An international pilots' group said Friday that airlines should monitor the level of potentially toxic fumes from the engines in their cabins.

The International Federation of Airline Pilots' Associations said regulators should immediately consider the issue of cabin air quality, after several incidents were reported of crew members falling sick from fumes in recent years.

"We'd like to see some proper independent scientific research into what is clearly ... a serious issue," group spokesman Gideon Ewers said.

The London-based group — which represents more than 100,000 pilots worldwide — has instructed cockpit crews to immediately don oxygen masks whenever fumes are detected.

The problem of fumes from engine oil or hydraulic liquid seeping into cabin air has been recognized by the industry for at least 25 years since the introduction of the latest generation of civil airliners that now dominate the skies.

Although no statistical data is available, European and American regulators say they believe that hundreds of incidents involving low-level air contamination occur annually worldwide. Most of these incidents pass with no lasting effects, doctors say.

But in December an eight-person American Airlines crew was treated for dizziness and nausea after landing in Fort Lauderdale, Florida. In August, two Flybe cabin crew members became ill on a flight between Birmingham and Belfast from suspected toxic gas poisoning.

The pilots group said it was concerned that passengers and crew were at risk of developing chronic sickness.

Experts said there are no data to prove that toxic chemicals are present in levels high enough to be dangerous to people, though they said more study was warranted.

"We don't have sufficient evidence to conclude that pilots or passengers are being harmed," said Alan Boobis, a professor of biochemical pharmacology at London's Imperial College.

A study of cabin air samplings prepared for Britain's Department of Transportation in January found hazardous substances characteristic of hydraulic fluids and heated engine oils, according to the pilots group. It said the effects of inhaling such substances were comparable to those experienced by breathing in volatile chemicals such as fresh paint.

Scientists said exposure to some of those fumes — either in high levels or for long durations — might affect the normal functioning of the nervous system, but that such a link would be hard to prove.

"The priority should be to do more research into this and to do some on-board monitoring," said Sarah Mackenzie Ross, a neuropsychologist at University College London.

Ross said she examined more than two dozen pilots who thought they had suffered cognitive impairment after being exposed to contaminated air. She said it was possible fumes could be hazardous to pilots and passengers, but that few preventative measures could be taken without more evidence.

"Low concentration of such contaminants generally do not have a lasting effect, although a prolonged exposure could help develop diseases such as asthma in some people," said Dr. Roland Reynaert, a Brussels-based toxicologist. He also suggested that oil fumes, which could reduce blood oxygen levels, could affect reaction times or cause pilots to become confused.

When the world's first jet airliners were introduced in the late 1950s, they were fitted with separate intakes for cabin air. But with the high-bypass jet engines in the 1970s, designers focused on a more efficient system that bleeds air into the cabin system from the engine itself.

Experts say this has allowed fumes from the engine's turbine bearings ahead of the cabin air intake to get into the air conditioning system.

Aviation experts say some aircraft, such as the Boeing 757 and the BAe146 — both widely used by commercial carriers — are considered particularly susceptible to such contamination.

In recent years, airframe designers have reverted to the original separate intakes on new planes.