Radioactive materials occur naturally and, as a result, everyone is exposed to a low-level of radiation every day. This exposure comes from a mixture of natural and manmade sources.
The actual amount of radiation that a person is exposed to depends on where they live, what job they do and many other things.
Scientists must always take into consideration the amount of background radiation when working or experimenting with radioactive sources and discount it from their results.
The simplest measure of radioactivity is the Becquerel (Bq). This is a measure of the activity of the nucleus. The activity is the number of unstable nuclei which decay per second in a source.