One source of natural radiation that presents a danger is radon — a radioactive gas with no colour, smell or taste. It is released from bedrock material and passes through the soil.
What are the Symptoms of Radon Exposure? - American Radon Mitigation
What are signs of radon poisoning? - Quora
What is Radon? — TJRadon
Living in the 'Granite State' means elevated risk for radon exposure, and in turn, lung cancer – New Hampshire Bulletin
Internal Exposure to Radon and Thoron through Inhalation [MOE]
Standard Water - Understanding How a Radon Mitigation System Works
Is Radon Testing Necessary? - Branch Property Investigations
Occupational exposure monitoring for radon in various manufacturing workplaces and underground public-use facilities in Korea - Environmental Science: Advances (RSC Publishing)
How climate change is putting millions at risk of radon exposure
Characterizing occupational radon exposure greater than 100 Bq/m3 in a highly exposed country