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May 1, 2011
The radiation fallout levels in the U.S. from the Japan Fukushima reactors remain low. The EPA runs a national RadNet system that collects radiation samples from air, water, milk and other sources. In addition, most states (e.g. Maryland) have independent collection systems, as do nuclear regulatory agencies.
We spoke with Clifford S. Mitchell, MD, MPH, of the Maryland Department of Health and Mental Hygiene.
The data are collected by measuring the activity level of the disintegrating isotopes. Each isotope emits a unique spectrum of energy, so the type of radiation, as well as total energy is measurable in picocuries (pCi/m3). However, those measurements are not clinically relevant.
One needs to then estimate a clinical dosage, which depends on the length of exposure and route of ingestion (i.e. inhaled, drank, touched, etc). Sieverts (Sv), or millisieverts (mSv), are the units of dosage one needs in order to compare fallout to other types of radiation, such as CT scans or X-rays. We do not have estimates for the mSv dose data.
The encouraging aspects of the data collected so far are that:
A) The total activity is not accumulating significantly
B) The long-half-life, most deadly, isotopes such as cesium and plutonium, are not yet detected, for the most part, in mainland U.S. Hawaii does have detectable levels of some of the more concerning isotopes, as does California rain water measurements (Te-132).
Given that I-131 has a short half-life of 8 days, one would not expect to see an accumulation of activity in pCi. However, a continued inhalation of low levels can translate into a greater dosage on terms of mSv.