High levels of an invisible gas which can cause lung cancer have been identified in parts of Devon by health officials.
Radon is a natural radioactive gas that comes from decaying uranium in rocks and soil and according to Public Health England (PHE), it is for most people the largest source of radiation they get exposed to.
The agency has published a geological map which shows which areas of England and Wales have higher levels than others.
Rules introduced during the 1990s state that the Government must act if levels of the gas in homes goes past 200 Bq m3, also known as "action level".
Radon has been proven to increase the risk of lung cancer, especially in people who smoke cigarettes.
In fact, the second leading cause of lung cancer in the UK is radon gas, which is sobering given that areas of Devon are in the highest category of radon-affected areas due to a large granite presence.
The number of homes at or above the action level in some areas in greater than 30%.
Looking nationwide, Cornwall and the Midlands are among the areas worst affected.
The amount of radon in the air is measured by placing a detector in the main living room of a home and in one bedroom.
Levels are usually higher in the winter, so the detectors are left in position for three months and can account for seasonal variations.
The amount of radon is measured in becquerels per cubic metre of air. The average level in UK homes is 20 Bq m3.
For levels below 100 Bq m3, your individual risk remains relatively low and not a cause for concern.
However, the risk increases as the radon level increases.
The radioactive elements formed by the decay of radon can be inhaled and enter our lungs.
Inside the lungs, these elements continue to decay and emit radiation - and, most importantly, alpha particles.
These are absorbed by the nearby lung tissues and cause localised damage. This damage can lead to lung cancer.
You can get a radon risk report from PHE for £3.90 and, if your home is in a high-risk area, the radon levels will need to be measured.
This costs £52.80 and you will get two detectors sent to you. These hollow plastic shells contain a piece of clear plastic that records the damage caused by radon.
After three months, you should post them back to PHE and they will measure the radon levels for you.
You can reduce your risk of radon exposure by sealing up any cracks in your floor, as nearly half of all radon comes from the ground.
Good ventilation can also help prevent it from building up inside your home.
You can read the full indicative atlas of radon levels in England and Wales here.
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