Perhaps our single most significant source of radiation exposure is:

Radon is indeed considered the most significant source of radiation exposure for the general public. It is a naturally occurring radioactive gas that is produced from the decay of uranium in the soil and rock. When radon gas infiltrates buildings, it can accumulate to levels that pose health risks, particularly because it can be inhaled and lead to lung cancer over time. The exposure from radon is quantified in terms of milliSieverts (mSv), and studies have shown that radon contributes to a larger percentage of total radiation exposure compared to other sources, such as cosmic radiation, alpha particles, and gamma rays.

Cosmic radiation, while it does contribute to our overall exposure, tends to represent a smaller fraction of the total dose received by individuals. The same is true for gamma rays, which are a form of electromagnetic radiation rather than a distinct source. Alpha particles are a type of radiation but are less penetrative in terms of health risk for the general population unless there is a specific scenario involving radon decay products. Overall, radon's prevalence and the potential for high concentrations within homes make it a significant concern in environmental health and radiation safety.