Radiation
Radiation and Air Travel
There is considerable interest in the effects of all types of radiation on health, including medical radiation such as from diagnostic imaging such as X-rays and thallium scans, waste from nuclear power plants, weapons testing fallout, industrial radiography for pipelines and tanks. Topics such as speculated harmful effects of mobile telephones and electrical power lines have also attracted much attention. A recent addition to the debate has been cosmic radiation, particularly with the vast increase in air travel in recent years. The following question and answer briefing is intended to be factual rather than speculative and may be useful to anyone with an interest in knowing more about radiation in general and cosmic radiation in particular.What is radiation?
Radiation can be defined as the emission and propagation of energy through space or through a material medium in the form of particles or waves. There are several different types of radiation. A useful way of classifying the different types of radiation is into ionising and non-ionising and electromagnetic and particulate.What is the difference between ionising and non-ionising radiation?
The basic difference is in the energy possessed. Non-ionising radiation (such as radio waves, radar and visible light) does not have sufficient energy to remove electrons from atoms while passing through matter; ionising radiation does.Are there different types of ionising radiation?
Yes. Ionising radiation can be electromagnetic or particulate. Electromagnetic radiation travels in the form of waves at the speed of light. Shorter wavelengths such as g-rays are more damaging to organisms. Particulate radiation consists of different types of particles such as a and b-particles, protons, and neutrons.How is ionising radiation measured?
Various instruments are used to measure ionising radiation. In general, instruments for measurement of particulate radiation are more complex, bulky and heavy than ones used for electromagnetic radiation (e.g. hand held counters or the small film badges worn by radiographers in hospitals).There are also several units used to quantify radiation, but the most useful for health effects is the dose equivalent, which takes into account the differing abilities of the types of radiation to interact with biological systems. Particulate radiation is generally more damaging in biological systems than electromagnetic. The unit is the sievert (Sv), but as it is a very large unit the millisievert (mSv) which is a thousandth of a sievert is more common as is the microsievert (mSv), a millionth of a sievert.
What does ionising radiation do to life forms?
The first biological effect of ionising radiation is the absorption of energy, leading to the removal of electrons from atoms. This occurs almost instantaneously and is followed by chemical damage to cells, which can include damage to DNA. Components of cells such as chromosomes, membranes and lysosomes may be damaged over a period of seconds to hours.Biological damage to cells, tissues and whole organisms takes place over hours to several years. This can include the immediate death of cells, reduction of capacity for cell division, mutation or cancerous change.
What is the added risk of cancer from ionising radiation?
When considering radiation induced cancer, it must be remembered that naturally occurring cancer is unfortunately very common anyway. Regrettably, in developed countries, about 230,000 persons in every million (more than 1 in 5) die from naturally occurring cancer.The International Commission for Radiological Protection (ICRP) estimates that an additional death rate of 12.5 persons per million for every mSv of exposure. This means that if a million people were given 2 mSv of radiation, the expected additional death rate from cancer would be 25 additional persons or 230,025 instead of 230,000 persons out of a million dying from naturally occurring cancer.
What levels of ionising radiation are safe?
This subject is far from simple because there are two types of harmful effects from radiation that must be protected against. High doses (such as seen in those involved in the fire fighting at Chernobyl) will cause inevitable harm, and these effects, such as bone marrow and intestinal damage, are known as deterministic and do not appear if the dose does not exceed a threshold value.Effects such as cancer and mutation occur randomly with both high and low doses of radiation and are known as stochastic effects. The probability of occurrence is proportional to the dose but there is no threshold below which it can be stated that the probability is zero.
The fact that there is no level of radiation which can be proven to cause no harm means that the International Commission for Radiological Protection (ICRP) recommends that no practice involving radiation be undertaken unless it is likely to bring a net benefit and that for any such practice, exposure should be kept As Low As Reasonably Achievable (ALARA principle), economic and social considerations being taken into account.
Can we completely avoid being exposed to ionising radiation?
Unfortunately no. Most people at some time or other have X-rays for dental or medical reasons and small amounts of radioactivity are still present from fallout from nuclear weapons tests. Even without these man made sources of radiation, exposed to natural background radiation. The rocks and soil of the earth's crust contain variable amounts of natural radioactive elements such as uranium and thorium. This varies with geography - for example the 'hottest' places in Britain are Cornwall and around Aberdeen. Radioactive matter is incorporated into plants, which are then eaten by animals with the result that most foodstuffs have a measurable amount of radioactivity. Additional radioactivity in food comes from the naturally occurring isotope carbon 14. Some cosmic radiation also reaches sea level.It is therefore completely impossible to avoid ionising radiation altogether. The average background radiation as about 2 mSv per year.
Are there any limits to the amount of radiation a person can be exposed to?
The International Commission for Radiological Protection (ICRP) sets certain recommended limits for whole body radiation exposure.For the general public, i.e. those persons who do not have contact with radiation as part of their work, the recommended limit is 1 mSv per year. For persons whose work requires that they have contact with radiation, the limit is 20 mSv per year. Workers whose annual dose is likely to exceed 6 mSv are required to be classified as radiation workers. This requires that their exposure be measured and regularly monitored and that they be subject to periodic medical examinations.
A special case exists for pregnant workers because of the potential for radiation to damage the developing foetus. After the pregnancy is declared, the dose limit is 1 mSv for the rest of the pregnancy.
It must be emphasised that the above limits are in addition to the expected levels of background radiation.
What is cosmic radiation?
Cosmic radiation is a naturally occurring ionising radiation that arises from outside the earth. It has two components, solar radiation (from the sun) and galactic radiation from outside the solar system. It primarily consists of particles such as protons, neutrons and a-particles. Secondary radiation is produced when these particles interact with the atoms in the earth's atmosphere. Galactic radiation has higher energy and is therefore the dominant component of cosmic radiation.What is the solar cycle and how does it affect cosmic radiation?
The activity of the sun varies on an 11 year cycle. When solar radiation is highest, the magnetic field associated with it deflects some of the galactic radiation away from the earth. As the galactic radiation is the main component of cosmic radiation, this means that the when solar activity is high the total intensity of cosmic radiation is lower and vice-versa.An exception to the above pattern occurs during solar flare activity, which takes place on an occasional basis during periods of high solar activity. Solar flares are caused by magnetic storms on the surface of the sun and consist of eruptions of highly charged particles that travel at high speed through space. If they happen to hit the earth, the cosmic radiation dose may be significantly increased for a short period (from hours to a few days).
Is cosmic radiation the same everywhere?
No. The high-energy particles collide with atoms of the earth's atmosphere as they travel towards the surface, losing about half their energy with each collision. This means that there is more cosmic radiation increases with altitude.Because most of the particles are charged, the magnetic field of the earth tends to sweep them towards the poles, so radiation is more intense at high latitudes, though there appears to be little variation at greater than 60 degrees north or south.
Are there any regulations concerning exposure to cosmic radiation?
Yes. The Council of the European Union has adopted a directive known as the EURATOM directive, one part of which requires the assessment and limitation of exposure of aircrews to cosmic radiation and for information to be given to aircrews.The Hong Kong Civil Aviation Department had set up a working group on cosmic radiation in the Hong Kong aviation industry where representatives from government, airline and union representatives participated. In September 2002, the group adopted the Euratom Directives. Dragonair was able to implement the key elements prior to that date, around April 2002.
What are the requirements of EURATOM for assessment of cosmic radiation exposure?
EURATOM requires that that aircraft capable of operating at altitudes of more than 49,000 feet should be equipped with active radiation monitors. Dragonair does not currently have any aircraft of this type.In the case of aircraft operating at 26,000 to 49,000 (all of the Dragonair fleet), EURATOM requires that the dosage to crew members should be assessed by computer program prediction. For annual doses calculated as less than 6 mSv, no further action is required, but it is recommended that where annual doses could exceed 4 mSv, the monitoring of individual exposures by computer program is conducted to ensure that no individual dose exceeds 6 mSv. Dragonair has also adopted an additional internal action level to ensure that crew will not exceed the recommended exposure dose.
A special case is made for pregnant aircrew. Once pregnancy is declared, the maximum dose equivalent permitted is 1mSv for the remainder of the pregnancy.
It must be emphasized that the above limits are in addition to the expected levels of background radiation.
Why don't aircrew wear dosimeters?
The types of dosimeters worn by most radiation workers are not suitable for particulate radiation. The best kind of detector for cosmic radiation is a Tissue Equivalent Proportional Counter (TEPC), but these are far too large and heavy to wear on the person.Do air passengers have an unacceptably increased risk of developing cancer?
As all air travel is associated with increased exposure to ionising radiation, and the any exposure to ionising radiation carries an increased risk of developing cancer, it therefore follows that there must be some increased risk. The increased risk is however, extremely small, given that naturally occurring cancer accounts for 23% of deaths anyway in many developed countries.For example, a person who flies direct return flights from Hong Kong to New York every two weeks for 20 years, has their risk of death from cancer increased from 23% to between 23.11% and 23.14%. This represents an increase in percentage over the normal of around 0.5%, which most people would probably not consider unacceptable.
