While measurable amounts of radiation from Japan's stricken nuclear reactors seem to have reached the East Coast of the United States, there is not yet any cause for alarm.

Public health officials have found traces of radioiodine-131 — an isotope produced in nuclear reactors — in rainwater in Boston and snow in New Hampshire. The levels detected — 79 picocuries per liter in Boston, 40 picocuries per liter in Concord, N.H. — are barely a trace and below what officials consider dangerous. A picocurie is an extraordinarily small measure of radioactivity.

A water sample also has been taken from the Merrimack River in Dracut.

While officials say the level of radioactivity detected so far is no cause for concern, it is good to know that health experts are monitoring the situation, for a number of reasons.

The situation at Japan's Fukushima nuclear facility appears not to be getting better but rather worsening. Even as workers struggle to keep the plant's reactors cool and restore control, reports show radiation levels are rising. That implies that the stricken facility will continue to spew radiation into the environment for some time. And that means the low levels of radiation detected here now could rise.

The impact of radiation on human health depends on the nature of the radioactive material involved. Different radioactive materials have differing half-lives — the time it takes for the material to decay. Radioiodine-131 has a half-life of just eight days. However, cesium-137, another product of nuclear reactors, has a half-life of 30 years.

The strong likelihood is that any radioactive material from the Fukushima reactors will be too widely dispersed by the time it reaches the East Coast to pose any threat to human health.

But it is better to know than to guess. That's why the testing is a good idea.

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