The ground beneath Sydney emits radiation. But it’s nothing to worry about
- Written by Laura Manenti, Experimental particle physicist, Faculty of Science, University of Sydney
When most people hear the word radiation, their mind jumps straight to nuclear disasters, such as at Chernobyl or Fukushima.
But radiation is everywhere. In fact, right now, as you read this, you are being exposed to radiation from the ground beneath your feet, the air around you, and even your own body. Radiation is not inherently bad: what matters is how much you are exposed to.
To this end, my team and I have built the first radiation map of our home town, Sydney. This map provides a new perspective of the city, showing that the ground beneath the city is constantly emitting a small amount of natural radiation. Spoiler: it’s nothing to worry about.
What is radiation?
At its most basic level, radiation is energy travelling through space.
In nature, it is often produced by radioactive elements – atoms that are unstable and so prefer to convert into other elements by releasing energy, ending up in a more stable state. This process is called radioactive decay.
When Earth formed around 4.5 billion years ago, it contained radioactive elements, such as uranium, thorium and potassium. Some radioactive elements decay in a fraction of a second; others decay so slowly they are still present today.
For example, natural uranium has a half-life of about 4.5 billion years. That means it takes 4.5 billion years for half of a given amount of uranium to decay, eventually turning into lead, which is stable.
Uranium, thorium and potassium dominate natural background radiation because they combine two key features: they were abundant when Earth formed, and they have half-lives comparable to, or even longer than, the age of Earth. Many other radioactive elements either decayed away long ago or were never present in significant amounts.
Because of this, these elements are everywhere. They are found in rocks and soil, taken up by plants, eaten by animals, and ultimately end up in our bodies. That is why we are, in a very literal sense, mildly radioactive.
We said that radiation is energy. But if you zoom in far enough, that energy starts to look like it’s being carried around by tiny particles: alpha particles (helium nuclei), beta particles (electrons or positrons), and gamma rays – photons, just like light, but far more energetic.
The key difference between the types of particle is how far they manage to travel. Alpha and beta particles don’t get very far before they run out of steam. A bit of air, clothing, or skin is usually enough to stop them. For that reason, they are mostly a concern when the radioactive material ends up inside the body – for example if it is inhaled, as can happen with radon gas.
Gamma rays, on the other hand, travel easily through air and out of the ground.
That makes them more relevant for external exposure, but also extremely useful: they escape from rocks and soil and reach our detectors. This is why gamma radiation is the type we can use to map what is happening beneath our feet.
Measuring Sydney’s radiation
When I moved from Abu Dhabi to Sydney in 2024, I observed something unexpected. The natural radioactivity I was measuring around the city with a small handheld gamma-ray detector was about five times higher than what I had been used to in the United Arab Emirates.
That raised two questions: why was natural radiation higher in Sydney than in Abu Dhabi? And was it safe?
Australia does have national radiation maps. But these are mostly based on surveys carried out from aircraft flying tens of kilometres apart. They are excellent for understanding broad geological patterns, but far too coarse to tell you how radiation varies from one neighbourhood, park or suburb to the next.
My students Tengiz Ibrayev and Matilda Lawtong and I set out to build the first high-resolution, ground-based map of natural gamma radiation for metropolitan Sydney. We carried out a radiation survey across a 10 by 10 kilometre region of the city, dividing the area into a grid and visiting almost every square on foot.
At each location – usually in public parks or open green spaces – we placed a gamma-ray detector on the ground and let it measure radiation for several minutes.
This gave us reliable averages rather than quick snapshots.
We also took measurements over open water in Sydney Harbour on a ferry. Water blocks radiation coming from the ground, so this let us measure cosmic radiation from space – high-energy charged particles originating from the Sun and deep space that constantly hit Earth). We then subtracted this background radiation, so we could focus on the radiation coming from the ground.
To understand why radiation levels changed from place to place, we also collected soil samples at selected locations and analysed them in the laboratory using very sensitive gamma detectors. This allowed us to measure how much uranium, thorium and potassium were present in the soil – the elements responsible for most natural radiation.
Sampling locations and rock types of the study area in Sydney. The red and yellow circles represent the gamma dose rate measurements on land and water, respectively. The black shovels correspond to the soil sampling locations.
Author provided, CC BY-NC
The pattern follows geology
Radiation levels across the city do vary, but not randomly. Areas built on sandstone and shale tend to show higher natural radiation than areas dominated by younger sediments.
In other words, the pattern follows geology, not human activity.
Radiation exposure is usually measured in units called millisieverts (mSv). Your own body contributes about 0.03mSv each year, mainly from potassium naturally present in your tissues.
Across the part of Sydney we mapped, the average terrestrial gamma radiation from the ground is about 0.24mSv per year. Even the highest values we measured are well within the range of natural background radiation seen worldwide.
We are hoping to expand this work to other cities around Australia through citizen science in schools. Doing so helps us turns something abstract and invisible into something we can measure, compare and understand.
Measuring radiation replaces fear with context. It doesn’t make the world more dangerous – it makes it clearer.
Authors: Laura Manenti, Experimental particle physicist, Faculty of Science, University of Sydney
