Radiation - Onslaught from Outer Space

Acute radiation poisoning is never a possibility unless the airplane flies through a nuclear explosion or there is highly radioactive material in the aircraft. For Indian aircrew, it would be simple and best to adhere to ICRP, FAA and European exposure limits.

Flying at altitudes above 35,000 feet in a passenger jet has the potential to expose passengers and crew to harmful ionising radiations (IRs). By definition, IRs are high-energy waves that are capable of producing ionisation in substances through which they pass. Structural changes to the substances passed through, including our deoxyribonucleic acid (DNA), are a potentially harmful consequence. IRs can cause extensive damage to the atomic structure of a substance, either as a result of the direct transfer of energy to its atoms or molecules or as a result of the secondary electrons and other atomic particles, released by ionisation. In biological tissues, the effect of ionising radiation can be very serious. This is the consequence of the forcible ejection of an electron from a water molecule and the oxidising or reducing effects of the resulting highly reactive end product. Damage to our DNA is always the currently unqualified and unquantifiable danger.

Sources of IRs in Aviation

These energy-filled ionising radiations that travel across the cosmos are also called galactic radiations. Ionising radiations from the sun also strike the earth. These solar energies fluctuate over 11-year cycles and when at their peak affect radio communications apart from inflicting spacemen and flight crews with higher than routine doses. The atmosphere attenuates much of this radiation energy and at sea level where detectable, it is virtually harmless. Somewhat disconcertingly, ionising radiations cause increasing incidence of cancer amongst flight and cabin crews. Passengers and unborn babies are also seen to be at some risk, although the outcome of exposure to this group is more difficult to validate than with flight crew.

At a cellular level and from a clinical perspective, DNA could get damaged with one of the following outcomes:

• Cells experience DNA damage and are able to detect and repair the damage.
• Cells experience DNA damage and are unable to repair the damage. These cells may go through the process of programmed cell death thus eliminating the potential genetic damage to the larger tissue.
• Cells experience a non-lethal DNA mutation that is passed on to subsequent cell divisions. This mutation may contribute to the formation of cancer.

IRs, Aviation and Occupational Safety

In the case of the Concorde supersonic passenger aircraft, it was appreciated that cosmic radiation (both galactic and solar) could present a hazard at its operating altitudes which was in the region of 59,000 feet. As such, ionising radiation monitoring equipment was permanently installed in the Concorde airliners and data was collected. The introduction of aircraft such as the Boeing 747-400 and the Airbus A330 and A340, accorded the airlines the capability to undertake ultra-long haul flights of 18 hours or more. Many of the routes flown are trans-Polar or trans-Siberian where geomagnetic and atmospheric shielding from galactic cosmic radiations is less than for routes at lower latitudes or around the equator, thus increasing the risk of exposure. But available evidence precludes IRs monitoring unlike with the Concorde. Also, such flights are not monitored for IRs exposure due to lack of a costeffective model.

How does one quantify exposure and who monitors ionising radiation activities globally? In 1991, the International Commission on Radiation Protection (ICRP), the principal agency that lays down universally accepted exposure norms, declared aircrew exposures to IRs as an occupational hazard. And this norm is valid even today. The US Federal Aviation Administration (FAA) and the European Union (EU) Joint Aviation Authorities (JAA) follow ICRP edicts on exposure limits. The FAA recommends the limit for an aircrew member of a five year average effective dose of 20 milli-Sievert (mSv) per year, with no more than 50 mSv in a single year. For a pregnant aircrew member, the recommended limit for the foetus is a dose of 1 mSv with no more than 0.5mSv any month. ‘Dose equivalence’ essentially takes account of the dose absorbed by the tissue and the quality factor of the radiation. Earlier on, the ‘rad’ and ‘rem’ (roentgen equivalent man), were popular units of measurements of IRs. Essentially, what is measured is the amount of radiation absorbed, (the ‘rad’) and the ‘rem’, which is an indicator of how some forms of radiation (alpha particles, neutrons, protons) are more efficient at transferring their energies than others.