The environmental factors of waste oils include fire and explosion hazards, stability of composition of properties, toxicity, carcinogenic and mutagenic effects and biodegradability. These factors come into the picture when waste oil is interacting with the environment in the broad sense of the word: direct contact with the atmosphere, water and soil, with plants and animals, as well as indirect effects occurring in storage, transportation and disposal or recycling.
The toxic and carcinogenic effects are caused by decomposition of oil (most importantly, the additives in the oil) during use. Lead compounds and the products of incomplete fuel combustion in motor oils, fine metal particles, various solvents, fungi and bacteria. Toxic substances may originate in the process of oxidation and nitrogenization of lubricants or thermal decomposition.
Research indicates the growth of some used mineral oils compared to new oil due to accumulation of polycyclic arenes, which result from incomplete fuel combustion and thermal decomposition of the oil.
Nitroso compound, nitrosamines in particular, which result from the reaction between sodium nitrate (used as anti-corrosion additive in some oils) and amine type additives (secondary and tertiary amines). The reaction is catalyzed by some microbes and contaminants.
Halogens in waste oils are a substantial hazard; these can damage the immune and reproductive systems of humans and animals, cause infertility, cancer and a range of other illnesses. Their toxicity is only inferior to that of heavy metals. Chlorine compounds are especially common. These substances may come from mineral oils, halogenic additives to oil and fuel and synthetic oils, polychlorinated biphenyl (PCB) oils in particular.
Recently, entirely new contaminants originating from synthetic oils and alternative fuels (such as methanol, gases of various origins, some vegetable oils) may be found in used oil. The environmental effects of these are not yet clear.