It was developed to monitor weapons testing in the 1950s. There are now stations in several SoCal communities.
The Radiation Monitoring system, known as RadNet, was installed in cities across the country as well as California. RadNet says it has monitors in San Jose, San Francisco, Sacramento, Los Angeles, Riverside, San Bernardino, and Anaheim.
The systems are maintained by the Air Quality Management District but all the data goes to a computer in Alabama which is watched around the clock by the Environmental Protection Agency.
The EPA looks for beta particles which could indicate increased levels of radio activity. They also take into account natural radiation that comes from the sun, space, and from the earth's crust.
Technicians swap out special air filters weekly from monitoring equipment and test them for any signs of radiation. The filters are then mailed to the EPA for more testing.
In the wake of Japan's nuclear disasters, the EPA says it does not expect to see radiation at harmful levels reaching this coast. An EPA spokesman says that Japan's nuclear plants are too far away but the system will keep a careful watch over levels.
The monitoring system was upgraded after 9-11 and the EPA says it will add more monitors to the network as part of the U.S. response to events in Japan.