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Propylene glycol (PG) has been described as responsible for contact allergy and irritant reactions, but there are a lot of non-conclusive studies reporting a different percentage of patients irritated, with diverse concentrations and doses applied. As this substance is contained in a large amount of products, we reviewed its irritant capacity and the appropriate concentration to be tested. PG from two different origins was tested in 230 healthy volunteers. We applied 25 µl of PG at two different concentrations (70 and 100%). PG from origin B (PGB) was tested furthermore with two different qualities. Biophysical measurements were performed in those subjects patch-tested with PGB and its acute dermal irritation was tested in human skin culture. After 48 h results did not show any difference between PG from both origins and qualities. After 96 h PGB was slightly more irritant than PGA. Biophysical measurements only increased significantly in the subjects with a positive allergic reaction and there were no differences in the in vitro test. PG is not valid as pattern of irritation even at 100% concentration because it irritates a small percentage of patients. The recommended concentration should be between 30 and 70% in water, and the 100% concentration could be used in non-reactive patients.