Environmental radioactivity is produced by radioactive materials in the human environment.  While some radioisotopes, such as strontium-90 (Sr) and technetium-99 (Tc), are only found on Earth as a result of human activity, and some, like potassium-40 (K), are only present due to natural processes, a few isotopes, e.g. tritium (H), result from both natural processes and human activities. The concentration and location of some natural isotopes, particularly uranium-238 (U), can be affected by human activity.