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Organophosphates and hazards to health – aircraft cabin air in the spotlight

Organophosphates and hazards to health – aircraft cabin air in the spotlight

9 March, 2015

David Barden (Cropped)

Dr David Barden

Technical Copywriter

David Barden received his Ph.D. in Organic Chemistry from Cambridge University in 2004, and during his time as an editor at the RSC wrote news pieces for Chemistry World on various scientific topics. He is now Technical Copywriter at Markes International, where he draws on the expertise of his colleagues to explain how new thermal desorption and mass spectrometry technologies can be applied to analyse volatile organic compounds in a wide variety of situations.

Have you ever been exposed to poor air quality on a plane?

Did you complain? If not, and you’ve been tempted to silently put up with unpleasant smells at 35,000 feet, then perhaps next time you might want to make your voice heard, in light of recent news regarding a pilot who had died in 2012 after claiming he had been exposed to poor-quality cabin air (also picked up by the mainstream media, including The Telegraph and The Independent).

Organophosphate compounds

Before anyone jumps to conclusions, I should point out that a final verdict on the death of the pilot Richard Westgate has not yet been reached. However, following initial investigations, Sheriff Stanhope Payne, the Senior Coroner for Dorset, UK, has released a report on ‘matters of concern’, because he believes there is a risk of other deaths occurring in similar circumstances. Specifically, he stated that the pilot’s body had “disclosed symptoms consistent with exposure to organophosphate compounds in aircraft cabin air”.

So just as unpleasant smells from new office furnishings have become a serious health matter and the subject of EU-wide regulation, it seems the time might now be coming where the matter of air-cabin ‘fume events’ is taken much more seriously.

The issue is highly contentious – campaigning groups and former airline pilots (Susan Michaelis and Tristan Loraine, to name just two) and are levelling accusations of negligence or wilful indifference at airlines, while aviation authorities and the airlines themselves have said little other than the usual reassurances. Even the Daily Mail – no stranger to simplistic, sensationalist coverage of ‘chemical’ stories – has this time released what seems to be a pretty balanced article on the topic.

To present a couple of hard facts in this maelstrom of claim and counter-claim, the UK government’s Department for Transport reported an estimate that fume events occur on roughly 0.05% of flights. They have also said that 207 contaminated air events were reported in 2010, out of 1.12 million passenger and cargo flights by UK carriers (0.018%). These seem like small numbers, but when it comes to matters that affect the safe piloting of a plane and the health of its passengers, surely a little application of the precautionary principle would not go amiss?

Tricresyl phosphate (TCP) esters

This is particularly because the compounds at the centre of the current news frenzy – organophosphates – have long been recognised to be pretty unpleasant, having been variously applied as insecticides, plasticisers, and of course the infamous nerve agents. However, just one group of organophosphates are the focus of attention here – tricresyl phosphate (TCP) esters. These are used as antiwear additives in aircraft lubricants, but are not widespread in other industries. This, combined with the fact that overall levels of VOCs in cabin air have been found to be very low, means that attention has fallen on these little-investigated organophosphates rather than the well-known ‘usual suspects’ for poor air quality, such as benzene and toluene.

These organophosphate additives are believed to enter the airstream because of the design of the cabin air system, which takes external air after compression (but before fuel injection), and mixes it with recirculated air before it passes into the cabin. It is clear that contaminants can occasionally get into this so-called ‘bleed air’, perhaps as a result of air backflow during engine start-up (as a current pilot suggests on his blog), component age or inadequate maintenance. Only one commercial aircraft – the Boeing 787 Dreamliner – has a design that completely avoids use of bleed air and thus the risk of contamination. However, the short time since its introduction means it’s probably too early to say whether fume events have been eliminated as a result.

Complex situation

Adding to the complexity is that the known mode of action of organophosphates doesn’t seem to tally with the reported symptoms. In a report on the website of the Aerotoxic Association, Professor Michael Bagshaw, Professor of Aviation Medicine at King’s College London, says that “The toxic effects of organophosphates [give] rise to muscular weakness and paralysis [...] it is impossible to explain the wide range of symptoms and signs reported by some crew members as a unified result of TCP exposure.” He goes on to say “as far as scientific evidence has been able to establish to date, the amounts of organophosphates to which aircraft crew members could be exposed, even over multiple, long-term exposures, are insufficient to produce neurotoxicity.”

The situation is obviously highly complex, with many factors at work, and much work remains to be done in identifying the precise chemicals involved and appropriate limit levels. Until then, if you’re the victim of poor on-board air quality ... kick up a stink.