"Pilots challenge pesticide cover-up" Yet another aviation scare story hits the press. What is it all about?
Most commercial and military aircraft engines use a synthetic lubricant containing an anti-wear additive called Tricresylphosphate (TCP). This is one of a group of chemicals known as Organo-phosphates (OPs) which have been used as insecticides since the 1950s, most commonly for sheep dipping or crop spraying.
In humans, OPs can damage, either temporarily or permanently, an enzyme called acetylcholinesterase. This plays a vital role in the transmission of messages between nerve cells and thence to muscles. One original use was as a nerve gas in warfare. It probably acts at other sites in the body as well. The variety of symptoms it can cause are therefore endless, but after a single exposure to a high dose the commonest are headaches, tiredness and weakness, eye irritation, sweating and twitching, poor memory, anxiety, depression. They may last for a few days to several weeks. Immediate admission to hospital is essential, as the effects may be fatal.
Absorption of OPs may be via ingestion (carrots and winter lettuce have been implicated in the past), through the eyes, the skin (those sheep-dippers); but relevant to aviation is by inhalation. It is postulated that a leaking oil seal can occasionally allow oil fumes inside the aircraft via the air conditioning system. The CAA has a list of such reports. The fumes presumably contain some OPs and if the lubricant has been overheated these may form yet more toxic esters not present in the original product.
There is no dispute that acute and repeated exposure can produce harmful effects, but what of chronic exposure at lower doses? No consensus exists. Studies are hampered by difficulty in measuring the actual exposure levels, length of time between exposure and symptoms, and the diverse nature of the symptoms themselves. Various epidemiological studies have been conducted but failed to deliver proof of cause and effect, most have involved high skin exposure in humans or feeding them to rats. (Hens weren't too clever afterwards either). One report by the Medical Research Council concluded that chronic exposures "are not likely to be responsible, in themselves, for any adverse health effects large enough to be subjectively apparent". Another suggested there was ample evidence to suggest that "long-term exposure to OP pesticides has a profound and deleterious effect on mental health".
As in many areas where aviation and medicine interface, more research needs to be done. Better studies are being conducted, but these will take some years to complete. Meanwhile, a "Group on OPs" has been formed of various experts to report back to Government and an "OP Information Network" collects all available data. Specialist treatment centres for acute poisoning are also to be established.
It would seem obvious that the problem would go away if the oil companies replaced OPs with another additive, though the Secretary of State for Defence says no viable alternative is available at present. Leaded petrol for old cars has been banished. The most readily available replacement additive contains Phosphorus! What might the oil companies replace OPs with? We managed to get Rain-Boe rain repellent removed from aircraft after it was thought to make some pilots unwell. Others complain of illness after smelling de-icing fluid. What of pesticides sprayed on the runways, or when disinsecting aircraft? The same chemical used in some countries has been banned from treating head lice in children for health reasons, though the d-phenothrin used by UK airlines is said to have shown no long term effects on high exposure to animals.
In this day and age it is impossible to avoid everything which might prejudice health, but when a risk is flagged up then it must be assessed as carefully as is possible and recommendations made to reduce it.
BALPA has given evidence to the House of Lords investigation into cabin air quality and added Organo-phosphate exposure to the ever lengthening agenda of their Medical Study Group who will report any fresh information. Meanwhile, anyone suddenly exposed to excessive aviation oil fumes should report to a hospital and express their concerns about Organo-phosphates. Those who believe they are suffering from ill health due to long-term exposure are unlikely at present to gain redress in the courts due to lack of evidence.
Stay away from sheep!
Dr S A Goodwin
Airport Medical Services Limited