Neurotoxic poisons in bleed air is not a new subject on this blog, but the more information we get on it, the more serious looks the aviation industry’s studied decision to ignore the dangers associated with contaminated cabin air, or to obfuscate.
To read about the human misery caused by cabin air contamination in airliners, go to The toxic subject that won’t die, and also, to read the personal testimony of suffering pilots and cabin crew, visit our AirSpace forum.
Meanwhile aviation journalist Tim van Beveren has conducted an interview with toxicologist Prof Dietrich Henschler of Wuertzburg University, Germany, who has been one of the world’s leading experts in workplace contamination since the 1950s. Then he was doing research on the precise group of chemicals that are often pumped into aircraft cabins when engine oil seals become faulty – tri-cresyl phosphates (TCP).
Such industry studies as are being done are looking for one particular variant, tri-ortho-cresyl phosphate (TOCP) when in fact, Henschler reveals, other variants are present and are far more harmful.
In his interview, Henschler reveals that TCP is much more dangerous than widely realised when it has been broken down into isomers, which is what happens when aircraft engine oils are heated and vapourised:
Tim van Beveren: Is it a major concern to you as a toxicologist that aircrews and passengers are being exposed to TCP, including MOCP (mono-ortho-cresyl phosphate) and DOCP (di-ortho-cresyl phosphate) at higher levels than TOCP in the aircraft cabin?
Prof. Dr. Henschler: As long as the mixture, which is almost a technical product, contains ortho-cresyl in whatever concentration, it is a matter of concern. So therefore [all efforts] should be made to lower the concentration of ortho-cresyl as far as possible.
Tim van Beveren: Is is acceptable for the aviation industry to focus on the TOCP content of the TCP only?
Prof. Dr. Henschler: No, this is completely misleading because this underestimates the real toxic potency of the mixture of the product.
Tim van Beveren: Given that TCP in jet oils contains a variety of isomers of TCP including MOCP and DOCP, is it acceptable for those monitoring for TCP to state levels are acceptable as they are below government-set exposure standards?
Prof. Dr. Henschler: To my information there is a lack of threshold limit values with TCP. One has been elaborated by the American Governmental Hygienists group who establish occupational exposure standards. They call it tri-ortho-cresyl phosphate and the value is 0.1 mg / m³. I think this has been elaborated on a very vague basis of data. The publication they are referring to comes from two Englishmen who have been paralysed during World War II, and there have been two or three air analyses performed there. This is trivial data to establish an official occupational standard. To my information no other country has established such a value up to now.
Tim van Beveren: You were heading the MAK commission (Maximale Arbeitsplatz Konzentration = maximum workspace concentration – this German government commission established/s the limit values for toxic substances in the working environment) in
Prof. Dr. Henschler: I was always against establishing an exposure standard in view of the lack of relevant data we have at hand.
Tim van Beveren: Would it be more appropriate to look at the mixture of contaminants in the jet oils rather than to the individual chemicals?
Prof. Dr. Henschler: You have to look at the mixtures because they are very complicated, they are varying in the content of the individual compounds and they are changed in the course of being heated up on metal surfaces, so that decomposition products will result. So you always have to look at what in the exposure air. And my recommendation is to establish competent analytical procedures to look at what is that exposure.
Tim van Beveren: Is it possible with today’s existing technology?
Prof. Dr. Henschler: It certainly is. It’s a matter of what you invest in, in such methodology. Modern analytical techniques are so sensitive, so reliable, so competent.
Tim van Beveren: So it’s just a matter of money?
Prof. Dr. Henschler: Maybe, yes
Tim van Beveren: Have you ever discussed your findings with oil manufacturers, governments or the military?
Prof. Dr. Henschler: Oil manufacturers, at least in our country are familiar with what I’ve published. My only contact with military people has been in
Tim van Beveren: What initiated your research into TCP back in 1956?
Prof. Dr. Henschler: Interesting question. After World War II the economic miracle in Germany ended up with an explosive increase in goods transportation by railway and they needed a paint which is resistant to UV irradiation and to meteorological influences, and this company who tried to develop such a product as an additive to lacquers asked me to have a look at what they had at hand at present with tricresyl phosphate with low content of ortho-cresyl. So this was the very beginning of the whole thing, and much to my surprise the mono-ortho esters were by far the most toxic components of the mixture of ten isomers.
Tim van Beveren: Would you say that breathing synthetic jet engine oils containing organophosphates such as TCP are likely to be harmful?
Prof. Dr. Henschler: These are harmful compounds.
Tim van Beveren: Why?
Prof. Dr. Henschler: Because they exert a toxic activity which we are well aware of, which is very nicely explainable how the mechanism of action is with these simple compounds. So, in view of the severity of the clinical symptoms and the ensuing fate of the patients involved, I would say it’s a dangerous material – it should be avoided as far as possible.
Tim van Beveren: When you started your research, what for you were the most interesting or maybe surprising findings?
Prof. Dr. Henschler: The extremely high toxicity of the mono-ortho ethers, which went completely against expectation. If you had at hand the three symmetric ethers tri-ortho, tri-meta, tri-para, you would have expected that with a lowering of the ortho-cresyl content of the mixture that toxicity will go down. But the contrary is right – it goes up. This was the most interesting and surprising finding I had.
Tim van Beveren: And is this common knowledge since?
Prof. Dr. Henschler: Hopefully, yes. Some people know, others do not, I don’t know the reason why. Sometimes chemistry is a little bit complicated, and thinking into molecular structures is, to some people an awful business, and though they resist a little bit in going deeper into the matter.
Tim van Beveren: Would you think that proper scientific studies of the phenomena we’re facing here, of contaminated cabin air should be done, and can it be achieved with today’s technology?
Prof. Dr. Henschler: They should be done on a large scale where you monitor with analytical techniques what is the outlet, what comes into contact with the individuals at exposure, and what is the composition of these complex mixtures. And this in relation to the complaints brought forward by the cabin personnel and the passengers on a large epidemiological basis.
Tim van Beveren: So if TCPs shouldn’t be in the engine oil, there should be other substances which are available?
Prof. Dr. Henschler: Less toxic or non-toxic ones! Better alternatives, yes.
Certainly TCP is a very toxic compound, but not the only one. There is another candidate for being eliminated, beta-naphthylamine which is a proven human carcinogen of high potency. The others I don’t know of.
Tim van Beveren: What makes these chemicals so dangerous to the human being?
Prof. Dr. Henschler: Cancer is one of the irreversible phenomena in medicine. If cancer comes into consideration, the evaluation changes from non-irreversible. Cancer in humans is in some way inevitable, the efficiency of treatments is still a little low.
Tim van Beveren: So given the fact that some claim that TOCP is the most toxic but there is MOCP and DOCP as well in the oil, what would you say as a toxicologist about just focusing on the TOCP?
Prof. Dr. Henschler: This is a crude underestimation of the toxic potential. If you focus on TOCP, tri-ortho-cresyl phosphate which has the lowest toxicity of all isomers containing ortho-cresyl. So, we have to focus on the di- and particularly the mono-ortho ethers. For regulatory purposes I would recommend to say an amount of TCP as a whole containing ortho-cresyl to a certain extent. This is a clear-cut definition.
Tim van Beveren: Would you say that there are differences between inhaling TCP and ingesting TCP?
Prof. Dr. Henschler: Principally not, because what is active is not the compound itself but a metabolite which has to be taken up either through the digestive tract or the skin or the pulmonary tract, to be transported to the blood and from there to the liver where it is enzymatically converted to the highly reactive intermediate. So it doesn’t matter through which entry the compound gets access to the circulation.
Tim van Beveren: Would you inhale heated engine oil?
Prof. Dr. Henschler: Me? No, never! I was very much surprised to read these reports that people get exposed to these compounds.
Tim van Beveren: You did research back 50 years ago. You initiated research, you found that there is a high toxicity. What does it tell you now 50 years later? It seems that people do not draw the right conclusions?
Prof. Dr. Henschler: It tells me that they haven’t picked up the recommendations of careful toxicologists. I have always recommended to keep emission exposures as low as possible and look for better alternatives, so it was much of a surprise to me to be informed of those types of incidents.