The proportion of cross-sectioned dentin tubules with subsurface occlusion ( occluded to a mean of 5 2 m , range : 1 to 9 m below the surface ) for the 8 % strontium group on days 1 and 2 ( pre-acid ) was 82 % ( SD : 9 % , 95 % confidence interval [ CI ] = 78 % to 86 % ) and on days 3 and 4 ( post-acid ) was 88 % ( SD : 10 % , CI = 83 % to 93 % ) .