This has been covered at least once before with you. There are other hindrance effects in play quite possibly invisible smoke. As hindrance is cumulative the los becomes blocked.
Look at it this this way when concidering smoke cover.
There is no border,it flows with the wind,mainly NE,but can effect adjecent hexs too,depending on your LOS.
Smoke can be a double-eged sword though.
It does disperse in time,and if used at the right time it(wether which side uses it)to hide,ambush or attack