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Japanese Nuclear Emergency: Radiation Monitoring

About RadNet Air Monitoring Data

EPA no longer updates the information at this link, but it may be useful as a reference or resource. This site contains information and data from March 11, 2011 to June 30, 2011. EPA has returned to routine RadNet operations. This site will continue to be available for historical and informative purposes.

For real-time air monitoring data, please visit the EPA RadNet website and Central Data Exchange. To view both current and historical laboratory data, please visit our Envirofacts database.

EPA's RadNet Air Monitoring Data Map

EPA continuously monitors environmental trends in radiation and has been doing so for 50 years. EPA has permanent or "stationary" monitoring stations throughout the nation that form the RadNet System.

EPA recognizes that RadNet provides complex data. The data can be difficult to understand at first glance, but, in their simplest form, the data show radiation trends over time. Tracking trends allows us see changes to radiation in the United States. EPA will continue to work with its world-class scientists to make these data understandable and relevant to citizens.

RadNet Air Data

Background Information:

What information does the RadNet data provide?

To make the monitoring results easier to visualize and follow over time, we have developed charts from the RadNet stationary air monitoring data. We have included two sets of data;"beta gross count rate" and "gamma gross count rate ranges.” Tracking beta and gamma radiation, which are particles and rays that come from radioacitve material, helps us to identify the type and amount of the radioactive material in the air.

  • The beta gross count rate measures the radiation from all radionuclides that emit beta particles, which is indicated by the term gross or total. The term count rate tells us how quickly beta particles are being detected, which indicates how much radioactivity the monitor is seeing.
  • The gamma data measures radiation from all radionuclides that emit gamma rays and splits them into ranges of energy. The word gross, or total, indicates that the measurement is from all gamma emitting radionuclides. Not all gamma rays have the same amount of energy. Breaking the data into discrete energy ranges helps scientists to determine which radionuclides may be present.

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Why are there fluctuations in the data?

Spikes in data can be caused by a variety of situations, including fluctuations in naturally occuring radiation levels like from radon, rain concentrating natural radiation, and changes in atmospheric (barometric) pressure.

Occasionally, you may see brief gaps in the data. Scientists remove any data points from the database that are caused by instrument error.  

Larger gaps generally mean the RadNet monitor was temporarily taken offline for maintenance or repair. In response to the Japan incident, we have prioritized maintenance to the west coast monitors. Having a monitor offline is not cause for concern. Even if the closest monitor is not operating, the RadNet system as a whole continues to provide a national view of airborne radiation in the environment.

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Why do I see higher levels of radiation at some monitor locations?

It is important to note that there are often large differences in normal background radiation across the nation. Background radiation levels depend on factors including altitude and the amount of naturally occurring radioactive elements in the soil. What is natural in one location may be different from what is natural in another.

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About RadNet Stationary Air Monitors

RadNet stationary (permanent) air monitors sample continuously at a nominal flow rate of 60 cubic meters per hour (Adults typically breathe at a rate of about 20 cubic meters per day.) The monitors collect any particles in the sample on a filter. Radiation detectors continuously measure the beta and gamma radioactivity from particles on the filter. Every hour, the stationary monitor sends an electronic report to EPA's National Air and Radiation Environmental Laboratory.

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How EPA uses RadNet Data

Beta particles and gamma rays, like all radiation, are natural and part of our environment. Tracking beta and gamma radiation, which are particles and rays that come from radioacitve material, helps us to identify the type and amount of the radioactive material in the air.By looking at this data over time, scientists recognize what is "normal" or "background" radiation in that location. Any reading above normal will trigger an alert to EPA scientists to review the data. Scientists remove any data points from the database that are caused by instrument error from the data sets.

Environmental monitoring data can be used to confirm that no excess radiation is present in an area or to help decision-makers decide whether appropriate actions need to be taken to protect the public.  The monitoring data can also be used to confirm that the data generated by computer models are accurate.

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How to Access RadNet Data on EPA's Central Data Exchange (CDX) and Envirofacts

In an effort to make the data more easily accessible and understandable, we have created a webpage with graphs of the monitoring data from across the country. You can access that information at https://epa.gov/japan2011/rert/radnet-data-map.html. Data designed for scientists and technical experts is available as explained below.


Near-real time air monitoring data are available from EPA's Central Data Exchange (CDX). You no longer need to register.


You can find air filter data, as well as historical radiation data on drinking water, milk, and precipitation on Envirofacts.

  1. Go to https://www.epa.gov/enviro/
  2. Select the "Radiation Topic" tab.
  3. Scroll down to the "Radiation" topic.
  4. Select "GO TO RadNet SEARCH."

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