One day, shortly before boarding a flight from Paris to Montreal, I began to think about the risks of flying for the first time. It was not the fear of engine failure or crashing into a mountain that worried me. Rather I realised I was about to make my 39th plane journey of the year, and as a result was exposing myself to higher than normal levels of radiation from space.

Like most holidaymakers, I had checked the weather forecast. But now, as I waited to board the plane, I wondered whether I and other frequent flyers should be more concerned with checking the space weather before we take off.

The Earth is constantly being bombarded by high speed, sub-atomic particles. These interact with the atmosphere and our planet’s magnetic field to generate cosmic radiation which rains down on us. Our exposure levels rise when we travel by plane, especially at higher altitudes and latitudes.

What do scientists know about the dangers that cosmic radiation might pose during regular flights, and is there anything that aviation authorities or passengers can do to minimise risk?

Cosmic radiation consists mainly of protons and helium nuclei that originate outside our galaxy. Scientists have long speculated over their origins, with likely candidates being powerful events such as star collisions, gamma ray bursts, black holes and supernovae – explosions that mark the death of large stars. Earlier this year US astronomers concluded supernovae were indeed a significant source of cosmic radiation hitting Earth. Particles thrown out by our Sun are another source.

These sub-atomic particles can be both low- and high-energy. Many are deflected by the Earth’s magnetosphere, without which cosmic rays would wipe out complex life forms pretty quickly by damaging tissue, DNA and causing lethal radiation sickness. Only very high-energy cosmic rays can reach our atmosphere at latitudes close to the equator, however lower-energy ones can reach polar latitudes.

Those that do penetrate the Earth’s magnetic shield collide with nitrogen, oxygen and other atoms in the air, generating highly energetic and invisible showers of ionised “secondary particles”, which cascade down on us in vast numbers, penetrating everything and everyone. The atmosphere provides good protection for those on the ground because particles hitting them will have undergone more collisions with atoms, but exposure is greater at high altitudes because the air is thinner.

Body shock

What can this do to the body? Cosmic radiation is ionising, which means the particles involved are energetic enough to knock charged particles from atoms – potentially causing chemical changes in body tissue that can increase risks from cancers and genetic abnormalities.

While this might sound scary, it should be made clear that we are regularly exposed to low doses of ionising radiation in other forms with no apparent health consequences in the overwhelming majority of cases – from radon in the air, naturally occurring radioactive substances in the ground such as uranium, building materials and during medical procedures.

The risks of individuals suffering health effects as a result of being exposed to ionising radiation of any kind – whether from cosmic rays, a nuclear power plant, an X-ray machine, or airport full body scanner – are measured in sieverts or rems (1 sievert equals 100rem). “The same potential risks exist,” says Major Alan Hale at the US Air Force School of Aerospace Medicine, based at Wright-Patterson Air Force Base, Ohio. “Health risk assessments are based on frequency, duration, and intensity level.”

The average person on Earth is exposed to around 350 millirems (mrem) per year. The average annual dose for US citizens is 620mrem , according to the US National Council on Radiation Protection and Measurements. About half of this comes from man-made sources such as X-ray, mammography and CT scans, while the other half comes from natural sources, of which only about 9% comes from cosmic radiation.

Cosmic radiation exposure levels during flights vary according to altitude, latitude and the space weather at the time. Typically, passengers flying from London to Chicago could expect to be exposed to around 4.8mrem, and those travelling from Washington DC to Los Angeles would be exposed to close to 2mrem. This compares to an airport body scanner which delivers around 0.1mrem and a chest X-ray that can vary between 2mrem and 10mrem.

As people travel more often and further away, frequent travellers should be aware of their exposure levels, says Mike Lockwood, professor of space environment physics at Reading University in the UK. “No need to panic, but cosmic radiation should not be ignored,” he says.

Your flight route is particularly important to consider, because exposure rises at higher latitudes. Because cosmic radiation particles are charged, they are deflected towards the North and South Poles by the Earth’s magnetic field lines. At these latitudes the magnetic field lines are closer to vertical, making it easier for cosmic ray particles to enter the atmosphere.

Airlines, however, prefer polar routes because they are shorter with lower head winds, meaning shorter journey times and lower fuel costs. A number of flights from the US to northern Europe and Asia pass directly over the North Pole – for example from San Francisco to Paris. The same goes for flights from, say, Santiago in Chile to Sydney in Australia, which cross the South Pole. “Airlines rotate staff around flight routes so nobody does exclusively polar routes,” says Lockwood.

Risky business

In the US, pilots and flight attendants have been officially classed as “radiation workers” by the Federal Aviation Administration since 1994. Staff regularly working on high-latitude flights are exposed to more radiation than workers in nuclear power plants. Despite this, the airlines don’t measure the radiation exposure of their staff, or set safe limits on the doses they can safely receive.

Among flight crews, there has been a lot of research into links between cosmic radiation and health risks, especially cancer. However, attempting to work out whether small additional doses of ionising radiation are linked to actual disease is far from straightforward.

In 2002, Scandinavian researchers analysed data from 10,000 male airline pilots over 17 years, and found they were at greater risk of developing melanoma and prostate cancer. However the charity Cancer Research UK says this may be related to other lifestyle factors such as the pilots spending more time sunbathing than the average person.

Two different groups of scientists from Japan and Italy combined their efforts in 2006 to look at health risks to female flight crew members from cosmic radiation. They found that women working on planes were more likely than average to develop breast cancers and melanomas, but again the authors admitted they could not be sure this was to do with cosmic rays. A meta-analysis published last year in the Journal of Radiological Protection concluded that overall cancer risk was not elevated, but that “malignant melanoma, other skin cancers and breast cancer in female aircrew have shown elevated incidence.”

Most plane passengers, however, needn’t worry too much, unless they fly regularly over the poles, says Lockwood. Even though their exposures might take them over the recommended annual dose, these limits have been set well below the level likely to cause actual health problems, he says. More dangerous would be spending a lot of time in Cornwall, in the UK, where naturally-occurring radon gas seeping from the ground means inhabitants are exposed to 780mrem per year, nearly three times the national average.

And while some fear that unborn babies could be at risk from cosmic radiation during flights, this is unlikely to be the case unless the women are flying several times a week, according to the US Aerospace Medical Association.

Sun trap

There are however times when cosmic radiation becomes more of a concern because of emissions from our Sun. Usually the solar energetic particles (SEPs) that reach us are of low energy, but the Sun is temperamental.

Levels of radiation and brightness from our star vary, with peaks in both occurring approximately every 11 years. The more sunspots appear on the surface, the more active the sun becomes and the more protons it sends our way. But the sun also has much longer phases, and currently, it is at a grand solar maximum – a phase that began in the 1920s. During this phase, the peaks of the solar cycle are larger and huge magnetic storms on the surface of the Sun, called coronal mass ejections (CMEs) or solar flares, are more frequent. These events fill space with streams of high-energy protons and electrons, some of which quickly reach Earth.

Measurements show solar activity has begun to calm recently and past experience indicates that it will continue to fall over coming decades. Once the Sun leaves its grand maximum, there will be fewer solar storms, but theory suggests those that do occur could be more powerful, ejecting more dangerous high energy particle in our direction. In short, passengers should expect higher exposures in the coming decades.

“Solar energetic particles events are difficult to assess, but being aloft at high latitudes during a big solar storm would be a large dose,” says Lockwood. “There are no studies that give the actual risk factor, but you certainly wouldn't want it to happen twice to one individual.”

People who have been unlucky enough to get caught in such an event should be informed, he adds. “It would not be wise for them to risk a second such exposure, and more regular health checks would be a good idea, as we already do for recognised radiation workers”. According to NASA, a strong solar storm in late October 2003 subjected passengers on polar flights – from Chicago to Beijing, for instance – to radiation well above the limit recommended by the International Commission on Radiological Protection.

Cosmic forecasting

Very few passengers check the space weather when they fly, but airlines do. In some instances they have varied flight paths to lower latitudes because of predicted solar activity, particularly SEP events during solar flares. “The most pressing reason for this is that the SEP cause radio blackouts,” says Lockwood.

So what can passengers concerned about their exposure to cosmic radiation do, short of stopping flying? Could they perhaps choose to sit in parts of planes that are subject to a lower dose of particles, or fly only at night, in order to put the Earth between them and the Sun? Unfortunately all seats on aircraft are equally affected and exposures are just as high at night.

Would it be possible to shield planes? After all, crew quarters onboard the International Space Station, which is located at the outskirts of the Earth’s magnetosphere, are lined with high-density polyethylene several centimetres thick. The hydrogen atoms in it are great at absorbing and dispersing radiation.

The airline industry is increasingly using carbon fibre-based composites to build planes because of their strength and low weight. These are much better protection against cosmic radiation than standard aluminium, and metals in general, says physicist Nasser Barghouty at Nasa’s Marshall Space Flight Center in Huntsville, Alabama.

In the meantime, the US Federal Aviation Administration’s Civil Aerospace Medical Institute has an online tool that allows individuals who are concerned to calculate their cosmic radiation exposure levels on specific routes.

Cosmic radiation comes in a wide variety of forms at varying energy levels, and calculating the health effects of low doses of radiation on specific individuals is complex, and inevitably involves simplification and estimation. The research that has been carried out on those who fly most frequently – airline crew – is far from conclusive.

The available evidence suggests that those who fly occasionally have little to worry about. Likewise most frequent flyers are also probably fine but could protect themselves with more frequent medical check-ups if they are worried.

So next time you fly, consider the galactic radiation from supernovae all around you – but try not to let it spoil your trip.



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