People receive some natural or background radiation exposure each day from?

There are several sources of natural or background radiation for people.

First, there are naturally occurring radioactive elements in the crust of the earth which have been present since the earth's formation. These can irradiate people in several ways.

• They can irradiate people externally because the soil and rocks all around us (and some building materials such as concrete and brick) act as radiation sources.
• Plants and animals that we use for food take in the radioactive elements and we ingest them along with our food.
• Some of the radioactivity is present in the air and we inhale it along with the air.

Second, there are radioactive elements which are produced by interactions of cosmic rays with the atmosphere, the best known being carbon-14 and hydrogen-3 (or tritium). These can irradiate us in much the same ways as the elements discussed above.

Third, we are constantly irradiated by cosmic rays.