Are granite worktops radioactive and a threat to health?

Posted by Rock Solid Worktops on 23 January 2017 at 5:50pm

This article explores the impact - perceived or scientifically proven - of radiation from granite when used in the home: is it a threat to public health, are the radiation levels harmful in any way, or are they so low as not to warrant concern?

Much has been published online about the question of radiation and the associated levels of risk in using granite worktops in the kitchen, and most of it is unsubstantiated. There’s also a fair degree of bias towards choosing alternative materials to granite for use as a kitchen worktop. At worst, this casts the natural stone industry in a poor light; at best, it plants doubt in the mind of anyone looking to use natural stone or composite quartz in their kitchens.

At Rock Solid Worktops, we like to think that we know a thing or two about both natural granite and composite quartz. However, before we start answering questions about safe levels of radiation in granite, it is worth considering the naturally occurring emissions of radiation in our everyday lives which go largely un-noticed, let alone have horror stories written about them. For example, in a world driven by digital technology, we subject ourselves to questionable levels of radiation on a daily basis through the use of mobile phones. Our favourite gadgets aside, remember that x-ray you had following your skiing accident? or that chatty dentist who took an image to get to the bottom of your blinding tooth-ache? In some cases, being subjected to radiation is a necessary evil, but hardly ever life threatening.

Like it or not, ionising radiation in our everyday lives is unavoidable since we’re surrounded by it. Our houses are built on ground which naturally emits radioactive radon gas, making up for around 50% of the radiation we are subjected to, on average. Most comes from natural sources and it is present in all living organisms. But we rely on food - be it organically produced, processed or preserved - as well as the air that we breathe, water, and the sun for our most basic levels of survival. According to information on the website of the World Health Organisation (WHO), no single member of the population of the world is without exposure to radiation from natural sources, absorbing an average of 2.4 millisievert (mSv) each year.

Harm to health through ionising radiation is governed by the amount of energy (radiation) that is absorbed by the human body, the calculation for which is known as an absorbed dose. In the UK, the average absorbed dose is 2.7 mSv per year, based on the different types of radiation to which we are exposed:

  • x-rays
  • gamma rays
  • neutrons
  • alpha particles
  • beta particles

Despite the low degree of natural radioactivity in building materials, including granite, sources to which we are regularly exposed are our homes, schools and workplaces. Different sources of exposure are compared by Public Health England, showing that a dental x-ray usually emits a dose of 0.005 mSv, whilst around 20 Brazil nuts (100g) deliver 0.01 mSv, and a transatlantic flight can expose passengers to a dose of 0.08 mSV.

So, what is radiation?

Radiation is the emission of energy as electromagnetic waves or moving subatomic particles, especially high-energy particles which cause ionisation.

Whilst the effects of radiation have been proven to be harmful to health if we’re exposed to large doses over time, we are ordinarily not at risk as we go about our normal daily lives. Naturally-occurring radioactive materials (NORM) are commonplace and are typically regarded as those materials which originate from the ground. Activities such as oil and gas production, mining, the burning of coal, and the manufacturing and use of fertilisers are among materials which result in NORM. When these materials come into contact with atmospheric gases, they become known as ‘cosmogenic’. Cosmic radiation has varying degrees of radioactive properties but is generally less harmful to us. This is because most cosmic radiation is absorbed into the earth’s atmosphere or deflected by its magnetic field.

Heads up for geological science!

Much of what causes radiation in granite begins deep within the earth’s crust and mantle, the latter being nearly 2,000 miles thick and making up a colossal 84% of its volume. This mass of molten rock is constantly on the move, whilst under tremendous pressure from heat and the build up of gases. Radiation occurs with the breakdown of the two most common naturally occurring materials found in rock: uranium (U) and thorium (Th). Another source of radiation is potassium-40 (K40), which is necessary to life and found in many food stuffs. Our bodies regulate the amount of potassium absorbed into our bodies; some of which is absorbed into our bones, but the rest is expelled. A 70 kg person usually has about 4,400Bq of potassium-40 (K40) in their body.

These materials take billions of years to break down. In order to get some idea of how long radiation takes to decay, we consider the amount of time it takes for the level of radiation to be reduced by half from its original value, which is known as its ‘half-life’. In the case of uranium, this can be around 4.5 billion years, and it will eventually become stable as lead (Pb206). Some isotopes of thorium have a half-life of a staggering 14.05 billion years, whereas potassium K40 has an even longer half-life of 1,251 billion years, despite having the capacity to undergo thousands of decays every second.

Radiation risks with granite worktops - are they justified?

Substantiated evidence confirms the presence of radioactive ‘biotites’ in granite. All those little black crystals found in many types of granite are actually biotites derived from the effects of the movement of radioactive inert 222Rn gas (Radon) and 210Po and 218Po (Polonium) ions along tiny fractures in the surrounding structure of elements and crystals. Polonium haloes, as they are called, are rarely, if ever, found, but uranium haloes are commonplace. What this tells us is that whilst granite and other types of solid stone (including composite stone) will emit radon gas as the uranium it contains breaks down; as a non-porous, solid rock, granite (and composite stone encased in resin) traps nearly all of that uranium within. The result is a level of radon emission from the surface so insignificant that it can hardly be measured.

Do all types of granite work top have the same levels of radiation?

Granite is sourced from around the world, across all continents. Some rocks are infinitely older than others, some are harder than others, and all contain a myriad of compounds which affect their colour and appeal for use as a counter top. No two pieces are alike, and they all have varying levels of radioactive materials. The Health and Safety Executive (HSE) has advice on what they term as an ‘effective dose’, the limit for which is calculated as an average “over an area of skin not exceeding 1 cm2”, and is equal to not more than “1 mSv in a calendar year”. 1 mSv is equal to 1 milligray (mG) of radiation, which is not enough to cause sickness.

If we take a sample of granite with a radiation of 40 Ra226, the radioactive dose equivalent is 400 mSv, which is equal to 0.4 gray (gamma) radiation.

Can I use a Geiger counter to measure radiation from my granite countertop?

A Geiger counter can be used to measure general radiation in your home environment, but it cannot measure specifically for radon in your granite worktop. A Geiger counter will measure gamma radiation either as an exposed rate or a dose rate, but you need to consider the environment in which you are attempting to get an accurate measurement, and what type of radiation you are looking for.

For a start, radiation emitting from the ground on which a slab of granite is resting, will affect the outcome of any measurement since it will pick up naturally occurring radon in the soil or concrete, for example. If you live in Devon or Cornwall, this would indeed prove problematic, since this is a region of the British Isles recognised as having high levels of radon gas in the ground due to the area’s geology. The natural background radiation would need to be measured before making a judgement on any measurement taken from a single piece of granite. For a map of the British Isles showing radon affected areas, and copies of downloadable reports, visit http://www.ukradon.org/information/ukmaps

Scientific measurements of radiation levels in granite look for uranium, radium, thorium and potassium, and require tests to be conducted in a controlled environment against specific criteria using gamma spectronomy: “a branch of science concerned with the investigation and measurement of spectra produced when matter interacts with or emits electromagnetic radiation.” (Oxford Dictionary)

Granites sourced from geographical areas known to have a high concentration of monzonites, (course-grained igneous rock solidified from lava or magma, and typical of the coastline sands of Southern India) will have a higher concentration of natural uranium radioactivity. Bianco Crystal is an example of such granite, typically from Spain; whereas Zimbabwe black granite is a course grained plutonic rock (solidified deep below the earth’s surface) with considerably lower concentrations of uranium.

Is food preparation safe on a granite worktop?

Granite worktops cannot affect the uptake of radiation in food; however, many foods such as meat, potatoes, bananas, and seafood - the latter having undergone research of its own with regard to the risk to public health from radiation - have plenty of harmless radioactive properties of their own, derived from the soil or water in which the food is farmed or harvested.

So, are granite worktops radioactive and do they pose a threat to health?

Whatever type of granite you choose for your kitchen, you are no more likely to subject yourself to harmful levels of radiation as you would by devouring bowls of steaming mussels harvested off the coast of India every day of the week. It is more likely that you will be subjected to questionable levels of radiation living in a house built of concrete, since this material is known to emit high levels of radiation. It is not, however, enough to cause harm to health. Recommendations set out in a document produced by the European Commission (Radiological Protection Principles Concerning the Natural Radioactivity of Building Materials, 1999) suggest that natural building stones are not likely to lead to exposure above 1 mSv, be they used superficially or in other minor use. Natural stones used in bulk are noted as “possibly” posing a risk of exposure above 1 mSv, but even the largest granite, or other solid stone worktops, will not present a risk of exposure to radiation in excess of the recommended dose of between 0.3 and 1 mSv per year.

Typical activity concentration of radiation measured in Becquerels per kilo (Bq kg-1) in three common building products are shown in the table on the right.

We are all subject to varying degrees of radiation and different types of radiation have different effects on our health. Some radiation is good for us, whilst some needs to be treated with care. Since all stone and certain man-made building materials such as concrete emit radiation, albeit in small amounts, it is safe to use these materials since the active dose to which we are subjected is not a threat to public health.

Granite and other natural stone such as marble, is quarried from all over the world, so it follows that the geology of the rocks will differ widely. This is also because the earth’s crust and mantle has been through a multitude of transitions over billions of years giving us metamorphic, igneous and sedimentary rocks as a result. Each have inherent characteristics from colour and pattern, to shape and size of grain, and ultimately varying degrees of radiation properties.

Of the top three most common materials found in rock, uranium (U) is the one which gets the most attention. Consequently, it is the one with which we are perhaps more familiar with regard to the compounds which are derived throughout its decay - namely radon gas. Soil, sand and the rocks beneath our homes are primary sources of radon gas, which need to be monitored more in some areas than in others.

Conclusion

Solid rocks of all kinds have traces of uranium that release radon with varying levels of intensity. Emissions from granite, whilst being a source of radon gas, are not high enough to justify any concern, and certainly not high enough to discourage its use in kitchens and bars for counter tops. Since the statutory guidance for Building Regulations 2010 (England), seeks to maintain good air quality by insisting on compliance of approved levels of ventilation in domestic and commercial properties (approved document F1 Means of Ventilation), it is highly unlikely that indoor radon concentrations from granite and other building materials will ever present a risk to public health.

Postscript

Not convinced? At Rock Solid Worktops, we take seriously any concerns that our customers may have about radiation in our granite or quartz. A moderately priced Geiger counter will measure gamma radiation levels in and around our workshop and showroom, just as it could in and around your home. In the first instance, a reading of background radiation is taken into consideration before readings from individual pieces of stone are recorded. To help in making informed choices about the levels of radioactivity from our granite and other stone products, we are more than happy to assist by taking readings for you using our Geiger counter - you can even visit our workshops and take them for yourself.


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