Methylisothiazolinone in rinse-off products causes allergic contact dermatitis: a repeated open-application study.

BACKGROUND: In recent years, the prevalence of contact allergy to the preservative methylisothiazolinone (MI) has increased dramatically. Cosmetic products are one of the major sources of exposure.

OBJECTIVES: To examine whether allowed concentrations of MI in cosmetic rinse-off products have the potential to cause allergic contact dermatitis.

METHODS: Nineteen MI-allergic subjects and 19 controls without MI allergy applied two liquid hand soaps five times per day on areas of 5 × 10 cm(2) on the ventral side of their forearms. One soap contained 100 ppm MI, the maximum allowed concentration in cosmetics, and was used by 10 allergic subjects and all controls. Another liquid soap with 50 ppm MI was used by nine allergic subjects. As the negative control, all subjects used a similar soap that did not contain MI. The repeated open applications proceeded until a positive reaction occurred or up to 21 days. The study was conducted in a randomized and blinded fashion.

RESULTS: Ten out of 10 MI-allergic subjects developed positive reactions to the soap with 100 ppm and seven out of nine reacted to the 50 ppm soap, while none of the 19 controls had a positive reaction during 21 days of application. No reactivity was seen to the soap without MI. The difference in reactivity to MI between MI-allergic subjects and controls was statistically significant (Fisher's exact test, P ˂ 0.0001).

CONCLUSIONS: Rinse-off products preserved with 50 ppm MI or more are not safe for consumers. No safe level has yet been identified.