Radon. Our Major Source of Radiation Dose

Radon: Our Major Source of Radiation Dose

Introduction

Radon-222 is a natural, radioactive isotope of element number 86, that

occurs in the uranium-238 decay chain (see table 1). Its immediate parent is

radium-226, and radon-222 itself decays by alpha particle emission through a

series of short -lived decay products (mainly isotopes of polonium, lead and

bismuth) to lead-210 and on eventually to stable lead-206.

Element Radiation Half-Life

Two other isotopes of radon occur in nature; radon-220, which occurs in the

thorium-232 decay chain (table 2) and radon-219 in the uranium-235 chain

(table 3).

radon detector

English: Decay chain 4n+2, Uranium series

English: Graph of lead-210 deposited per month in ...

Both these isotopes have half-lives of under a minute and are less

important than radon-222 which is the subject of the rest of this essay.

Element Radiation Half-Life

Table 2. Thorium-232 Decay Chain

Table 3. Uranium-235 Decay Chain

Radon is one of the group 0 elements, the noble gases, and is, therefore,

chemically virtually inert. However, it has been reported that fluorine

reacts with radon, forming a fluoride.

On average, one part of radon is present in 1 x 1021 parts of air. At room

temperature radon is a colourless gas; when cooled below its freezing

point (-71 C), radon exhibits a brilliant phosphorescence which becomes

yellow as the temperature is lowered and orange-red at the temperature

of liquid air. Liquid radon boils at -61.7 C.

It is formed in the environment by the decay of the trace amounts of

naturally occurring radium present in all rocks and soils. With a half-life of

3.8 days, it can migrate considerable distances through the ground, and

escape into the air. Outside, radon quickly disperses and levels are low -

about 4 Bq m-3, but indoors levels are higher - about 20 Bq m-3 on

The radioactivity of radon is measured in Becquerels per cubic metre of air

(Bq m-3). The Becquerel is named after Henri Becquerel (see essay number

1). A Becquerel is one radioactive disintegration per second.

average and can reach over 1,000 Bq m-3. When we breathe in air containing

radon, and its short-lived decay products, they irradiate our lungs. When

radon levels are high this causes a significant increase in the risk of lung

cancer.

This essay explains the concern about radon, explores the evidence that it

causes harm to humans, and provides details of the concerted effort by the

authorities in the UK to tackle the existing situation, and to reduce the risk in

the future. But first a little bit of radon history.

History of Radon

Friedrich Dorn, a German scientist, discovered in 1900 that radium was giving

off a gas which he called radium emanation. A few years later, in 1908,

William Ramsey and R.W. Whytlaw-Gray isolated enough of the gas to study

its physical properties. As well as finding it was the densest gas known (9.73

g dm-3), they called it niton. In the 1920's, the name radon, symbol Rn, was

adopted for all the isotopes of element 86.

Friedrich Dorn William Ramsey

Though radon was not discovered until 1900, the effects of prolonged

exposure to high levels had been noted over 300 years earlier. Two

researchers in the first half of the sixteenth century, Georgius Agricola (a

German physician and geologist, 1494 to 1555) and Paracelsus (a Swiss

physician, alchemist and scientist, 1493 to 1541) studied the diseases of

underground miners in Europe. They found that many miners died early

because of lung diseases, and concluded that the causes were dust and gases

in the mines. Studies in more recent times have shown that high radon levels

in mines in many parts of the world are linked to a higher risk of lung cancer.

Agricola Paracelsus

Concern about Radon

Everybody is exposed to radiation from a variety of sources (see essay

number 6 for more details). In the UK, 50% of the annual dose to the

average person comes from inhalation of radon and its short-lived decay

products, so radon is the major source of radiation exposure for almost all the

population. In addition, a significant number of people, perhaps a quarter of a

million or so, receive annual doses of 10 mSv.

Annual dose of radiation in UK (NRPB)

It is well known from studies of the survivors of the atomic bombs dropped on

Japan, some early medical procedures, and events such as the Chernobyl

accident that radiation can cause cancers. Using the data from these studies,

it is possible to estimate risk factors for the much smaller environmental

radiation exposures we all receive. However, these risk factors are so small it

is normally not possible to observe them directly for each source of exposure

against the natural background cancer rate. However, the exception is radon

as the exposures are often more bigger for some sections of the population.

The Uses of Radon

Radon is still produced for therapeutic use by a few hospitals by pumping it

from a radium source and sealing it in minute tubes, called seeds or needles,

for application to patient. This practice has been largely discontinued as

hospitals can get the seeds directly from suppliers, who make up the seeds

with the desired activity for the day of use.

There are still places where bathing in radon laden water is thought to be

healthy for the body and soul. One such place is the Rudolf-Stollen mine that

uses radon inhalation as a healing tool.

A radon monitoring method is employed in the Chuko fault zone in south

central Taiwan for earthquake prediction. Soil gas radon is monitored

continuously with a solid-state detector and recorded with a data logger. The

detector assembly is housed in a PVC pipe to reduce the influence of

environmental factors. The fault zone is known to have deep source gases

and sensitive to earthquake activities. The quantities of natural gas releases

are known to vary with earthquake activities. Data retrieval from the end of

October 2000 to the end of February 2001, showed that spike-like radon

anomalies, i.e. rapid increases in the amount of radon, occurred before every

major earthquake with a magnitude of more than 4.0 on the Richter scale.

The strong correlation between spike-like anomalies and major earthquakes

suggests that this might become a method of earthquake prediction.

The Evidence for the Risk from Radon

The first populations to be studied in detail were groups of underground

miners. Some 60,000 miners were involved in 12 main epidemiological

studies. The miners worked underground for significant periods between the

years 1941 and 1990 in 12 different groups of mines in 8 different countries.

Many of the mines produced uranium ore, but others were iron, tin and

fluorspar mines. Over 2,600 lung cancers were observed in these miners,

which is far more than the 750 predicted on the basis of the number of

cancers in the appropriate general population.

Subsequently, studies have been carried out to look for a direct link between

radon in the home and lung cancer. Two of the biggest were in Sweden and in

the Southwest of England. The study in Devon and Cornwall involved 982

individuals with lung cancer and 3,185 matched controls. A number of

analyses were carried out and most indicated that higher lung rates were

found in those exposed to higher levels of radon. However, in most cases the

result did not reach statistical significance so the conclusion was that the

overall result was compatible with the results of the analyses of the study of

the miners. This indicates that 5% of all lung cancers in the UK are caused by

radon.

Another study was performed in Gansu province, China, of people who move

house very rarely and suffer high mean radon levels. Also many dwellings

are below ground. Measurements were made with two one-year alpha track

detectors. The mean radon concentrations were 230 Bq m-3. These are

approximately ten times higher than average UK values.

While the general picture was of a clear association between domestic radon

exposure and lung cancer, one observation was striking. The association

seemed stronger in those living in below ground dwellings (439 cases), rather

than above ground houses and apartments (329 cases). The study concluded

'that effects of residential radon may equal or exceed miner-based estimates,

which are currently used to evaluate risk'.

The Mechanism for Harm from Radon

Radon is present in air in very small concentrations and it moves in and out of

our lungs with the rest of the air in the normal process of breathing. When

radon in the air decays into its daughter products, it forms atoms of solid

elements which are negatively charged. These anions attach themselves to

the small dust particles in the air to form a radioactive aerosol. When we

breathe these particles into our lungs, they stick to the lung-lining and are

not exhaled. As they are still radioactive, they irradiate the lung tissue. The

polonium-214 and -218 emit highly energetic alpha radiation causing damage

to the DNA of cells lining the lungs. Most of the damaged cells are killed.

However, some cells are partially damaged and get replicated. These cells can

induce lung cancer.

The Radon Programme in the UK

It was not until the latter half of the twentieth century that it was realised

that high radon levels in homes were a matter for concern. In the last thirty

of so years, much work has been done in the UK by the National Radiological

Protection Board (NRPB) with the full support of successive governments. A

strategy to control and reduce excessive exposure to radon has been

developed and maps, based on many measurements in homes, show the

areas with the greatest risk of high radon levels. Each local council will have

such a map for their area.

Radon Map of England and Wales from the NRPB

How to Measure Radon

There are many different methods of measuring radon levels. The two most

important are activated charcoal adsorption and alpha track detection.

For activated charcoal adsorption, an airtight container with activated

charcoal is opened in the area to be sampled, and radon in the air adsorbs

onto the charcoal granules. At the end of the sampling period, the container

is sealed and may be sent to a laboratory for analysis.

In alpha track detection, the detector is a small piece of special plastic or film

inside a small container. Air being tested diffuses through a filter covering a

hole in the container. When alpha particles from radon and its decay products

strike the detector, they cause damage tracks. At the end of the test the

container is sealed and returned to a laboratory for reading.

The detector is treated to enhance the damage tracks and then the tracks

over a predetermined area are counted using a microscope or optical reader.

The number of tracks per area counted is used to calculate the radon

concentration of the site tested.

Reducing Radon Levels in Existing Houses

One or more of the following methods may be used to reduce the radon level

in an existing building. A 'sump' and extract pipe can be installed beneath

the floor, from outside. A fan can be fitted to draw out the radon and blow it

into the atmosphere above the roof of the house. Gaps between the ground

floor and walls and gaps around service pipes can be sealed. Positive

pressurisation of the house using a fan in the roof-space prevents gas

entering, i.e. making the air pressure inside slightly higher than outside. Also

natural or forced ventilation of the void under the ground floor will reduce

radon levels.

How to Prevent High Levels in New Buildings

New buildings are generally protected by full protection to a suspended

concrete ground floor. In this the radon-proof barrier is positioned over the

floor structure and linked to cavity trays at the edges. Supplementary

protection is also provided by locating under floor vents on two or more sides

of the under floor space. If necessary the rate of ventilation and radon

dispersion can be increased by fitting an electric fan at a later date.

The radon barrier comprises of a cavity tray through the wall linked to a

membrane across the floor. This is then sealed to a 300 μm polyethene

membrane laid across the beam and block floor. To make it easier to seal the

two materials the cavity tray is laid so that it laps about 300mm over the

edge of the floor. The membrane over the floor can then be sealed to the

cavity tray using a double sided butyl jointing strip just prior to installing the

floor topping. Airbricks are installed where possible on all sides of the

building at intervals at least as frequent as would be normal for an ordinary

suspended timber floor.

Estimates of radon concentrations at different dis...

Apparatus used by Ramsay and Whytlaw-Gray to isola...

English: Radon test kit