Relevance of animal studies in the toxicological assessment of oil and wax hydrocarbons. Solving the puzzle for a new outlook in risk assessment.

Paraffin waxes and white mineral oils are distinct petroleum products separated from a common feedstock by crystallization, where only n-alkanes, iso- and cyclo-alkanes with a linear backbone of ∼ 20 carbon atoms long, selectively crystalize out from the oil to form the wax, which is solid at room temperature, whereas oils remain liquid. Up until the 90's, these differences were reflected in separated regulatory assessments. A paradigm shift occurred when Fischer 344 rats (F-344) developed liver epithelioid granuloma following exposure to low and medium viscosity oils or waxes. This lesion was used as common denominator between these products to be jointly assessed under the common term "mineral hydrocarbons - MHC", obviating compositional differences. This regulatory paradigm dominated for the next 30 years, exacerbated by the EFSA 2012 evaluation using the analytical term "MOSH" (mineral oil saturated hydrocarbons) which encompassed these products under single chromatography fraction. The reconstruction of historical developments, together with recent EFSA-sponsored studies of toxicity and accumulation and supporting literature, has allowed us to understand the etiology of the F-344 rat hepatic epithelioid granuloma, which is presented in an adverse outcome pathway (AOP). Considering chemical composition, it clearly demonstrates that the hepatic effects in F-344 rats caused by linear alkanes of waxes are irrelevant for humans. Waxes are thus not MOSH and should thus be evaluated on their own merit. The term MOSH should not include n-alkanes and be exclusively used to mineral oil fractions when considering their chemical makeup for a relevant human hazard assessment.