How much radiation is too much?
Oct 17 2013
In space, astronauts are exposed to protons, helium nuclei, and high-Z high energy ions (HZE ions), as well as secondary radiation from nuclear reactions from spacecraft parts or tissue.
Space radiation is composed mostly of high-energy protons, helium nuclei, and HZE ions. The ionisation patterns in molecules, cells, tissues and the resulting biological insults are distinct from typical terrestrial radiation (x-rays and gamma rays, which are low-LET radiation). GCRs (galactic cosmic rays) from outside of the Milky Way galaxy consist mostly of highly energetic protons with a small component of HZE ions.
The space radiation appears to be one of main roadblocks to interplanetary travel. There is the risk of cancer caused by radiation exposure. The largest contributors to this roadblock are: (1) The large uncertainties associated with cancer risk estimates, (2) The unavailability of simple and effective countermeasures and (3) The inability to determine the effectiveness of countermeasures. Operational parameters that need to be optimised to help mitigate these risks include: length of space missions, crew age, crew gender, shielding, and biological countermeasures.
Major uncertainties regarding the space radiation include effects on biological damage related to differences between space radiation and x-rays, dependence of risk on dose-rates in space related to the biology of DNA repair, cell regulation and tissue responses, predicting solar particle events (SPEs), and individual radiation sensitivity factors (genetic, epigenetic, dietary or “healthy worker” effects).
While the risk of radiation exposure is high for astronauts, people travelling in aircrafts may also be exposed to more ionising radiation than they would be exposed to on the ground. According to the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA), the increased exposure occurs because the earth’s atmosphere provides less protection from cosmic radiation at the typical cruising altitudes of commercial aircraft, which are usually between 7,000 and 12,000 metres. According to ARPANSA, aircrew and frequent flyers get the most additional exposure because of the extra time they spend at cruising altitudes. However, large studies of pilots and aircrew have generally shown no significant association with an increased health risk.
This raises the question: How much radiation is too much? The National Council of Radiation Protection recommends a maximum 1 mSv (millisieverts) the unit of measure for radiation) dose of radiation for the general public. They also recommend a limit on “controllable sources” of exposure (of which flying could be included) to 0.25 mSv. A person would have to spend 22 hours in the air annually to get this level of exposure.
It is commonly agreed upon that frequent travellers are not at greater health risk because of their exposure to cosmic radiation. There is, however, more concern for pregnant women as high levels of radiation exposure can affect foetal development. Still, experts agree that babies who receive small amounts of radiation (less than the equivalent of 500 chest X-rays) are not at greater risk for birth defects.
Before you vow never to step on a plane again out of fear of putting yourself at risk for cancer, compare the amount of radiation you are exposed to in your normal daily life: CT Scan: 10 mSv, breathing radon gas found in the air: 2.28 mSv, consuming food: 0.4 mSv, smoking half a pack of cigarettes a day: 0.18 mSv, chest X-ray: 0.02 mSv, and flying in an airplane for one hour: 0.005 mSv.
So, what’s the bottom line? Statistically, the risk posed by exposure to radiation from passing through scanners in airports and during commercial flight is exceedingly low. Relatively speaking, the radiation people receive from medical imaging and procedures poses a much greater risk. For that reason, when physicians are counselling patients who are concerned about the amount of radiation they will receive going through airport security or during flight they might do better to focus the discussion on issues that are more likely to ensure safety such as appropriate use of medical radiation and developing habits including use of seat belts.
The American College of Radiology has developed educational materials for patients, providers, and radiologists through their Image Wisely and Image Gently campaigns in an effort to reduce the amount of medical radiation patients receive for routine procedures. In addition, manufacturers of imaging equipment are developing new technologies that will significantly reduce the amount of radiation delivered during imaging — in some cases by more than tenfold. Although the amount of radiation delivered during a CT scan may never be as low as that delivered by an airport security scan, we are likely to see substantial reductions in medical imaging radiation during the next three to five years.
(The writer is on the faculty of Indian Institute of Technology, Mandi, India)