This article may require cleanup to meet Wikipedia's quality standards. The specific problem is: This article reads as if it is a research paper and not an encyclopedia article (in part because it is mostly cut and paste from the public domain study by NASA). It needs to be moved to a more encyclopedic topic heading, perhaps something like Cancer and spaceflight, and re-written in an encyclopedia article style.(July 2012) |
Astronauts are exposed to approximately 72 millisieverts (mSv) while on six-month-duration missions to the International Space Station (ISS). Longer 3-year missions to Mars, however, have the potential to expose astronauts to radiation in excess of 1,000 mSv. Without the protection provided by Earth's magnetic field, the rate of exposure is dramatically increased. [1] [2] [ failed verification ] The risk of cancer caused by ionizing radiation is well documented at radiation doses beginning at 100 mSv and above. [1] [3] [4]
Related radiological effect studies have shown that survivors of the atomic bomb explosions in Hiroshima and Nagasaki, nuclear reactor workers and patients who have undergone therapeutic radiation treatments have received low-linear energy transfer (LET) radiation (x-rays and gamma rays) doses in the same 50-2,000 mSv range. [5]
While in space, astronauts are exposed to radiation which is mostly composed of high-energy protons, helium nuclei (alpha particles), and high-atomic-number ions (HZE ions), as well as secondary radiation from nuclear reactions from spacecraft parts or tissue. [6]
The ionization patterns in molecules, cells, tissues and the resulting biological effects are distinct from typical terrestrial radiation (x-rays and gamma rays, which are low-LET radiation). Galactic cosmic rays (GCRs) from outside the Milky Way galaxy consist mostly of highly energetic protons with a small component of HZE ions. [6]
Prominent HZE ions:
GCR energy spectra peaks (with median energy peaks up to 1,000 MeV/amu) and nuclei (energies up to 10,000 MeV/amu) are important contributors to the dose equivalent. [6] [7]
One of the main roadblocks to interplanetary travel 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. [6] Operational parameters that need to be optimized to help mitigate these risks include: [6]
Source: [6]
Source: [6]
Quantitative methods have been developed to propagate uncertainties that contribute to cancer risk estimates. The contribution of microgravity effects on space radiation has not yet been estimated, but it is expected to be small. However as microgravity has been shown to modulate cancer progression, more research is needed into the combined effects of microgravity and radiation on carcinogenesis. [8] The effects of changes in oxygen levels or in immune dysfunction on cancer risks are largely unknown and are of great concern during space flight. [6]
Studies are being conducted on populations accidentally exposed to radiation (such as Chernobyl, production sites, and Hiroshima and Nagasaki). These studies show strong evidence for cancer morbidity as well as mortality risks at more than 12 tissue sites. The largest risks for adults who have been studied include several types of leukemia, including myeloid leukemia [9] and acute lymphatic lymphoma [9] as well as tumors of the lung, breast, stomach, colon, bladder and liver. Inter-sex variations are very likely due to the differences in the natural incidence of cancer in males and females. Another variable is the additional risk for cancer of the breast, ovaries and lungs in females. [10] There is also evidence of a declining risk of cancer caused by radiation with increasing age, but the magnitude of this reduction above the age of 30 is uncertain. [6]
It is unknown whether high-LET radiation could cause the same types of tumors as low-LET radiation, but differences should be expected. [9]
The ratio of a dose of high-LET radiation to a dose of x-rays or gamma rays that produce the same biological effect are called relative biological effectiveness (RBE) factors. The types of tumors in humans who are exposed to space radiation will be different from those who are exposed to low-LET radiation. This is evidenced by a study that observed mice with neutrons and have RBEs that vary with the tissue type and strain. [9]
The measured change rate of cancer is restricted by limited statistics. A study published in Scientific Reports looked over 301 U.S. astronauts and 117 Soviet and Russian cosmonauts, and found no measurable increase in cancer mortality compared to the general population, as reported by LiveScience. [11] [12]
An earlier 1998 study came to similar conclusions, with no statistically significant increase in cancer among astronauts compared to the reference group. [13]
The various approaches to setting acceptable levels of radiation risk are summarized below: [14]
NCRP Report No. 153 provides a more recent review of cancer and other radiation risks. [19] This report also identifies and describes the information needed to make radiation protection recommendations beyond LEO, contains a comprehensive summary of the current body of evidence for radiation-induced health risks and also makes recommendations on areas requiring future experimentation. [14]
Astronauts' radiation exposure limit is not to exceed 3% of the risk of exposure-induced death (REID) from fatal cancer over their career. It is NASA's policy to ensure a 95% confidence level (CL) that this limit is not exceeded. These limits are applicable to all missions in low Earth orbit (LEO) as well as lunar missions that are less than 180 days in duration. [20] In the United States, the legal occupational exposure limits for adult workers is set at an effective dose of 50 mSv annually. [21]
The relationship between radiation exposure and risk is both age- and sex-specific due to latency effects and differences in tissue types, sensitivities, and life spans between sexes. These relationships are estimated using the methods that are recommended by the NCRP [10] and more recent radiation epidemiology information [1] [20] [22]
The as low as reasonably achievable (ALARA) principle is a legal requirement intended to ensure astronaut safety. An important function of ALARA is to ensure that astronauts do not approach radiation limits and that such limits are not considered as "tolerance values." ALARA is especially important for space missions in view of the large uncertainties in cancer and other risk projection models. Mission programs and terrestrial occupational procedures resulting in radiation exposures to astronauts are required to find cost-effective approaches to implement ALARA. [20]
Organ (T) | Tissue weighting factor (wT) |
---|---|
Gonads | 0.20 |
Bone Marrow (red) | 0.12 |
Colon | 0.12 |
Lung | 0.12 |
Stomach | 0.12 |
Bladder | 0.05 |
Breast | 0.05 |
Liver | 0.05 |
Esophagus | 0.05 |
Thyroid | 0.05 |
Skin | 0.01 |
Bone Surface | 0.01 |
Remainder* | 0.05 |
*Adrenals, brain, upper intestine, small intestine, kidney, muscle, pancreas, spleen, thymus and uterus. |
The risk of cancer is calculated by using radiation dosimetry and physics methods. [20]
For the purpose of determining radiation exposure limits at NASA, the probability of fatal cancer is calculated as shown below:
The cumulative cancer fatality risk (%REID) to an astronaut for occupational radiation exposures, N, is found by applying life-table methodologies that can be approximated at small values of %REID by summing over the tissue-weighted effective dose, Ei, as
where R0 are the age- and sex- specific radiation mortality rates per unit dose. [20]
For organ dose calculations, NASA uses the model of Billings et al. [23] to represent the self-shielding of the human body in a water-equivalent mass approximation. Consideration of the orientation of the human body relative to vehicle shielding should be made if it is known, especially for SPEs [24]
Confidence levels for career cancer risks are evaluated using methods that are specified by the NPRC in Report No. 126 Archived 2014-03-08 at the Wayback Machine . [20] These levels were modified to account for the uncertainty in quality factors and space dosimetry. [1] [20] [25]
The uncertainties that were considered in evaluating the 95% confidence levels are the uncertainties in:
The so-called "unknown uncertainties" from the NCRP report No. 126 [26] are ignored by NASA.
The double-detriment life-table approach is what is recommended by the NPRC [10] to measure radiation cancer mortality risks. The age-specific mortality of a population is followed over its entire life span with competing risks from radiation and all other causes of death described. [27] [28]
For a homogenous population receiving an effective dose E at age aE, the probability of dying in the age-interval from a to a+1 is described by the background mortality-rate for all causes of death, M(a), and the radiation cancer mortality rate, m(E,aE,a), as: [28]
The survival probability to age, a, following an exposure, E at age aE, is: [28]
The excessive lifetime risk (ELR - the increased probability that an exposed individual will die from cancer) is defined by the difference in the conditional survival probabilities for the exposed and the unexposed groups as: [28]
A minimum latency-time of 10 years is often used for low-LET radiation. [10] Alternative assumptions should be considered for high-LET radiation. The REID (the lifetime risk that an individual in the population will die from cancer caused by radiation exposure) is defined by: [28]
Generally, the value of the REID exceeds the value of the ELR by 10-20%.
The average loss of life-expectancy, LLE, in the population is defined by: [28]
The loss of life-expectancy among exposure-induced-deaths (LLE-REID) is defined by: [28] [29]
The low-LET mortality rate per sievert, mi is written
where m0 is the baseline mortality rate per sievert and xα are quantiles (random variables) whose values are sampled from associated probability distribution functions (PDFs), P(Xa). [30]
NCRP, in Report No. 126, defines the following subjective PDFs, P(Xa), for each factor that contributes to the acute low-LET risk projection: [30] [31]
The accuracy of galactic cosmic ray environmental models, transport codes and nuclear interaction cross sections allow NASA to predict space environments and organ exposure that may be encountered on long-duration space missions. The lack of knowledge of the biological effects of radiation exposure raise major questions about risk prediction. [32]
The cancer risk projection for space missions is found by [32]
where represents the folding of predictions of tissue-weighted LET spectra behind spacecraft shielding with the radiation mortality rate to form a rate for trial J.
Alternatively, particle-specific energy spectra, Fj(E), for each ion, j, can be used [32]
The result of either of these equations is inserted into the expression for the REID. [32]
Related probability distribution functions (PDFs) are grouped together into a combined probability distribution function, Pcmb(x). These PDFs are related to the risk coefficient of the normal form (dosimetry, bias and statistical uncertainties). After a sufficient number of trials have been completed (approximately 105), the results for the REID estimated are binned and the median values and confidence intervals are found. [32]
The chi-squared (χ2) test is used for determining whether two separate PDFs are significantly different (denoted p1(Ri) and p2(Ri), respectively). Each p(Ri) follows a Poisson distribution with variance . [32]
The χ2 test for n-degrees of freedom characterizing the dispersion between the two distributions is [32]
The probability, P(ņχ2), that the two distributions are the same is calculated once χ2 is determined. [32]
Age-and sex-dependent mortality rate per unit dose, multiplied by the radiation quality factor and reduced by the DDREF is used for projecting lifetime cancer fatality risks. Acute gamma ray exposures are estimated. [10] The additivity of effects of each component in a radiation field is also assumed.
Rates are approximated using data gathered from Japanese atomic bomb survivors. There are two different models that are considered when transferring risk from Japanese to U.S. populations.
The NCRP recommends a mixture model to be used that contains fractional contributions from both methods. [10]
The radiation mortality rate is defined as:
Where:
Identifying effective countermeasures that reduce the risk of biological damage is still a long-term goal for space researchers. These countermeasures are probably not needed for extended duration lunar missions, [3] but will be needed for other long-duration missions to Mars and beyond. [32] On 31 May 2013, NASA scientists reported that a possible human mission to Mars may involve a great radiation risk based on the amount of energetic particle radiation detected by the RAD on the Mars Science Laboratory while traveling from the Earth to Mars in 2011-2012. [15] [16] [17] [18]
There are three fundamental ways to reduce exposure to ionizing radiation: [32]
Shielding is a plausible option, but due to current launch mass restrictions, it is prohibitively costly. Also, the current uncertainties in risk projection prevent the actual benefit of shielding from being determined. Strategies such as drugs and dietary supplements to reduce the effects of radiation, as well as the selection of crew members are being evaluated as viable options for reducing exposure to radiation and effects of irradiation. Shielding is an effective protective measure for solar particle events. [33] As far as shielding from GCR, high-energy radiation is very penetrating and the effectiveness of radiation shielding depends on the atomic make-up of the material used. [32]
Antioxidants are effectively used to prevent the damage caused by radiation injury and oxygen poisoning (the formation of reactive oxygen species), but since antioxidants work by rescuing cells from a particular form of cell death (apoptosis), they may not protect against damaged cells that can initiate tumor growth. [32]
The evidence and updates to projection models for cancer risk from low-LET radiation are reviewed periodically by several bodies, which include the following organizations: [20]
These committees release new reports about every 10 years on cancer risks that are applicable to low-LET radiation exposures. Overall, the estimates of cancer risks among the different reports of these panels will agree within a factor of two or less. There is continued controversy for doses that are below 5 mSv, however, and for low dose-rate radiation because of debate over the linear no-threshold hypothesis that is often used in statistical analysis of these data. The BEIR VII report, [4] which is the most recent of the major reports is used in the following sub-pages. Evidence for low-LET cancer effects must be augmented by information on protons, neutrons, and HZE nuclei that is only available in experimental models. Such data have been reviewed by NASA several times in the past and by the NCRP. [10] [20] [34] [35]
The sievert is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing radiation, which is defined as the probability of causing radiation-induced cancer and genetic damage. The sievert is important in dosimetry and radiation protection. It is named after Rolf Maximilian Sievert, a Swedish medical physicist renowned for work on radiation dose measurement and research into the biological effects of radiation.
Ionizing radiation (US) (or ionising radiation [UK]), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel up to 99% of the speed of light, and the electromagnetic waves are on the high-energy portion of the electromagnetic spectrum.
The gray is the unit of ionizing radiation dose in the International System of Units (SI), defined as the absorption of one joule of radiation energy per kilogram of matter.
Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingested or inhaled radioactive substances, or externally due to irradiation by sources of radiation.
Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Exposure can be from a source of radiation external to the human body or due to internal irradiation caused by the ingestion of radioactive contamination.
Absorbed dose is a dose quantity which is the measure of the energy deposited in matter by ionizing radiation per unit mass. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection, and radiology. It is also used to directly compare the effect of radiation on inanimate matter such as in radiation hardening.
The linear no-threshold model (LNT) is a dose-response model used in radiation protection to estimate stochastic health effects such as radiation-induced cancer, genetic mutations and teratogenic effects on the human body due to exposure to ionizing radiation. The model statistically extrapolates effects of radiation from very high doses into very low doses, where no biological effects may be observed. The LNT model lies at a foundation of a postulate that all exposure to ionizing radiation is harmful, regardless of how low the dose is, and that the effect is cumulative over lifetime.
Radiation hormesis is the hypothesis that low doses of ionizing radiation are beneficial, stimulating the activation of repair mechanisms that protect against disease, that are not activated in absence of ionizing radiation. The reserve repair mechanisms are hypothesized to be sufficiently effective when stimulated as to not only cancel the detrimental effects of ionizing radiation but also inhibit disease not related to radiation exposure. It has been a mainstream concept since at least 2009.
in space we cannot breath and we have no gravity
Health threats from cosmic rays are the dangers posed by cosmic rays to astronauts on interplanetary missions or any missions that venture through the Van-Allen Belts or outside the Earth's magnetosphere. They are one of the greatest barriers standing in the way of plans for interplanetary travel by crewed spacecraft, but space radiation health risks also occur for missions in low Earth orbit such as the International Space Station (ISS).
A gamma ray, also known as gamma radiation (symbol γ or ), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically shorter than those of X-rays. With frequencies above 30 exahertz (3×1019 Hz), each gamma ray imparts the highest photon energy of any form of electromagnetic radiation. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; in 1900 he had already named two less penetrating types of decay radiation (discovered by Henri Becquerel) alpha rays and beta rays in ascending order of penetrating power.
David A. Schauer, ScD, CHP, is executive director emeritus of the National Council on Radiation Protection and Measurements (NCRP). During his tenure a number of updated and new publications were issued by the Council.
Effective dose is a dose quantity in the International Commission on Radiological Protection (ICRP) system of radiological protection.
Studies with protons and HZE nuclei of relative biological effectiveness for molecular, cellular, and tissue endpoints, including tumor induction, demonstrate risk from space radiation exposure. This evidence may be extrapolated to applicable chronic conditions that are found in space and from the heavy ion beams that are used at accelerators.
Epidemiological studies of the health effects of low levels of ionizing radiation, in particular the incidence and mortality from various forms of cancer, have been carried out in different population groups exposed to such radiation. These have included survivors of the atomic bombings of Hiroshima and Nagasaki in 1945, workers at nuclear reactors, and medical patients treated with X-rays.
HZE ions are the high-energy nuclei component of galactic cosmic rays (GCRs) which have an electric charge of +3 e or greater – that is, they must be the nuclei of elements heavier than hydrogen or helium.
Travel outside the Earth's protective atmosphere, magnetosphere, and in free fall can harm human health, and understanding such harm is essential for successful crewed spaceflight. Potential effects on the central nervous system (CNS) are particularly important. A vigorous ground-based cellular and animal model research program will help quantify the risk to the CNS from space radiation exposure on future long distance space missions and promote the development of optimized countermeasures.
StemRad is an Israeli-American start-up company that develops and manufactures personal protective equipment (PPE) against ionizing radiation. Its first product was the 360 Gamma, a device that protects the user's pelvic bone marrow from gamma radiation. StemRad has also developed the StemRad MD, a protective suit designed to provide whole-body radiation protection to physicians, and the AstroRad vest for radiation protection in space, which is currently being tested on the International Space Station and is one of the primary payloads onboard NASA's Artemis 1 lunar mission.
Radiation exposure is a measure of the ionization of air due to ionizing radiation from photons. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. As of 2007, "medical radiation exposure" was defined by the International Commission on Radiological Protection as exposure incurred by people as part of their own medical or dental diagnosis or treatment; by persons, other than those occupationally exposed, knowingly, while voluntarily helping in the support and comfort of patients; and by volunteers in a programme of biomedical research involving their exposure. Common medical tests and treatments involving radiation include X-rays, CT scans, mammography, lung ventilation and perfusion scans, bone scans, cardiac perfusion scan, angiography, radiation therapy, and more. Each type of test carries its own amount of radiation exposure. There are two general categories of adverse health effects caused by radiation exposure: deterministic effects and stochastic effects. Deterministic effects are due to the killing/malfunction of cells following high doses; and stochastic effects involve either cancer development in exposed individuals caused by mutation of somatic cells, or heritable disease in their offspring from mutation of reproductive (germ) cells.
Flight-time equivalent dose (FED) is an informal unit of measurement of ionizing radiation exposure. Expressed in units of flight-time, one unit of flight-time is approximately equivalent to the radiological dose received during the same unit of time spent in an airliner at cruising altitude. FED is intended as a general educational unit to enable a better understanding of radiological dose by converting dose typically presented in sieverts into units of time. FED is only meant as an educational exercise and is not a formally adopted dose measurement.
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: CS1 maint: multiple names: authors list (link)This article incorporates public domain material from Human Health and Performance Risks of Space Exploration Missions (PDF). National Aeronautics and Space Administration. (NASA SP-2009-3405).