The experts
tell you
more
Pierre Lantos (astrophysicist
at the Paris-Meudon Observatory)
The experts
tell you
more
|
|
 |
 |
Pierre Lantos (asrophysicist at the Paris Observatory)
- The "solar cycle" and its influence on the Earth
"The solar cycle", of an average duration of eleven years but which can range from 9 to 14 years, is a phenomenon which is related to the frequency of the appearance of sunspots. These spots, due to the presence of strong magnetic fields, are regions which are slightly cooler than the rest of the sun's surface. The spots change in just a few days. They can produce solar flares, the most powerful of which, lasting a few hours, disturb the Earth's spatial environment.
These solar flares can have many effects: disturbances to telecommunications or positioning systems such as GPS, or to the orientation and operation of satellites. Protons coming from the sun, accelerated during a flare, result in doses which are added to those produced by the cosmic rays coming from the Galaxy. Special surveillance is necessary for manned space flights. This is the case on the International Space Station, and will be increasingly so when manned flights to the Moon or Mars are planned. Planes are better protected thanks to the Earth's atmosphere, but the strongest flares can be detected at their altitudes. Predicting solar flares a few days in advance is a common practice, but we cannot currently determine to what degree their protons will be sufficiently powerful as to produce particular effects at the altitudes used by aircraft.
On average, one flare per year can be detected in aircraft, using dosimeters, or on the ground, with the use of "neutron monitors",; which constantly monitor particle flows. In the last fifty years, sixteen flares have produced appreciable doses on board aircraft. Of these, only two truly stand out: the first, in September 1989, represented the equivalent of around one month of cosmic radiation, while the second, in February 1956, could have equalled - using the most pessimistic calculations - one year of this same radiation.
When the solar cycle is at its peak, as in the year 2000, sunspots are more frequent and, consequently, so are solar flares. Inversely, however, the solar activity makes it harder for cosmic radiation coming from the galaxy to spread throughout the solar system. On board an aircraft, the resulting dose of cosmic radiation is thus reduced by 30 to 50%. ”
- Means of observation
"Solar activity is constantly observed by ground stations (in the visible domain and through radio astronomy) and by satellites (like the European SOHO satellite). Cosmic radiation and major solar flares, in terms of the doses received on board aircraft, are monitored by a worldwide network of "neutron monitors". Some fifty such monitors exist around the world.
These instruments detect the secondary particles reaching the ground. The French Institute for Polar Research and Technology in Brest (IPEV) has two of them: one in the Kerguelen islands (Indian Ocean) and the other on Terre Adélie (Antarctica); each day, using a computerised connection by satellite, they provide the necessary data to the S.I.E.V.E.R.T. system. A particle value flow is recorded each minute, thus allowing detailed monitoring of changes in the eruptions. "
For more information...
Example of the solar flare observation on 15 April 2001
ITW Margot Tirmarche (epidemiologist at the IRSN)
- Why use epidemiology to assess
the effects of radiation on health ?
Setting aside very high exposure levels, which are sure to produce known syndromes in the following hours or days,
ionising radiation produces unpredictable long-term effects, primarily cancer.
It is impossible to come right out and say that exposure will produce long-term effects on a person;
at the most, one can speak of the probability of the appearance of pathologies. Long-term epidemiological studies are the only way to assess this probability, by determining the frequency of the occurrence of pathologies in the exposed groups. These are also the only studies which consider humans in actual situations, rather than cell cultures or laboratory animals.
Nevertheless, these studies have limits. First of all, radiation does not cause a specific type of cancer:
at present, it is not possible to distinguish between cancer caused by radiation or by something else.
We are therefore limited to searching for a possible increase in the frequency of cancer in the exposed group, in comparison with a non-exposed reference group. For weak doses, this increase is difficult to identify, due to the many other factors that can contribute to the appearance of the pathology.
Incidence of cancer after the
Hiroshima and Nagasaki bombardments
These figures present the values in death by cancer after the Hiroshima and Nagasaki bombardments. The figures identified as "observed" correspond with the actual number of cancer cases observed in the exposed group. The "expected" figures correspond with the number of cancer cases in a group comparable with the residents of the Japanese cities, but not exposed. We can also note that the 200,000 immediate victims primarily died as a result of the mechanical and thermic effects of the explosions.
- What are the main lessons ?
Most of the information on radiation-induced cancer is obtained from the studies on the survivors of the Hiroshima and Nagasaki bombardments, and from studies on groups exposed to radiation in a work setting, such as uranium miners.
In Japan, the determination of the individual doses received during the atomic explosions led to values which vary from 0
to more than 3 Gray (average 0.2 Gy) on a single occasion. Compared with a similar but non-exposed group, an excess of
87 leukemia cases was observed (i.e. a 40% greater frequency), as well as 334 solid cancers (4%) amongst the 86,500
monitored people, affecting the lungs, stomach, colon, oesophagus, bladder and breast cancer, amongst women.
There is certainly a linear relation between the received dose (estimated) and the likelihood of the appearance of cancer,
as confirmed by the study on uranium miners (lung cancer due to radon).
- Have such studies been undertaken on the flight personnel
on commercial aircraft ?
Yes, but the number of people was smaller, and the studies had obvious methodological limitations,
if for no other reason than the difficulty in determining the dose received by a person during his/her career.
One must remember that flight personnel receive, on average, a few millisieverts per year (in addition to the natural
radioactivity). With this dose, the additional risk of the appearance
of cancer, if there is one, is very low, and thus very difficult to detect.
Several European studies are currently in progress in an effort to identify a possible risk of cancer which could relate
to the professional exposure suffered by flight personnel.
Laurence Lebaron (medical doctor at the IRSN)
- How does radiation affect the system ?
At very high doses, the damage to the DNA is such that the cells die. The severity of the effects therefore depends directly on the dose, and can range from temporary problems to serious or even fatal syndromes.
On the other hand, for the smaller doses which we are looking at here, the cells can repair the DNA and restore the entire genetic message. But errors are possible, and the cells can mutate and become cancerous (though not all mutations lead to cancer, of course). These are unpredictable effects: the more the cells are affected, the greater the probability that one of them will result in cancer several years later on. Thus, the probability of the occurence of these effects increases with the dose. However, if cancer occurs, its severity will be independent of the dose.
There is nothing particular about the strictly medical effects: radiation-induced cancer is not different from any other cancer. Quite simply, as DNA is more vulnerable during its replication, tissues or organs where the cells are actively dividing are the most sensitive. We therefore may find leukemia (due to damage to the bone marrow), and cancers of the lungs, colon or stomach. Also very sensitive are the breasts, for women, and the thyroid amongst children.
If an affected gamete mutates and then takes part in fertilisation, the genetic anomaly will be transmitted to the embryo. Genetic effects have thus been observed in the progeny of animals, but never in the offspring of man, even amongst the survivors of atomic bombardment (Hiroshima and Nagasaki) or atomic accidents (Chernobyl). If it exists, the risk is very low.