected.
On the other hand, tough, I found that in some instances, the area around the unburnt spot tends to show small, but significant, PIE/PIF degradation over time. So there is a possible concern in terms of life expectancy.
I choosed to dust all my blanks before burning.