Contaminated air on board
For several reasons, numerous toxic substances can infiltrate the cabin via the air conditioning system. This happens almost continuously, even in new and well maintained aircraft. The concentration of this contamination however, is normally too low to be observed directly (no smell, not visible). Most people will not be affected by it. Medical issues after a flight (like a soar throat, coughing, flu-like symptoms) are often wrongly not linked to contaminated air on board. Nevertheless, this so-called Long Term Low Level Exposure can eventually lead to more serious health problems. This depends on one's personal immune system, but it goes for both crew members and frequent flyers.
Under certain conditions, in particular during certain technical malfunctions like worn or broken oil seals in the engine, the concentration of harmful substances can suddenly rise. This is called a Fume Event. Many airlines prefer to camouflage this as a "Smell Incident", as this sounds a lot less serious. It is however about a serious incident, because the high concentration of (highly) toxic fumes that occurs will often cause acute symptoms of poisoning. A Fume Event can be accompanied by visible smoke or mist and/or a bad smell ("dirty socks" or "wet dog"). Tha is however not always the case, since a number of the poisonous substances involved are odourless and invisible (e.g. Carbon monoxide).
Can a Fume Event happen any moment?
Fume Events could happen any moment of course, e.g. when an oil seal in an engine fails. But fortunately those occasions are rare. Cances of a Fume Event are considerably higher during "power transients", when engine power is increased or reduced. For example, during engine start, Take Off and cling, descent and landing. That is because the air flow through the engine is not stable at those times.De used oil seals are of the Labyrinth-type, and will always allow tiny amounts of oil to pass. The engine design is such that at each oil seal the air pressure outside is higher than the oil pressure inside. In theory, any oil in between the seal should always flow inwards instead of outwards. That works very well as long as conditions are stable, with a constant air flow. During power transients this is unfortunately a lot less reliable.
Incidents are still underreported
The reporting by crew of Fume Events has been discouraged by many airlines for years. The (sometimes serious) complaints about cabin smoke and/or odours are often reported in the Technical Logbook as an "Odour Incident". Only too often, they are not reported at all, to avoid long delays, time consuming maintenance and high costs.
Maintenance schedules drastically expanded
Until the nineties, engine manufacturers would prescribe a Heavy Maintenance inspection every 5.000 flying hours for their engines. The engine needed to be taken off the aircraft for that and be completely disassembled in the work shop. Part of this check was the replacement of all oil seals. Obviously a costly operation!
The later generations of jet engines however, became more and more reliable and less prone to malfunctions. As the airline companies were pressing to cut maintenance costs, maintenance intervals were gradually extended. These days the engine condition is monitored via a datalink from home base. Maintenance planning is based purely on (wear) condition. Not too long ago, manufacturer General Electric proudly announced that one of its engines had served for 40,000 flying hours without a single shop visit!
Those engines may have improved, but...
Although modern jet engines are undoubtedly much more reliable, their oil seals have barely changed over time, and they wear just as fast as before! As they stay in place for longer periods, they will gradually start spilling more oil, especially during power transients. As long as the oil consumption is within manufacturer's limits (upto one liter per hour!), this will be no reason for an overhaul. The contamination of the Bleed Air will however increase, and so will chances of a Fume Event.
...even those maintenance instructions are not followed up!
For example, take the Airbus A320, one of the types frequently involved in Fume Events. In their maintenance instructions, Airbus prescribes a thorough cleaning of the entire air conditioning system's interior after each Fume Event. A costly procedure, that takes at least 2-3 days in a hangar. A Fume Event is however often written down as an "Odour Incident". And so the aircraft can be released within a few hours... But as a consequence, that aircraft will become more and more contaminated. It will repeatedly get involved in such "Odour Incidents", with all the consequences of that for crews and passengers. These days, on the internet lists can be found of notorious "stincker planes", aircraft that are repeatedly involved in Fume Events, without proper preventive measures being taken.
Over the last few years, an increase in the number of reported Fume Events can be noticed. The bad news is that Fume Events do indeed occur more frequently. The good news is that crew awareness of the dangers is definitely rising. The media are reporting more frequently about such incidents, and they hear more often about colleagues which serious, often permanent health issues. In the Netherlands alone several hundreds of cockpit- and cabin crew members were incapacitated during their daily work, some permanent. World wide you're looking at thousands of them.
More often appropriate measures
A crew that is aware of this and ends up in such a situation, will approach it differently. Chances are for such a crew to immediately take proper measures to guarantee flight safety. Those can vary from applying cockpit oxygen masks to making a precautionary- or emergency landing. Until recent years, especially cockpit crew would play down the problem only too often, with all the consequences of that..