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Lubricating oil base stocks (LOBs) are substances used in the manufacture of finished lubricants and greases. They are produced from residue remaining after atmospheric distillation of crude oil that is subsequently fractionated by vacuum distillation and additional refining steps. Initial LOB streams that have been produced by vacuum distillation but not further refined may contain polycyclic aromatic compounds (PACs) and may present carcinogenic hazards. In modern refineries, LOBs are further refined by multistep processes including solvent extraction and/or hydrogen treatment to reduce the levels of PACs and other undesirable constituents. Thus, mildly (insufficiently) refined LOBs are potentially more hazardous than more severely (sufficiently) refined LOBs. This article discusses the evaluation of LOBs using statistical models based on content of PACs; these models indicate that insufficiently refined LOBs (potentially carcinogenic LOBs) can also produce systemic and developmental effects with repeated dermal exposure. Experimental data were also obtained in ten 13-week dermal studies in rats, eight 4-week dermal studies in rabbits, and seven dermal developmental toxicity studies with sufficiently refined LOBs (noncarcinogenic and commonly marketed) in which no observed adverse effect levels for systemic toxicity and developmental toxicity were 1000 to 2000 mg/kg/d with dermal exposures, typically the highest dose tested. Results in both oral and inhalation developmental toxicity studies were similar. This absence of toxicologically relevant findings was consistent with lower PAC content of sufficiently refined LOBs. Based on data on reproductive organs with repeated dosing and parameters in developmental toxicity studies, sufficiently refined LOBs are likely to have little, if any, effect on reproductive parameters.

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Early subchronic and chronic inhalation toxicology studies on various petroleum mineral oils and formulated lubricants supported the ACGIH TLV of 5 mg/m(3) for mineral oil mist. Additional subchronic studies with aerosolized mineral base oils and lubricants during the last 15 years demonstrated that exposures to aerosols of mineral base oils (often >100 mg/m(3)) resulted mainly in concentration-related accumulation in the lung of alveolar macrophages laden with oil droplets. Inflammatory cells were observed with higher aerosol concentrations, consistent with the clinical literature from highly exposed workers. These pulmonary changes appeared to be a nonspecific response to the presence of deposited aerosol. Studies on sensory irritation during exposures of lab animals to mineral oils indicate possible effects only with very high aerosol concentrations. Coupled with changes in refining to remove carcinogenic polynuclear aromatic hydrocarbons from mineral oils several decades ago, this information indicates that current aerosols of mineral oils have a profile of low toxicity from acute to long-term exposures. Available information suggests that additives in some formulated products and/or maintenance of mineral-based metalworking fluids may play a much more significant role in potential health effects.