Radiation Environment and Medicine 2022 Vol.11, No.1 1–6

Review

Some Aspects of the Natural Radiation Environment
James Mc Laughlin*
School of Physics, University College Dublin, Ireland
Received 26 October 2021; revised 15 November 2021; accepted 16 November 2021

Human exposure to the natural radiation environment and the consequent radiation doses canserve as a benchmark against which different artificial radiation exposures can usefully becompared. In radiation risk communications with the public such comparisons can help the public
to have an informed perspective of radiation exposures and risks. A short overview is given
here of the main components of natural radiation to which humans are exposed both externallyand internally. The average annual global effective dose from radiation has been estimated by
UNSCEAR to be about 3.0 mSv of which approximately 80% (2.4 mSv) is due to natural radiation.
At the level of the individual, however, a wide variability of doses from natural radiation exists.
This is true in particular of the doses received in the indoor environment from the inhalation of
airborne progeny of radon and thoron gases. This account of some aspects of natural radiation in
the environment is based on the 1st IRSCC (International Radiation Science Collaboration Centre)
Seminar of the Institute of Radiation Emergency Medicine, Hirosaki University, Japan which wasgiven by the author of this paper in February 2021.
Key words: natural radiation, doses, risks

1. Introduction
When life first appeared on earth is still a matter of debate
and conjecture but based on the age of the oldest fossil
containing rocks life was present on earth at least 3.5
billion years ago. From the time life first appeared until
the present all life forms have been continually exposed
to natural radiation coming from the environment1). Fromthe moment of our conception until our death exposure
to natural radiation is part of our life experience. In
this short account the principal sources of such naturalradiation exposures, their properties and health impact
will be briefly described and discussed in the following*James Mc Laughlin: School of Physics, University College Dublin, IrelandE-mail: james.mclaughlin@ucd.ie
Copyright © 2022 by Hirosaki University. All rights reserved.

sections. This account is based on the 1st IRSCC Seminar2).
More detailed accounts of natural radiation sources andhealth effects can be found, for example, in the reportsof UNSCEAR and in such compendiums as the European
Atlas of Natural Radiation (EARN)3, 4)．
The sources of natural radiation in the environment
may for convenience be divided into two main categories:
primordial radionuclides and cosmic radiation. Primordialradionuclides were principally formed by nucleosynthesis
in stars and supernovae and were present in theinterstellar material from which the solar system wasformed. Over geological time scales and following theformation of the primeval earth the outer layers of
the earthʼs crust became enriched in the radioactive
primordial radionuclides principally those of the Uranium(238U, T1/2 = 4.47 x 109 y) and Thorium (232Th, T1/2 = 14
x 109 y) series and of Potassium-40 ( 40K, T 1/2 =1.25 x
109 y). Due to geological processes which control the2James Mc Laughlin / Radiation Environment and Medicine 2022 Vol.11, No.1 1–6
Table 1. Main sources of doses from natural radiation4)
SOURCESOF EXPOSURE
COSMIC RADIATIONDOSESINTERNAL
Cosmogenic
radionuclides

EXTERNAL
Directly ionising, photon andneutron components

U and 232 Th Series and 40K

238

TERRESTRIAL
RADIONUCLIDESRadon ( Rn), Thoron ( 220Rn) and their progenies. (For these
species, the internal doses are more significant than the external
doses.)
222

Fig. 1. Global Doses from Radiation3, 4)

partitioning of primordial radionuclides on earth and due
to radioactive decay external terrestrial dose rates today
are considerably lower than at the time of the earthʼsformation. Due to its shorter half life compared to 238U
and 232 Th the reduction in the contribution to dose rates
from 40K has been more pronounced1).
Human exposures to natural radiation take place by
three main pathways. These are:(a)By external exposure to radiation from naturalradionuclides present in our surroundings such as insoil, rocks, air, building materials etc.
(b)By inhalation of natural radionuclides in the air.
(c)By ingestion of natural radionuclides in solid andliquid foods.
The partitioning of the doses received via these threepathways are given in Table 1.
UNSCEAR has over many years compiled global dataon sources and doses from both natural and artificial
radiation3). Figure 1 shows the estimated mean global
ef fective doses from these sources based on the
UNSCEAR 2008 Report to the UN General Assembly3, 4).
In the context of this paper two features of this are most

relevant to note. As shown in Figure 1 approximately
80% of the mean annual estimated dose of 3 mSv/a tothe global population comes from natural sources withthe remaining approximately 20% being due to medical
exposures3, 4) The second point of relevance here is that at
2.4 mSv/a approximately half of the natural doses are due
to radon (222Rn) and thoron (220Rn ) or more correctly theirshort-lived progeny. While the dose pie charts are being
continually revised in the light of new data becoming
available it must be emphasised that these estimateddoses are average or mean values. At the level of anindividual dose contributions can be quite variable fromthese mean values. This is true in particular of medical
exposures where quite high doses are received during
radiotherapy compared to routine x-ray procedures suchas at the dentist etc. This is also the case with naturalradiation where there is great variability particularly
for radon exposures. Radon concentrations in air are
known to range over many orders of magnitude from< 1 Bq/m3 over the polar ice caps or mid oceanic air tomany thousands of Bq/m3 in indoor air in a minorityof dwellings. In contrast to this where most people liveJames Mc Laughlin / Radiation Environment and Medicine 2022 Vol.11, No.1 1–6

doses from cosmic radiation have in relative terms amuch lower variability. For example, while the typicalcosmic radiation dose at sea level is between about 0.3
and 0.4 mSv/a in the city of La Paz at an elevation of 3900
m in the Andes it is approximately 2 mSv/a which is only
about a factor of five greater.
2. Cosmic Radiation
In the period 1909 to 1912 investigators of ionisation of
the air found experimental evidence which suggested
the existence of an extra-terrestrial or cosmic source ofionisation. Unequivocal and quantitative evidence forthis was finally obtained by the Austrian physicist Victor
Hess during a series of pioneering balloon flights in 19111912. In these flights his measurements showed thatabove a height of about 1 km the rate of ionisation of
the air began to steadily increase as a function of height
above ground thereby verifying the existence of ionisingradiation coming from the cosmos5). Hess together with
Carl Anderson was awarded the Nobel Prize in Physics
in 1936 for his discovery of cosmic radiation. It should
be noted that the work of the Italian physicist Domenico
Pacini in 1911 on comparing air ionisation rates in sealed
chambers above and below the sea also suggested theexistence of a cosmic component or source of radiation in
the atmosphere6).
It is now known that primary cosmic radiation
originates in outer space both in our own galaxy and in
distant galaxies and that it is composed mainly of 87%
protons, 11% alpha particles (helium nuclei) and about 1%
of nuclei of atomic nuclei (Z) from 4 to 26. These primarycosmic rays travel at speeds close to that of light. They
are highly penetrating and their mean energy is about
1010 eV. Maximum energies appear to be in the region of
1019 eV. The typical flux of cosmic rays at the top of theearthʼs atmosphere is about 104 m-2s-1. In addition to theprimary cosmic radiation there is also cosmic radiation ofsolar origin. Solar cosmic rays have much lower energies( <100 MeV) than the primaries but have a higher proton
component (98%) and a lower alpha particle content (2%)7).
At ground level the largest component of cosmic radiation
are muons at energies generally in the range about 1 to
20 x 109 eV.
When cosmic radiation enters the atmosphere it
interacts with molecules in the air to produce a cascadeor shower of secondary particles consisting of electrons,
neutrons and mesons. Cosmogenic radionuclides are also
produced as a result of these interactions most notably
14 8)
C . Cosmic radiation globally on average accounts forabout 0.4 mSv (16%) of the annual dose from all naturalradiation sources. The actual dose rate from cosmic
radiation is a function of altitude above sea level andgeomagnetic latitude. For example, while the average3

dose from cosmic radiation at sea level is estimated to bein the range 0.3 – 0.4 mSv/year for populations living athigh altitudes in the Andes or Himalayas doses are in therange 1.0 – 2.0 mSv/year. For example, in Japan the dose
rate from the ionising component of cosmic radiationon the summit of Mount Fuji (elevation 3776m) was
found to be 3.6 times that at sea level while that from its
neutron component was about ten times that at sea level9).
Various programmes are available such as the Excel
based EXPACS Programme to calculate cosmic ray dose
rates at any location on the surface of the earth based on
input data of elevation, geomagnetic latitude, heliocentric
potential and soil moisture10). The significance of moistureis because cosmic radiation reflected from soil is affectedby its moisture content as the water molecules in wet soil
will absorb neutrons.
Travelling by air increases the dose received fromcosmic radiation. On a typical high altitude flight routefrom Tokyo to Frankfurt the return trip cosmic ray dose
received is about 100 μSv while on a typical lower altitude
and much shorter flight from Tokyo to Seoul it is only
about 6 μSv. Therefore for members of the public these
non-occupational cosmic radiation doses do not present
a radiation protection problem. On the other hand foraircrew, the annual cumulative doses can be of the orderof a couple of mSv and therefore require the application ofradiation protection procedures in particular for pregnant
aircrew11, 12).
While it is likely only to be relevant to a small numberof people there is growing scientific interest in the doses
and possible consequent health effects due to cosmic
radiation exposure of astronauts on long term missions inspace stations and in interplanetary travel. For the Apollo
and Skylab space missions it has been estimated that
doses in the range 1.2–1.4 mSv/d were received by the
astronauts. In longer duration missions to the International
Space Station for example the dose rate would be
typically about 150 mSv/a. Of particular interest areplanned manned missions to Mars where the total journey
time to and from Mars will be about a 1year duration
followed by perhaps a comparable time on the planet. For
such an exposure scenario estimated cumulative effective
doses of up to 1 sievert (1000 mSv) may be experienced
by astronauts. Additional exposures from solar flares or
major stellar events during the mission which cannot be
predicted could give rise to significant additional doses.
In spite of the high doses that astronauts have in thepast received there is up to the present no evidence of
spaceflight radiation carcinogenesis. One reason for thislack of evidence is that estimates of the rate of cancer
in astronauts and cosmonauts due to cosmic radiation islimited due to low statistics. A recently published studyof cancer mortality in US astronauts, Soviet and present
day Russian cosmonauts which showed no significant

4James Mc Laughlin / Radiation Environment and Medicine 2022 Vol.11, No.1 1–6

increase in cancer mortality when compared to thegeneral population only approximately 320 subjects werestudied13). In addition there are the inherent difficulties in
the application of radiation risk factors largely based on
low LET exposures on earth, such as from studies of theJapanese atomic bomb survivors to that of the completely
different space radiation environment.
Currently (2021) the U.S space agency NASA is
proposing a new space radiation health standard for
astronaut13). NASA intends to use a mean 3 percent REID
(Radiation Exposure Induced Death) as the basis for adose-based limit. On this basis, for all astronauts, themaximum allowable space radiation exposure would
be the ef fective-dose for a 35-year-old female astronaut
whose mean REID is at 3 percent. This standard if
adopted would delineate an effective-dose career limit ofapproximately 600 mSv that applies equally to male andfemale astronauts, regardless of an astronautʼs age.
In the context of effects of cosmic radiation during
space flight It is of some interest to note that during theApollo lunar missions, starting with Apollo 11 astronauts
reported flashes of light in their eyes which were
subsequently attributed to the passage of relativistic
particles though their heads. Various mechanisms
were proposed for this phenomenon such as Cerenkov
radiation flashes in the vitreous humour of the eye dueto the passage of relativistic particles to direct action oneither the visual cortex or retina14). To date the latter
possibility is considered to be the most likely. Prior evento the Apollo missions experimental investigations at
sea-level on earth showed a positive correlation between
the passage of cosmic ray mesons through the humanhead and the detection of light flashes in the eyes of dark
adapted persons15). As a student in the 1960s the author ofthis paper was a volunteer participant in investigations of
this type.
3. Terrestrial Radiation
As noted in the introduction the surface of the earth
contains concentrations of primordial radionuclides
and their progeny. Radiation exposure of humans andassociated doses from these can take place principally by(a) direct external radiation of gamma and betas outdoors
either from soil or rocks or indoors from building
materials derived from geological material. (b) inhalationof airborne radon, thoron and their progeny both
outdoors and more significantly indoors. (c) ingestion of
the primordial radionuclides and their decay products inwater and foodstuffs.
In the case of outdoor exposure the external radiation
component of terrestrial radiation from primordialradionuclides depends primarily on the characteristics oftheir specific activity in the surface material to a depth ofabout one metre at the point of measurement. UNSCEAR
estimates that globally the mean annual dose at thesurface of the earth is about 0.48 mSv corresponding
to an hourly dose rate of 55 nSv/h. There are however
locations on the earth where elevated concentrations ofprimordial radionuclides can give rise to much higher
doses. Ramsar (Iran), Kerala (India), Yangjiang (China)
and Guarapari (Brazil) are examples of such locations.
In the case of Ramsar it is the elevated concentrationof 226Ra in the local geology and surface waters that is
the reason for this16). The author has measured outdoor
dose rates as high as 100 μSv/h in Ramsar and at a wall
surface in one dwelling there measured the dose rate tobe approximately 34 μSv/h.
In the case of indoor exposure to terrestrial radiation
the primordial radionuclide concentrations in the building
materials are the source of the exposure and resulting
doses.
Because of this many radiation regulatory agencies
control the use of building materials by means of anactivity concentration index (I) for the gamma radiation
emitted by building materials. In the case of Member
States of the European Union the EU Basic Safety
Standards uses an index value of 1 as a conservative
screening tool to identify materials that may cause thereference dose level of 1mSv/a to be exceeded11). The
index value is determined by means of the following
formula:I = CRa226 /300 +CTh332/200 +CK40 /3000 where CRa226, CTh332
and CK40 are in Bq/kg.
Naturally occurring radionuclides in the terrestrial
environment both on land and in the oceans areincorporated into food products. Apart from the inhalationof airborne radon/thoron progeny food intake is asignificant route for the intake of natural radionuclides
by the public. Due to food intake the body of a 70 kg
human adult contains about 140 g of the essential element
Potassium of which approximately 0.017 g is the naturally
occurring radioactive isotope Potassium 40 (40K). This
gives rise to the human body having an activity of about
5000 Bq. For adults the total annual effective dose dueto the ingestion and inhalation of natural terrestrial
radionuclides is estimated to be about 0.29mSv, of which
0.17mSv is due to 40K and 0.12mSv is due to the longlived radionuclides in the uranium and thorium series. In
contrast to this the average annual dose contribution fromthe ingestion in food of the cosmogenic radionuclide 14C ismuch smaller at about 2 μSv.
It is of interest to note that studies of representative
dietary intakes for Japanese adults have shown that thecommitted effective doses are dominated by naturalradionuclides in foodstuffs9). In this regard the three most

James Mc Laughlin / Radiation Environment and Medicine 2022 Vol.11, No.1 1–6

significant contributions are made by 210Po (0.73 mSv). 40K(0.18 mSv) and 210Pb (0.058 mSv). As the total committed
effective dose from all radionuclides is estimated to be
0.80 mSv the 210Po contribution accounts for 90% of this.
On the basis of food category 80% of the total committed
effective dose of 0.80 mSv is due to the consumptionof fishes and shellfish which are an important thoughdeclining component of the Japanese diet9).
4. RadonExcluding doses from radiotherapy UNSCEAR has
estimated that the largest and most variable contributor
to radiation doses received by the majority of the generalpublic during their lifetime is due to the two main isotopesof the gaseous element radon namely: 222Rn (T ½ = 3.825
days) and 220Rn (T ½ = 55.6 s) which are generally referred
to as radon and thoron respectively16). Radon is a memberof the 238U series and its immediate parent is radium(226Ra) while thoron is a member of the 232Th series andits immediate parent is another radium isotope (224Ra).
UNSCEAR has estimated that globally radon and thoron
account for more than half of the dose. Indoor radon
concentrations are mainly due to radon entering buildings
from the decay of radium in the soil or rocks beneath thebuilding. Outdoor concentrations of radon are typicallyin the range 3 to 10 Bq/m3 while in most countries meanindoor concentrations generally range from about 20
to 80 Bq/m3 3, 17). A small percentage of buildings exist
in many countries with indoor radon concentrations ofmany thousands of Bq/m3. For example, the author has
measured radon concentrations as high as 50000 Bq/m3
in a dwelling built on karstic limestone above uraniferous
geological strata in southwest Ireland18).
The principal epidemiologically confirmed healtheffect of radon is lung cancer not only in undergroundminers but also in studies of the general public exposed
to radon in their homes19, 20). The mechanism by which
inhalation of radon is considered to increase the risk oflung cancer is by alpha particle damage to sensitive cells
in the bronchial epithelium. When air containing radonis inhaled the radiation dose received is mainly due toalpha particles emitted by the short-lived progeny ofradon such as 218Po and 214Po deposited on the surface of
the airways rather than by radiation from the gas radon
itself 21). A detailed account of the dosimetry of radon andits progeny is given in the ICRU Report 8819).
5. Public Perception of Natural Radiation
Most members of the public have very little knowledgeof natural radiation. In recent decades due mainly to
public information campaigns on health risks fromindoor radon public awareness of natural radiation has

5been increasing in many developed countries. Even fortechnically educated members of the public, however,knowledge of natural radiation and its health effects arevery rudimentary and it is generally thought to be of little
health consequence. This is in part due to the common
use of the term “background radiation” as a descriptor
for natural radiation in the environment. In English, andperhaps in some other languages the word “background”
used in describing a phenomenon has connotationsof insignificance or of not being important such as in“background noise”. An additional aspect of how theterminology used affects public perception of naturalradiation arises because it is natural and in the present
zeitgeist something natural is often and incorrectly taken
to infer it to be safe or of little danger to health. When one
considers the many life threatening natural phenomena
such as earthquakes, tsunamis, volcanic eruptions,malarial carrying mosquitos etc. to uncritically view
natural phenomena as safe is illogical but it is a perception
held by many. Numerous studies of public perception
have shown that the anxieties and fears of the public forradiation from artificial sources such as nuclear power donot seem to apply to radiation from natural sources22, 23).
A good example of this is public apathy towards therisk from indoor radon even though the extensive bodyof epidemiological evidence available is unequivocal
that radon is a human carcinogen and according to the
WHO radon is the second known cause of lung cancer
after smoking. WHO estimates that exposure to radon
may be responsible for between 3% and 14% of lungcancer deaths globally24). Even allowing for the many
uncertainties involved in such estimates it is clear thatin the case of radon exposure the health effects aresubstantial. Here again the fact that radon and its progeny
are natural radionuclides in the perception of the publicmeans nobody is to blame for indoor radon exposures.
This misconception is a significant contributing factor
to apathy and inertia of the public towards controllingindoor radon. Studies have shown that even whenhouseholders are made aware that their homes have an
unacceptable level of radon only a small percentage will
remediate the problem25). The reality is that while radonis indeed natural indoor radon levels are not natural. They
are a direct consequence of the way we locate, design,
construct, use and maintain our dwellings. Radiation fromindoor radon is in fact a form of TENR (TechnologicallyEnhanced Natural Radiation) which is conceptually thesame as the enhancement of cosmic radiation doses
received by both aircrew and the travelling public due to
air travel.
In the aftermath of nuclear accidents such as occurredin Chernobyl in 1986 and more recently in Fukushima
in 2011 part of the reason for the fear and anxiety of the
public was that they had no benchmark or yardstick6

James Mc Laughlin / Radiation Environment and Medicine 2022 Vol.11, No.1 1–6

against which to compare the resulting doses. Doses fromnatural radiation in the environment could usefully play
the role of such a benchmark against which artificial
doses from accidental radionuclide releases and eventhose from medical exposures could be compared22).
6. Conclusion
As described in this paper exposures, both external andinternal, to radiation from the different components of thenatural radiation environment are and always have beenpart of the human experience. UNSCEAR estimates themean dose rate globally from natural sources to be about
2.4 mSv/year. At this dose rate level the health risks
to the public from natural radiation in most exposure
scenarios are considered to be insignificant. In the case,however, of long term elevated radon exposure numerous
epidemiological studies both of exposed workers and ofmembers of the public have unequivocally shown andquantified the increased risk of lung cancer related tosuch exposures19, 20). It is considered that increased publicawareness of natural radiation in all its forms can play auseful role in helping the public to compare radiation risks
and to make value judgements of the risks from radiation
exposures in other situations such as in accidental nuclear
energy accidents and medical exposures.

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Sato T. Analytical model for estimating terrestrial cosmic ray
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NASA. Space Radiation and Astronaut Health: Managing andCommunicating Cancer Risk. National Academies Press. 2021;
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Conflict of Interest
The author declares that he has no conflict of interest.
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