**7. Consequences of lubricant degradation**

Lubricant degradation has a significant effect on the properties of the lubricant which can have significant consequences in aerospace. Degradation results in an increase in the chemical reactivity of the oil through the formation of acid and bases, changes in viscosity and changes in thermal conductivity. All of these can result in reduced life of the engine and also decreased operational efficiency. It is important that all of these effects be minimized for safe air travel.

There is an additional safety concern associated with lubricants and their degradation products present in most commercial and military aircraft. Air used to pressurize the cabin is drawn from the engine through a bleed air nozzle. While under normal operation, the air is thought to be safe, seal leakage results in traces of lubricant directed into the cabin. In cases of seal failure, high concentrations of lubricants, additive and degradation products enter the cabin. Smoke events are caused by seal failures, as well as other causes. Fume events occur in 2.1 of every 10,000 flights [51] and oil fumes are noted in 1% of all flights. The health related concerns are indicated by the 30% of fume events where crew impairment has been recorded even though there is recognized under reporting of impairment [52].

Aerotoxic syndrome has been described as an occupational illness along with epidemiological evidence [53]. Possible toxicological mechanism leading to aerotoxic syndrome has been described by Howard et al. [54]. A possible cause for Aerotoxic syndrome is based on repeated low dose exposure to organo-phosphorus compounds derived from phosphate esters [55]. High doses of organophosphates are known to cause organophosphate induced peripheral neuropathy (OPIDN) [56], however the doses encountered here are much lower, suggesting other chronic mechanisms [57]. The toxicity evidence indicates the need for clean air requirements for aircraft using bleed air for cabin pressurization [58].
