Dave
You pose some basic questions that I suggest you should not be over concerned about.
We all breathe contaminated air in aircraft and our daily activities, the issue is the extent of any harm (if any) from contaminants – what are they and thus what are their concentrations. The current and quite rightly valid concern arises predominantly from oil in the air supply. Most modern oil formulations contain potentially harmful contaminants, but to date none of the science based investigations have identified any concentrations that would endanger health, even after a major oil leak. Some research suggests that there are several orders of magnitude of safety margin.
The immediate in-flight risks arise from the irritant effect of a contaminant and other constituents of oil. For example the ‘sweaty socks’ smell may come from valeric acid (Pentanoic Acid) in oil. Apart from the smell it is highly irritable even in the lowest concentrations and may effect vision and give a distinctly unwell feeling (sickness).
Oil contamination can result from overfilling APUs, engine leaks, or residual oil deposits from previous failures that have not been cleaned properly (plus de-icing fluid, acid rain, skin, and everything else in a conditioning system).
The defences are to apply threat and error management principles; avoid the problem in the first place with good maintenance and rectification; where problems are identified combat the threat with safety drills and then mitigate the effects; if the problem is serious then the aircraft safety is paramount - divert. Serious problems are rare events, although currently there is excessive attention on the subject. Perhaps like flight without hydraulics when the novelty of the event occurrence wears off the industry will revert to a more balanced view.
If more than a transient oil smell is detected then precautions against irritation should be taken; use oxygen masks, goggles, etc. Beware the human factor of adapting to the intensity of a smell, you think that it has decreased over time, but it has not. Also be aware of differences in personal susceptibility to both irritants and smells; some smells in the extreme can be disabling.
Illness or degraded performance due to irritants are rare, but can be self generating and infectious, this may result in pseudo illness in other crew members. Again human issues - ‘it won’t effect me’, but it could.
Fresh oil smell is an indicator of a transient or change; old oil smells and a range of other smells have been associated with longer term problems – residual oil, dirty air conditioning systems (or aircraft), or the original problem not being fixed. Beware of ‘ground tested found serviceable – no oils smell/leak detected’.
Jet engine fumes are predominately carbon monoxide; whilst this is potentially lethal your exposure risks are far higher on the journey to work than taxying behind another aircraft – it is the change of conditions that you are detecting.
As for cumulative effects the debate continues, but I think your best policy is to avoid smokers rather than worry about flying. Worry contributes to stress, which degrades human performance, which is central to safe operations.