The increasing use of methylisothiazolinone by itself in cosmetic and toiletry products has earned the preservative the title of contact allergen of the year from the American Contact Dermatitis Society.
Methylisothiazolinone (MI), a moderate-strong sensitizer, is important to know about, because a few years ago it was introduced into the market as a stand-alone preservative at up to 100 parts per million, Dr. Donald V. Belsito, professor of clinical dermatology at Columbia University, N.Y., said at the society’s annual meeting.
Previously, MI was used only in combination with methylchloroisothiazolinone (MCI), and the concentration limit of MI in that combination was 3.75 ppm for rinse-off products, and 1.8 ppm for leave-on products. Even at those lower limits, the MCI/MI combination in 2009-2010 was considered the fifth most common cause of preservative allergy.
The new 100-ppm limit (instituted based on the belief that MI was a weaker sensitizer than MCI) represents a 25-fold increase in the permitted concentration in cosmetics, Dr. Belsito noted.
At least one study, however, has shown that the eliciting concentration for MI reactivity can be as low as 5 ppm, Dr. Mari Paz Castanedo-Tardana of the Dartmouth-Hitchcock Medical Center, Lebanon, N.H., and Dr. Kathryn A. Zug of Geisel School of Medicine at Dartmouth, Hanover, N.H., noted in an article in the January/February issue of Dermatitis, which granted MI its dubious 2013 title.
Dr. Castanedo-Tardana and Dr. Zug noted that in 2010, data suggested that more than 2,400 U.S. cosmetic products contained MI, which represents a doubling since 2007. The preservative is also used in industrial products without limit, thus providing other opportunities for exposure, such as occupational exposure (Dermatitis 2013;24:1-6).
An important problem is that few data exist concerning the cross-reaction patterns of MCI and MI, the authors noted.
"Bruze et al. showed that a small proportion of patch test subjects sensitized to MCI/MI also reacted to MI. Isaksson et al. also suggested that patients with high patch test reactivity to MCI may also react to high concentrations of MI (1,000 ppm). The percentage of concomitant reactions between MCI/MI and MI in the patch test population is relevant with regard to the diagnosis of MI contact allergy," according to the authors. They also noted that in a Danish study, a Finnish study, and a German study, only 40%, 66%, and 67% of patients with a positive reaction to MI, respectively, also had a positive reaction to MCI/MI.
The limited experience with MI-only patch tests concentrations also helped the allergen gain its newly bestowed title.
Concentrations of 300 ppm (0.03% aq), 500 ppm (0.05% aq), 1,000 ppm (0.1% aq), and 2,000 ppm (0.2% aq) have been tested, and all have yielded relatively similar percentages of positive test reactions.
"As with any other allergen, the ideal patch test concentration should be able to detect as many cases of contact allergy as possible without causing irritant reactions or active sensitization," the authors said, noting that preliminary results from Denmark suggest that 2,000 ppm (0.2% aq) is an appropriate concentration.
For now, however, it is likely that since MI is not routinely tested in any standard screening series, allergy to this preservative is being widely overlooked.
In fact, reports from Europe show that the frequency of allergy to MI is already at the same level as other preservatives that have been available for many years.
"Methylisothiazolinone should be considered as a potential suspect allergen among patients with suspected cosmetic dermatitis, facial dermatitis, and sunscreen allergy," the authors said, concluding that the addition of MI to an allergen screening series "will likely uncover otherwise undiagnosed cases of preservative contact allergy."
The presenter and authors reported having no disclosures.