Michler's ketone (4,4'-Bis(dimethylamino)benzophenone)

Michler's ketone (MK) was synthesized in the 1870s by Wilhelm Michler alongside Michler's Base, and both compounds share the bis(dimethylaminophenyl) scaffold that gives them their photochemical and dyeing properties [1]. MK became important as a UV photosensitizer and photoinitiator in diazo and photographic processes and in radiation-curable printing inks [2]. Concerns about food safety arose when Michler's ketone and its related compound isopropyl thioxanthone (ITX) were found to migrate from UV-cured printing inks on food cartons through the carton walls into the food — a 2005 discovery in German milk cartons triggered investigations across the EU [1]. Regulatory action in the EU restricted Michler's ketone in printing inks and adhesives intended for food contact [2].

Occupational exposure in printing ink manufacturing, UV-curing operations, and photographic chemical production [1]. Consumer exposure through migration from UV-cured printing inks on food packaging — particularly fatty foods like oils and dairy products [2]. Laboratory chemists using Michler's ketone as a photosensitizer [1].

Michler's ketone is metabolically demethylated and N-hydroxylated to reactive carcinogenic intermediates similar to those from Michler's Base [1]. It induced liver tumors and hemangiosarcomas in rodent studies. EPA classifies it as a Group B2 probable carcinogen; IARC Group 2B [2]. The food packaging migration pathway made this a public health concern beyond just occupational exposure [1].

Printing industry workers, UV-curing facility workers [1]. Consumers of packaged food products using UV-cured inks, particularly fatty foods in paperboard cartons [2].

1. Occupational: fume hood and gloves for all MK handling [1]. 2. EU and many national regulations now restrict MK in food-contact printing inks [2]. 3. Choose products in glass, metal, or polyolefin packaging if concerned about printing ink migration [1].