Dr.  Ruth Lyons

The study of average radon levels in tourist caves being carried out by Dr Stephen Solomon (Australian Radiation Laboratory), Dr Julia James, (University of Sydney) and myself (University of Auckland) is the first step in addressing the second question of actual radon concentrations. Recognizing the relevance that this question has to the health of employees, the Occupational Health and Safety Group, Worksafe, have made funding available to study Australian tourist caves and the University of Auckland has subsidized the equivalent study of New Zealand tourist caves.  You will all be familiar with the little pottles which have been placed at strategic sites in tourist caves throughout Australia and New Zealand; thanks to assistance from many of you, these have been changed at regular intervals and are now being processed to obtain seasonal and annual data.  Summary results will be published in this journal shortly; detailed results will be sent to individual cave managements as well as reported to the funding bodies. 

At the request of several people at the 1lth ACKMA Conference in Tasmania in May this year - not least our persistent Editor - I have written this article to address the first question in lay terms, and hopefully to dispel some common myths and misapprehensions.  If there is cause for concern, let us at least be clear about what the potential problems are so we are neither vulnerable to the sort of hype the media are prone to, nor dismissive of a potentially serious environmental and workplace problem.  It will also equip us to deal with any concerns members of the public may raise with us.  I have written this article in question and answer format using the questions I am most commonly asked.  If I haven't made things clear or if there are other questions you would like answered, please feel free to ask - my contact details are at the end of this article. 

What is radon?
Radon is a radioactive gas.  It is colourless, odourless, tasteless and chemically inert, ie. it doesn’t react with other substances, so it isn't poisonous.  However it is radioactive, which means it spontaneously gives off small particles from its nucleus and these may damage tissues. 

What is the connection between radon and the bomb?
None.  Radon is not produced in nuclear tests or accidents.

Where does radon come from?
Radon occurs naturally, it is not a pollutant.  It is produced when uranium breaks down radioactively.  When uranium decays radioactively it becomes. thorium which also gives off radiation and becomes in turn, thorium, radium and then radon.  Radon itself becomes polonium, bismuth and finally lead, which is stable and non-radioactive.  This is known as the uranium series decay chain and the other elements which are produced are called uranium daughters - or radon 'progeny" for the politically fastidious!  Everything, including rocks, plants and you yourself, contains a small amount of uranium which gradually decays to produce radon.  However the amount of uranium (and hence radon) is usually in such small quantities it doesn’t concern us.  Radon only becomes a concern if it leaks out of rocks and sediments into a confined space where it can accumulate instead of being diluted and dissipated.

If radon is naturally occurring why do we worry about it?
Just because something is naturally occurring it doesn't mean it is good for you.  Arsenic is naturally occurring.  So is asbestos. 

Why radon?  Why aren’t we worried about the other naturally occurring radioactive elements like uranium?
Of course, if they occur in  sufficient quantities we are.  However, in normal environmental levels, most of the radiation given off by uranium and its daughters is absorbed by the host rock and is not a problem. Radon on the other hand, being an inert gas, is very effective at diffusing through rock and sediments to reach the cave air (this is why the radon can be detected even when the lids are screwed onto the pots).  Then radon and its radioactive products can be breathed into the lungs where they are in close contact with vulnerable tissues which may be damaged by the radiation given off.

Why do some places have high radon levels and others not?
There are two factors which control the levels of radon in any confined space such as a building, mine or cave: firstly, the amount of radon reaching the air and secondly, dilution of the radon by the air.  The amount of radon reaching the air is determined principally by the type of rock (volcanic rocks usually have higher levels of uranium and hence produce more radon than limestone does) and the size of the grains (more radon will reach the air from sands and gravels than from solid rock).  The buildup of radon will depend on how large and well ventilated the space is: in a confined space with poor ventilation the radon will not be diluted or dispersed as effectively as in a large space with a good flow-through of air.

Initially it was thought that radon would not be a problem in caves because caves are usually found in limestone which has a low uranium content.  However measurements in English and American caves showed that levels were sometimes very high and could even exceed those found in uranium mines.  This is because they may contain shales or volcanic sediments, which have more uranium than limestone, and may also be small and poorly ventilated ~ the two factors which, combined, give high radon levels.

Which caves are likely to pose a problem?
Generally speaking, small caves with a lot of volcanic sediments and poor ventilation will have higher radon concentrations than large clean well-ventilated caves However, levels, even in the same cave, may vary a great deal daily and seasonally - I have measured levels which are more than 10 times greater at some times compared to other times at the same site.  The reasons for this are complex and interlinked.  For example, ventilation patterns may be quite different summer and winter or day and night: when a cave is drier a sump may open, increasing ventilation and reducing radon levels, but on the other hand the drier mud will release more radon than the same mud when it is wet, increasing levels.  The only way to really know how much radon is there is to measure it.

Who is at risk?
Anyone who spends a lot of time in high radon environments.  Members of the general public are not at risk.

Do you get sick or glow in the dark if you have too much radon?
No and no.  If you are exposed to huge amounts of radiation you can get acute radiation sickness and die in a few days.  This is what can happen in nuclear accidents such as Chernobyl, or in the fall-out range of nuclear tests.  It doesn't happen with environmental levels of radon, and “a few days out of the cave when you feel sick” may be good if you have flu but are quite irrelevant to any effects radon may have on you.

If it isn’t poisonous and you don’t glow in the dark, what is the health risk of having too much radon?
If you have too much accumulated exposure to radon you have an increased risk of getting lung cancer, or cancer of other parts of the respiratory system.  It doesn’t mean you will get cancer, just that you have a slightly greater chance of developing it.  In this it is very like smoking.  We all know people who have smoked two packs a day and are happily wheezing their way into old age with never a cancer in sight.  But the odds of developing lung cancer are higher in both cases compared to non-smokers or to people who have not been exposed to radon.

How much higher is the risk of developing lung cancer?
That’s the $64,000 question and nobody can be quite sure because we simply don’t have the data to do statistical studies.  There are some studies being done on the cave dwellers in China who live in houses excavated from the loess deposits in the highlands but we don’t have results yet. The best available estimates are shown the chart, which also shows the relative effects of cigarette smoking. (Smoking does not release radon, radon and smoking are alike only in that they both increase the risk of getting lung cancer because they both damage lung tissue.)

What are these “becquerels” and “mSv” and “Working Levels” shown in the table?
Becquerets (Bq) are the number of radioactive decays that occur every second.  For example, 100 Bq per cubic metre means that, in every cubic metre of air 100 atoms of radon every second give off radiation and change to polonium. 
Millisieverts (mSv) are a measure of the total amount of radiation absorbed by the body and how much damage it is likely to do.  Several factors are taken into account when this is worked out, such as the amount of radiation given off with each radioactive decay, what type of radiation it is and where in the body it is absorbed.
Working Levels (WL) are a convenient way of managing the amount of radiation a worker may be absorbing by relating the radon levels to the number of hours a worker can be exposed to it before exceeding the regulations.

If we make certain assumptions (see later in the discussion about radon daughters) 3700 Bq is roughly equivalent to IWL and would give a dose of around 1 mSv if you were exposed to it for 70 hours (or 2 working weeks).

What are the current regulations?
The International Radiation Protection Board has recommended limits for workers of 1000 Bq per cubic metre of air, or an annual dose of less than 50 mSv in any one year and not more than 100 mSv in any 5 year period.  Looking at the chart, you can see this is nearly 10 times the average environmental radiation received during “normal” living.  If someone spent their working life receiving that amount of radiation per year they would be 5 times more likely to die of lung cancer than a typical non-smoker.

This is the same risk experienced by a “light” smoker having about half a pack a day or someone receiving several hundred chest x-rays a year.  However, we are becoming increasingly aware of the hidden dangers of radioactivity and limits are constantly being revised downwards, never upwards.

The IRPB does not have the power to enforce their recommendations: it is left to individual countries and states to enact legislation.  In Australia, the NHRMC and Occupational Health and Safety are jointly publishing guidelines which will confer a Duty of Care on employers; these should be available by the time you read this. 

Is it the levels of radon that are important or the length of time I spend there?
Both are important - the amount of radiation you receive is the level of radon multiplied by the time you spend there.  For example, 1 hour at 4000 Bq is the same as 2 hours at 2000 Bq or 8 hours at 500 Bq.  The regulations stipulating 1000 Becquerels per cubic metre of air as the maximum allowed level of radon for workers assume that the worker will be spending 8 hours a day or 2000 hours per year.  If you work only 400 hours a year underground you can have 5 times the level of radon and still receive the same annual dose from the workplace.  Acceptable residential limits are much lower because people usually spend more time at home than at work.

What if I smoke as well as go into radon-high environments?
The short answer is that if you smoke and worry about radon you've got things out of perspective, as you can see from the chart.  Give up smoking first, then worry about radon.  The combined risk of smoking and radon is likely to be much greater than just adding the two individual risks together, but there isn't any solid data on this yet.
 

We don’t legislate for smoking, why should we legislate for radon?
Smoking is usually voluntary and not a condition of employment (apart from passive smoking, which is increasingly unacceptable to many people).  Radon exposure in the workplace is not voluntary and employers have a legal and moral “duty of care” to their employees. 

Shouldn’t we take recreational caving and/or smoking into account when setting limits on work-related exposure?
A difficult ethical question indeed!  It is true that whereas there is a legal distinction between work-derived and voluntarily accepted risks, the combined health risk makes no such distinction.  It is also true that most of us ascribe to an ethic of individual freedom and personal responsibility, at least in our leisure activities.

How do you know you have had too much exposure to radon?
The short answer is you don’t.  You can't smell it or taste it, you will not feel sick, you will not have any immediate effects.  The only way to know how much you've had is to monitor it.  This can be done indirectly by knowing typical levels of radon in the caves in which you work and combining this with the hours you work.  If levels are high or vary a great deal, personal monitors can be worn to monitor individual exposures directly. 

What can be done if I’ve had too high an exposure to radon?
Reduce your exposure in the future (and give up smoking if you smoke).  It is the total dose you get that is important, not occasional peaks.  For example, if you need to work in a high radon environment for a couple of months or years, choose a low radon environment for the next couple of months or years.  This way your average level ie: the total dose you receive over the whole time, can be acceptable.  The regulations allow for this by setting limits for the dose that is acceptable in any one year higher than the average dose over a five year period.

How’s your own exposure to radon?
Mmmmm - interesting question.  Probably not too bad as Australasian caves have not so far had particularly high levels.  But I estimated that the people who took the measurements in one British cave exceeded their total yearly recommended maximum in the time it took them to collect the data!

How do you measure radon?
The radon pottles and personal radon monitors work by counting the number of radon decays in the air of the container.  Radon diffuses into the container and when it decays the radiation “scratches” the piece of special plastic in the bottom of the container.  This is then etched to make the tracks stand out so they can be counted.  From the number of counts and the time the monitor has been exposed we can work out the average concentration of radon.

The amount of radon at any one time can also be measured using a radiation detector, which may be linked to a computer readout.  These are relatively expensive, costing around $20,000; they are very useful for detailed data and immediate feedback but not for long term averages.  The instrument many of you have seen me using (and helped me carry!) measures radon daughters, not radon itself.

So is it radon or radon daughters that matter?
Radon daughters.  Radon doesn’t spend very long in your lungs anyway, you breathe it in and you breathe it out and the chances of it giving off radiation while it is in your lungs are very small.  Radon daughters, being solids instead of a gas like radon, and also being very prone to stick to small particles in the air, are much more likely to get stuck in your lungs.  All the radiation given off after that will be right next to vulnerable tissue.  So it is actually radon daughters that matter, not radon itself.  Other factors that matter are the size and number of small particles in the air and the percentage of radon and radon daughters that are attached to these particles, as these particles are more likely to stick in the lungs than unattached atoms.

Then why do we spend all this effort measuring radon when it’s really the radon daughters that matter?
Radon levels give an upper limit to the amount of radiation due to both radon and its daughters that may be present, so low levels of radon mean we don't have to worry about total radiation levels.

Average radon levels are relatively easy and cheap to measure.  The pots we use can be mass-manufactured and distributed widely whereas more sophisticated methods require expensive instrumentation and careful maintenance - the high humidity in caves is notoriously hard on high voltage instruments (these detectors are run at around 300OV).  Financially and logistically it would be quite impossible to achieve the coverage we have in the Worksafe study, which corresponds to Stage 1 recommended by the Radon in Caves Workshop held in Canberra in 1992.  Moreover, when levels can vary so widely in the same cave at different times, average levels give a better indication than spot measurements.  This is particularly important when we  are comparing caves and deciding where more concentrated effort is best spent.

How do you work out radiation damage from radon concentrations when it’s not actually radon doing the damage?
We have to make various assumptions, such as the number of radon daughter atoms present relative to the number of radon atoms. (This is called the equilibrium ratio, and is the most important factor in estimating the actual radiation dose.) We also assume that a certain proportion of these will be breathed in and lodge in the lungs.  This depends on the number of aerosols or small particles in the air to which the radioactive atoms may stick, the proportion which do stick to the particles, and the size of the particles and hence the chance of them staying in the lungs when they are breathed in.  We also need to know how long people have spent working in different concentrations, remembering that conditions almost certainly vary a great deal so that average conditions may not represent the conditions at the times when people are actually in the caves.  For the best estimate we need to know when as well as how long, and how levels vary with time.

With all these assumptions how accurate are your estimates of radiation dose from the levels of radon?
The Worksafe estimates are expected to be within the target set at the 1992 Workshop, i.e. within a factor of 5 of the true value.

How can the estimates be improved?
By actually measuring some of the contributing factors instead of guesstimating them.  For example the 3-day study in Buchan Caves by Dr Stephen Solomon and others from Australian Radiation Laboratory, which was reported at the 1992 Workshop at Buchan, measured radon, radon daughters, the number and size of the particles and the percentage of radon daughters attached to particles.

What can cave managers do if levels are high?
Increasing ventilation is the obvious solution when we’re dealing with high levels in mines but it is not generally an option in caves because of the delicately balanced micro-climate and the serious effect any changes may have on speleothems and cave fauna.  Nor can we seal off the sources of radiation as is sometimes done in buildings built directly on uranium-rich rocks.  And closing caves would be a particularly drastic “solution”!  Our options therefore lie in the realm of personnel management: in limiting the number of hours worked by any particular worker; in introducing personal monitors to ensure workers do not exceed permissible limits, and in supporting further research so that more accurate estimates can be made. Better estimates mean less margin has to be allowed when planning work rosters, and a better knowledge of variation in expected doses at different times and seasons mean we can schedule duties intelligently to reduce doses.

For example, if an area in a cave is known to have high radon levels in summer but low levels in winter, then carrying out maintenance duties in winter rather than summer in that area will give a lower dose rate and mean a worker can spend much longer at the site for the same total dose.  Similarly we may be able to modify tour routes to minimize time spent in areas which may have high concentrations generally or at particular times.

Are you planning any more scientific research on radon in caves?  And what do you hope to achieve by it?
I myself will be involved in two projects in 1996 looking for further scientific insight into radiation in caves.  They should have important spin-offs for cave management and also improve the estimates of radiation exposure that we already have.

One project is a year-long in-depth study similar to the one in Buchan Caves by Dr Solomon is being set up in Jenolan Caves as a scientific collaboration by Drs Stephen Solomon, Julia James, Stuart Whittlestone (ANSTO) and myself.  We will be gathering data on the various factors affecting radiation doses due to radon and its daughters, the temporal and spatial variation of radon in these cave systems and the relationship of radon with other environmental parameters. 

In the first half of 1996 I will be taking leave from the University of Auckland to take the study of radiation in Australasian tourist caves a step further.  I plan to take both radon and radon daughter measurements simultaneously at as many sites as possible where we have already measured average radon values in the Worksafe study.  This will give equilibrium values which are a very important parameter in converting radon levels to actual radiation dose and can be used to refine the Worksafe dose estimates by using a measured value instead of an assumed value.  I would very much appreciate your cooperation in giving me access to the caves, and in return I promise to try to answer any questions you like to ask and, of course, to give you a copy of the results when the study is completed.  I look forward to meeting lots of you again on your home territory - or should that be, in your own caves down under? 

RADON RISK EVALUATION CHART

This chart compares the risk of dying from lung cancer for different levels of exposure to radon and smoking. (The meaning of Bq, WL and mSv is given in the text.) Before you panic, note that the conversion from Bq and WL to mSv and hence to the risk in this table assumes that the person spends all their working life of 2000 hours per year for 35 years in an atmosphere with that concentration of radon, and needs to be adjusted for lesser periods of time.  For example, if you only spend 1000 hours per year, halve the number of mSv and look at the risk corresponding to that value. (The chart is derived from one in A Citizen’s Guide to Radon, published by The UK Environmental Protection Agency for radon in houses.)

Journal No 21. December 1995. Pages 4 – 11.