Galactic Cosmic Rays (GCR’s) are considered a threat to aviators, flight crews and frequent air travelers. This risk to astronauts is even greater (possibly even lethal) and continues to pose a significant obstacle to long expeditions into outer space. Cosmic radiation is comprised of high-energy subatomic particles from all of the natural elements in the periodic table. They exist in similar proportion to that of the elements found throughout the remaining solar system. According to NASA, “about 90% of the cosmic ray nuclei are hydrogen (protons), about 9% are helium (alpha particles), and all of the rest of the elements make up only 1%”. GCR’s are thought to originate from supernovae, though this remains uncertain.
The Environmental Protection Agency (EPA) has an interactive website called RadTown USA that educates the user on one’s daily sources of radiation exposure . They estimate that 18% of the average annual dose of radiation one is exposed to is man-made (medical procedures and consumer products), while 82% is from natural sources such as ground minerals, radon, and cosmic radiation. Cosmic radiation makes up 8% of this total annual average rate for most people. The average annual sea level dose has been estimated to be about 3.0 mSv/year.1 Aircrew and definitely astronauts are exposed to considerably higher levels.
Radiation exposure also comes from man-made sources. To put GCR values into context, it is helpful to compare their dose values to the quantity of radiation incurred thru medical imaging studies.
- Chest X-Ray: 0.1 mSv (100 ųSv)
- Body CT Scan: 10.0 mSv (10,000 ųSv)
- Chest CT Scan: 8.0 mSv (8,000 ųSv)
- Mammogram: 0.7 mSv (700 ųSv)2
A majority of cosmic radiation never reaches those of us living on the terrestrial earth due to the sun’s magnetic field, the earth’s magnetic field, and the earth’s atmosphere. There are a number of variables that affect one’s exposure to cosmic radiation:
- Latitude: Due to the magnetic and gravitational fields of our rotating planet, the earth’s atmosphere and magnetic field offers more protection over the equator than at the poles. Although the degree of this effect changes with latitude, at altitudes flown by most commercial airliners, GCR at the poles is increased 2.5 to 5 times the exposure values over the equator.3
- Altitude: The density of the atmosphere diminishes with increasing elevation. This allows less protection against cosmic radiation. Increasing latitude while holding altitude constant has a greater effect than increasing altitude with constant latitude.3 Therefore all other things equal, it is better to fly slightly higher than at latitude extremes.
- Sun Cycle: The sun’s magnetic field reverses its direction every 11 years (last flip was Jan 2014). During the time period around a reversal, the field is at a solar minimum. At the opposite ends of the spectrum are solar maximums. During maximums, the sun’s activity is stronger and more frequent, increasing the GCR dose by 1.2 to 2 when compared to periods of solar minimum.3 Additionally, solar flares occur from time to time, which may increase cosmic radiation dose for a shorter period of time. Military, aviation and space agencies like the FAA and NASA pay close attention to space weather and may publish advisories. To keep up to date on your space weather, the National Oceanic and Atmospheric Administration (NOAA) provides an online service.
- Duration: Most occupational exposures are a product of dose and time. Obviously, the longer one spends in an environment exposed to ionizing radiation, the larger the overall dose.
HEALTH EFFECTS OF COSMIC RADIATION
The biological consequence of radiation exposure is due to the damage incurred from ionizing radiation on living cells. The high energy particles and sub-particles contained in cosmic radiation can penetrate deep into tissues, affecting cellular DNA most significantly. A great deal was learned about the negative health effects of ionizing radiation following the atomic bomb detonations on Hiroshima and Nagasaki in 1945. When humans are exposed to very large doses of ionizing radiation (> 1,000-2,000 mSv) they may manifest symptoms of Acute Radiation Syndrome (ARS). The main body systems affected in ARS are the gastrointestinal tract and the bone marrow. At very large doses those exposed can suffer fatal neurovascular compromise. Exposures to this degree are fortunately quite rare and are really only observed following atomic explosive detonation or nuclear power plant meltdown, which are clearly exceedingly rare. Unpredictable solar particle events (SPE’s), have released radiation levels into space measured at doses that would be lethal to astronauts.
The principal health concerns to aircrew and astronauts who have frequent low-dose exposure to ionizing radiation are:
- Increased risk of cancer
- Increased risk for genetic defects to a developing fetus during pregnancy
- Increased risk for genetic defects to future offspring
- Degeneration of tissues – Cataracts and possibly other medical conditions
NASA: EFFECTS OF COSMIC RADIATION ON AVIATION
OCCUPATIONAL COSMIC RADIATION LIMITS
Radiation is often converted to a dose equivalent when considering it’s biological effect. This dose equivalent is measured in Sievert (Sv). Considering the small doses within cosmic radiation, the dose is more commonly displayed in micro-Sievert per hour (ųSv/Hr) or milli-Sievert (mSv) per year, where 1 Sv = 1,000 mSv and 1 mSv = 1,000 ųSv (1 Sv = 1,000,000 ųSv). The radiation effective dose is the tissue-weighted sum of all equivalent doses. It accounts for the type of radiation and the nature of each irradiated organ or tissue since different types of radiation do different degrees of damage. The U.S. Nuclear Regulatory Commission (NRC) explains that “for beta and gamma radiation, the dose equivalent is the same as the absorbed dose. By contrast, the dose equivalent is larger than the absorbed dose for alpha and neutron radiation, because these types of radiation are more damaging to the human body.” If you find measuring radiation a gripping topic, check out the NRC’s summary.
The International Commission on Radiological Protection (ICRP) has recommended that exposure of galactic cosmic rays to aircrew be considered an occupational exposure. The value of 50 mSv in a year was recommended by the Health Physics Society (HPS) in a 2010 position paper. And they further recommend a dose of 100 mSv accumulated over a lifetime. Many other countries have adopted the limits posed by the International Commission on Radiological Protection (ICRP) 2007 report recommendations of 20 mSv per year for occupational effective dose limit with allowances to go as high as 50 mSv per year so long as the average annual dose over five years does not exceed 20 mSv .
The FAA acknowledged the occupational exposure to ionizing radiation to aircrew in 1994. Since May 2000, European airlines have been required to monitor and record occupational exposureto comply with the European Directive. In October 2003, the FAA released a technical report providing education and recommended limits on cosmic radiation for aircrew. They chose to use the above ICRP guidance to recommend a 5-year average effective dose of 20 mSv per year, with no more than 50 mSv in a single year for aircrew members. For a pregnant aircrew member starting from date of reported pregnancy, the recommended limit for the developing fetus is an equivalent dose of 1 mSv, with no more than 0.5 mSv in any month. If any aircrew member’s annual exposure exceeds 6 mSv per year, medical surveillance is recommended. (NCRP – 1993).
In the European Union, European Directive Article 42 deals with ionizing radiation in aircrew. Their recommendation is the same as those proposed by the FAA except for pregnancy they recommend 1 mSv limit to a fetus during pregnancy, BUT use the phrase ‘as low as reasonably achievable’ (ALARA) exposures instead of imposing a monthly limit. Due to this difference, American pregnant aircrew can theoretically fly during their second and third trimesters, while their European counterparts are typically grounded from flying duties.
Space agencies approach radiation limits for astronauts in a different way. The National Academy of Sciences (NAS) made recommendations for career dose limits of astronauts to NASA in 1970. The number that they chose was based on an attempt to maintain astronauts’ probability of developing cancer on par with the general population. The NAS recommended a reference risk of 4 Sv for a career and NASA adopted this as their dose limit thru 1989. It was in this year that the National Council on Radiation Protection (NCRP) issued Report 98, which provided updated recommendations that were dependent on both age and gender. These limits only applied to low earth orbits (LEO). Appreciating the significant risks to life and health that astronauts take by the nature of the job, these guidelines attempted to set dose limits of no more than a 3% increased risk of (cancer) mortality. These limits were updated in 2000 and summarized in NCRP Report 132, significantly lowering the 1989 suggested limits. In light of the continued scientific research, large uncertainties remain regarding the cancer risks from exposure to space radiation.
NASA: EFFECTS OF COSMIC RADIATION ON ASTRONAUTS
ESTIMATING EXPOSURE LEVELS
Approximations of radiation exposure can be calculated by multiplying block hours by average exposures. The average air crew dose lies in the range of 3 to 6 mSv/Yr, with the amount of individual radiation depending on number of flight hours, flight altitude and latitude, and solar activity. Short hauls mission average 1-3 ųSv/hr, long haul 4-5 ųSv/hr, and the Concorde was averaging 12-15 ųSv/hr. Recent British Airways’ research looking at high altitude, long duration flights found an effective dose rate of 3.5 mSv/Yr. The FAA has calculated the highest exposure levels for any flight originating from the U.S. as that from NYC to Athens at 6.3 ųSv/hr.
Air travel passengers are held to the limit for the general public of 1 mSv/year. A large majority of all passengers will not have exposures near this limit. Frequent business travelers, however, may be able to reach or even exceed this number. This number could be reached after only 8 transatlantic flights per annum. The ICRP and others have argued that since these individuals are traveling for business, they should perhaps be held to the occupational limit of 20 mSv/year.
The FAA has developed a computer software program for public use, entitled CARI-6 that provides an estimated equivalent dose for a particular flight when certain parameters of the flight are supplied to it. There is an online dose calculator that that the FAA offers for aircrew and passengers to estimate their radiation dose prior to or after flight. The FAA also provides a free download of the CARI-6 software. Military and commercial aircrew can use this tool to estimate dosage.
Spaceflights above 300 nautical miles enter the Van Allen belts and have a dramatic increase in radiation levels. An astronaut in the Van Allen belt without shielding could be exposed to over 500 ųSv/Hr. (FAA.doc) For this reason, astronauts wear passive dosimeters and all of the space vehicles and international space station (ISS) are equipped with dosimeters to directly measure radiation dose and variations. Missions on the ISS or Russian Mir space station have registered exposures greater than 100 mSv to some astronauts. And long-term future missions to Mars estimate exposures exceeding 1,000 mSv.
EPIDEMIOLOGY OF COSMIC RADIATION
There have been a number of medical studies that have attempted to identify the clinical effect of increased radiation doses on aircrew and astronauts. Many of these studies have been marred by small sample size or other design limitations. Some of the largest studies comparing aircrew to the general population have found an increased risk for malignant melanoma and other skin cancers in aircrew. However, even the studies’ authors admit that this finding could also be explained by increased UV ray exposure during leisure activities. Other studies have alleged that female flight attendants have increased mortality from breast cancer. A large European study found that breast cancer mortality was slightly increased. This finding was not statistically significant though, and the study also found that the all cause and all cancer mortality in female flight attendants was lower than non-aircrew peer groups. Another area of research has tried to elucidate the association between GCR exposure and the formation of cataracts. Although some studies have found a small effect, others have refuted these findings.
A useful illustration of the math used to determine the increased risk on aircrew and their progeny can be found in the 2003 notification:
“At the radiation doses received by aircrews, an increased risk of fatal cancer is the principal health concern…Suppose a crewmember worked 700 block hours per year for 25 years flying between New York, NY, and Chicago, IL…Based on flight data in Table 2 and assuming the flight dose is the same in both directions, the effective dose of galactic cosmic radiation is 0.0039 millisievert per block hour. In 25 years, the dose to the crewmember would be 68 millisieverts.
25 years x 700 block hours per year x 0.0039 mil- lisievert per block hour = 68 millisieverts
As seen in Table 3, crewmembers receiving 68 milli-sieverts will, on average, incur an increased lifetime risk of fatal cancer of about 1 in 360 (0.3%). In the general population of the United States in 1998, about 24% of adult deaths were from cancer…
Genetic defects passed on to future generations are a possible consequence of exposure to ionizing radiation…A child is at risk of inheriting genetic defects because of radiation received by the parents before the child’s conception. Suppose one of the child’s parents worked 700 block hours per year for 5 years flying between New York, NY, and Chicago, IL, before the child was conceived. As in the previous example, the effective dose of galactic cosmic radiation received by the parent is assumed to be 0.0039 millisievert per block hour. In 5 years, the dose to the parent would be 14 millisieverts. As seen in Table 4, with a parental dose of 14 millisieverts between 1 in 25,000 (0.004%) and 1 in 13,000 (0.008%) first-generation children would be expected to inherit one or more radiation-induced severe genetic defects. In the general population, 2-3% of liveborn children have one or more severe abnormalities at birth.”1
Studies on military pilots are surprisingly sparse, but many of the research on commercial aircrews can be applied to their military counterparts. Rotatory wing pilots will not get high enough or accumulate enough hours to pose a significant risk. Fighter pilots will not accumulate nearly the number of block hours as their colleagues flying heavy, transport aircraft. And military long-haul aircraft should be able to use many of the same modeling software to estimate their dose and risks. High-altitude pilots of the cold-war era U-2 reconnaissance spy plane should pay attention. A study published in Neurology last year found that U-2 pilots had a significantly higher number of white-matter lesions in their brains on MRI when compared to a control group. This finding is thought to be due to frequent exposures of relative hypoxia. A large body of research has been conducted looked at the risk of DCS and hypoxia in these high-altitude flyers, however, exposure to cosmic ionizing radiation is also significant and warrants further exploration.
MITIGATION OF COSMIC RADIATION
The mainstay of reducing risk to standard military and commercial aircrew is to limit exposure. Given the relatively low levels of radiation exposure incurred by even those aircrews frequently flying trans-polar, high-altitude flights; limits can be managed effectively with computer modeling such as the CARI-6 described above. As referenced in the video above, another new program called Nowcast of Atmospheric Ionizing Radiation System (NAIRAS) is funded by NASA and aims to provide real-time, data-driven information to make predications about potentially harmful space weather. This information could be used to minimize exposure of radiation to both aviation aircrew and astronauts.
High-altitude military U-2 pilots and astronauts, however, will be subjected to levels of radiation that may likely be to the direct detriment to their longevity or quality of life. The material used to make space suits, space vehicles and space stations continue to be studied for their properties in reducing radiation effective dose. Aluminum at a density of 20 g/cm^2 has been found to reduce effective doses for SPE’s to below acceptable limits. A variety of plastic materials have also been studied with promising results for reducing radiation in deep space. These materials to limit exposures will likely need be combined with technologies that increase the speed of travel to limit exposure duration as well.
SPACE.COM EXPLAINS NEW PLASTIC FOR PROTECTION AGAINST SPACE RADIATION
Regarding standard commercial aircrew, Dr Bagshaw likely summarizes best:
“Whilst it is accepted that there is no level of radiation exposure below which effects do not occur, all the current evidence indicates that the probability of airline crew or passengers suffering any abnormality or disease as a result of exposure to cosmic radiation is very low.”2
Astronauts performing missions in space cannot avoid being exposed to dangerous levels of radiation. As humans attempt to explore deeper into space, the importance of finding solutions to reduce exposure will continue to grow. A number of scientific experiments on materials to limit radiation exposure to within acceptable limits remains underway. Novel materials to protect from radiation or technologies to improve speed of travel will likely precede space missions to Mars and the farther depths of our Solar System.
1. Friedberg, Wallace & Kyle Copeland. What Aircrews Should Know about their Occupational Exposure to Ionizing Radiation. DOT/FAA/AM-03/16. Oct 2003.
2. Bagshaw, Michael. Cosmic Radiation in Commercial Aviation. Aviation Medicine, King’s College London, UK. IAASM. 23 Nov 2007.
3. Davis, Jeffrey R. Fundamentals of Aerospace Medicine. 4th ed. Lippincott Williams & Wilkins. 2008.
4. ICRP Publication 103. The 2007 Recommendations of the International Commission on Radiological Protection
5. FAA. Section II, 2.14, Exposure to Radioactive Materials