The irritant potency of 5 types of insulation wools (2 types of rockwool and 3 types of glasswool) were studied on the basis of their differences in diameter by a standardized rubbing test. Assessment was done by laser Doppler flowmetry (LDF) and erythema scores. Insulation wools show a wide distribution of the diameter around the mean. Of the rockwool material, 30% consisted of small grains, called "shot". The control test site did not change over time; thus, the rubbing procedure itself does not seem to contribute to the skin reactions. All fibre types proved to be irritant to the skin. The irritant reactions did not correspond with the mean diameter, although the fibre type with the smallest diameter was the least irritant and the fibre type with the largest diameter the strongest. The presence of "shot" had a significant effect on the LDF responses. "Shot" and the distribution around the mean diameter may play a role in eliciting the skin irritation by insulation wools.