Flight attendant Kerstin Konrad still remembers her first "fume event" - as the industry calls air inside the aircraft that is possibly contaminated.

"On the flight, passengers were asking for headache tablets," Konrad told DW. 

Passengers had noticed a peculiar smell at the back of the plane and the captain asked her to check it out, she says. "I went back and sniffed the air nozzle."

"I immediately got a headache and a tingling sensation in my limbs that spread like numbness." She says she then became dazed and so confused she could not perform even the simplest task. "I could not sort the juices on the trolley anymore, which sounds crazy."

Konrad says that since 2013 she has experienced a total of four such fume events, sometimes accompanied by odd smells, sometimes without. She says she has on each occasion had headaches and abdominal pain, numbness and confusion and has also suffered permanent health damage meaning she can no longer fly. She has nerve damage and restricted lung functions, both confirmed by a doctor.

Konrad has become an activist in fume events, organizing petitions and demonstrations with others, most recently at the beginning of September in front of the Reichstag building in Berlin.

And she is not alone. Some 228 such fume events were reported to the German Federal Bureau of Aircraft Accident Investigation (BfU) in 2016 alone.

And the number of cases reported by pilots and cabin staff to their employers may be even higher, according to TV channel ZDF, referring to internal records held by the airlines, some of which are not passed on to the BfU.

What is a dangerous level? 

The German Flight Accident Investigation Act and an EU regulation determine what constitutes near-accidents, which need to be investigated by authorities. Here, the situation in the cockpit is crucial.

"Circumstances that force a pilot to use an oxygen mask are what can lead to a serious incident," Jens Friedemann, an accident inspector at the Federal Bureau of Aircraft Accident Investigation (BfU), told DW.

Using these criteria for the 228 fume events reported last year only two were considered serious incidents. In other words, there is no fundamental, and above all no particularly pressing, problem.

Contested cause and effect 

But others are less certain. Doctor Astrid Heutelbeck told DW, for example, that she can prove a causal connection using residues in the blood and urine of people who have suffered a fume event, by using a procedure called human biomonitoring.

"In human biomonitoring — in connection with the time of the event — we find substances and groups of substances that are not described as components of the normal environment, but as the ingredients of kerosene, oils or hydraulic fluid," Heutelbeck said.

It is proven that these substances can damage the nervous system and the lungs, Heutelbeck, who is the head of the work and environmental medicine outpatient department of the Göttingen University Medical Center, believes.

The air in the cabin is tapped from the turbines of the aircraft; it is also called bleed air. But it may possibly be contaminated with constituents of kerosene, oil or hydraulic fluid.

"People can still inhale and exhale the same amount of air as everyone else. That's not a problem, but to get the oxygen out of this air does not work anymore and they notice it in terms of physical strain. When they go upstairs, for example, people may say they feel like grandma."

Since the beginning of 2014, Heutelbeck has investigated "several hundred" patients from the aviation industry with these symptoms, using standardized medical procedures and ruling out other pathologies.

'No evidence' 

The aviation industry, meanwhile, is dragging its feet.

It has yet to recognize a scientifically proven link between the above outlined symptoms and what is called the bleeding of toxic air into plane cabins.

"The bleed-air system has been standard in the aviation industry since the 1960s and has proven to be reliable," Claudia Nehring of the Federal Association of German Air Transport (BdL), representing the interests of airlines and airport operators, told DW.

"In addition, the air is drawn off in front of the combustion chamber of the engine - the kerosene injection and the combustion take place later, which means that it is technically impossible that the air is connected with exhaust gases or residues from the combustion chamber," she goes on.

"There are different studies that have examined the quality of cabin air," she says. "None of them has demonstrated that cabin air is contaminated in such a way as to cause health damage."

The European Aviation Safety Agency (EASA) has also air-tested plane cabins for toxic substances in two studies and found that air quality in the aircraft is no worse than that in normal offices.

Skeptical of such doubts 

However, it has yet to be investigated whether a combination of certain poisons in low concentrations can be harmful to health under different air pressure conditions and at low humidities. In addition, the air was only checked on 69 flights.

"If one assumes that about every thousandth flight is affected and they measure only about 70 flights, then this disproportionality speaks for itself," Heutelbeck says.

British and Australian researchers have found a "direct link" between this aerotoxic syndrome, the health effects caused by fume events and the working environment, that is the aircraft.

This "new occupational disease" must be recognized as such urgently, according to the authors of a study published by the World Health Organization (WHO) in June 2017.

The BdL declined to comment.

And normal passengers? 

If a pilot is struck by a fume event, of course the situation can quickly become very dangerous. But pilots are able to draw on masks that supply them with pure oxygen, something flight attendants and passengers are unable to do. The masks that fall down in an emergency contain only oxygen-enriched cabin air.

Most passengers do not fly as often as flight attendants and pilots, but theoretically they can also suffer health damage from fume events.

The industry is feeling growing pressure and has committed to actively working on research. "We have a great interest in gaining further insights," BdL spokeswoman Claudia Nehring told DW. "Because the safety and health of crews and passengers is our top priority."

All the airlines represented by BdL are currently conducting tests with filter systems and sensors, but it will take another two years for the results to be evaluated and at least three years until the European Commission publishes a new study, which will be conducted with the help of the industry.

High costs 

Of course, fume events are also about money. It would be expensive to equip all machines with filter systems and sensors for permanent monitoring of air quality. It would be even more expensive were the cabin air not taken from near the combustion chamber of the engines, as is the case with the Boeing 787 ("Dreamliner"), an exception among the large passenger engines.

The costs of treatment for staff and, in the worst case, the insurance against incapacity to work after accidents at work have to be borne by the responsible professional association, the BG Verkehr.

Its spokeswoman told DW that professional associations are "legally obliged to recognize accidents only if there is a link between an accident and health problems," adding that no causal link has been established to date.

Nevertheless, it is probable that the initial costs for human biomonitoring are being borne by the BG Verkehr.

"I have a feeling there's no great interest in really investigating this issue," flight attendant Kerstin Konrad tells DW, adding that it's time for politicians and lawmakers to take the lead. "After all, it's about human lives."