By Alan Boyle, Science Editor, NBC News

Experts say that fish and other marine species shouldn't be as affected by Japan's nuclear crisis as species on land, in part because of differences in the ways radiation is dispersed.

But that doesn't mean authorities can ease up on monitoring the sea and its bounty for contamination. To the contrary: Inspectors around the world are keeping a close eye on food imports from Japan, and some countries have ordered special inspections or even outright bans on fish coming from areas near the plant.

Twenty days after Japan's earthquake and tsunami touched off a breakdown and partial meltdown at the Fukushima Dai-ichi nuclear complex, some radiation experts are still struggling to get an accurate read on the situation.

"My basic feeling is that they're going to come to grips with this, and at the end of the day, it's not going to be as bad as people fear," said Florida State University oceanographer William Burnett, an expert on the environmental effects of radioactivity. "Having said that, trying to follow this story has been difficult. I see almost no real data."

The most reliable measurements have been coming from the International Atomic Energy's daily updates on the situation, said Andrew Maidment, a professor of radiology and chief of the physics section at the Hospital of the University of Pennsylvania. So get ready for some real data.

The latest fish radiation readings are above background levels, but still nowhere near the safety limits. The highest radiation reading for fish from the Japanese port of Choshi was 3 becquerels per kilogram of cesium-137 — far below Japan's limit of 500 becquerels per kilogram, or Bq/kg.

"This confirms what scientists including myself have been saying: First of all, the water will dilute this, and the uptake will therefore be lower than it would be for a terrestrial animal," Maidment told me. "The greater the volume of water, the higher the dilution, and the lower the impact."

When radioactive fallout is dispersed on land, it collects on what is essentially a two-dimensional carpet of vegetation, to be ingested later by livestock or humans. But when the fallout reaches the sea, it's dispersed in a much deeper three-dimensional space.

Maidment said this phenomenon was seen clearly in the wake of the 1986 Chernobyl disaster, which still ranks as more of an environmental catastrophe than Fukushima: Wild boars that were tested within a 30-kilometer radius of the Chernobyl site registered 470,000 Bq/kg of cesium-137, while freshwater perch showed concentrations of 4,000 Bq/kg. Eight years later, the levels were reduced to 5,000 Bq/kg for the boars, and 200 Bq/kg for the perch. Even those levels are unpalatably high, but they illustrate Maidment's main points: Marine life tends to absorb less radiation, and contamination levels go down over time.

Yardsticks for radiation

Let's take a moment to talk about the radiation standards: When we're talking about the absorbed dose for humans, that tends to be expressed in terms of millisieverts. For example, the typical annual radiation dose from natural sources amounts to roughly 3 millisieverts.

But when we're talking about the radioactivity contained in various substances, the standard measure is becquerels per kilogram or per liter. The safety standards vary according to what type of radioisotope we're talking about, the type of substance we're talking about, and the type of person who might come in contact with that substance. That's because there's a wide range of variation in the uptake of radioisotopes and their effects on the body.

Here are the safety limits set by Japan's Food Safety Commission and reported by Bloomberg:

Iodine-131: 300 Bq/kg for drinking water, milk and dairy products. 2,000 Bq/kg for vegetables except for root vegetables and tubers.
Radioactive cesium: 200 Bq/kg for drinking water, milk and dairy products. 500 Bq/kg for vegetables, grains, meat, eggs and fish.
Uranium: 20 Bq/kg for infant foods, drinking water, milk and dairy products, 100 Bq/kg for vegetables, grains, meat, eggs and fish.
Alpha-emitting nuclides of plutonium and transuranic elements: 1 Bq/kg for infant foods, drinking water, milk and dairy products. 10 Bq/kg for vegetables, grains, meat, eggs and fish.

Materials exceeding 100 Bq/kg should not be used as the basis for powdered infant milk formula.

Close to the Fukushima plant, the radiation levels are alarming: The IAEA said samples of seawater collected 330 meters east of the nuclear complex's discharge point showed iodine-131 concentrations of 74,000 becquerels per liter, roughly equivalent to Bq/kg. The cesium levels were 12,000 Bq/kg for cesium-137 and another 12,000 Bq/kg for cesium-134. But those levels drop sharply with distance, due to the dispersion factor.

Radioactive iodine-131 is a huge concern for people living around the Fukushima plant, because that can be quickly taken up into the thyroid and pose a significant cancer risk. What's more, iodine is taken up readily by seaweed, which is a popular food item in Japan. Elevated (but still safe) levels of radioactive iodine have been detected in seaweed as far away as Vancouver, and in milk samples from Spokane, Wash. (The reading for the milk was 0.8 picocuries per liter, or roughly 0.03 Bq/kg.)

The flip side is that iodine-131 has a relatively short half-life of eight days, so as time passes, the iodine risk should drop significantly for fish as well as for people.

Cesium-137 has a longer half-life (30 years), so it poses a longer-lived threat. The fish tests suggest that the cesium radiation levels are just one-tenth of the iodine levels, Maidment said.

Stay focused on the fish

Authorities will have to be extra-vigilant about watching radiation levels from Fukushima for a long time — on the ground, in the air and at sea, said Edwin Lyman, a senior staff scientist at the Union of Concerned Scientists, an activist group.

"Even dilute levels of contamination can be enhanced in certain marine life, you know, just like mercury concentrates in large fish, like tuna," he told reporters during a briefing on Monday. "Also, plants like seaweed are known to concentrate certain isotopes, and so are certain types of shellfish. But I would think certainly in the fishing industry in the region, they're most likely going to have to take measures to inspect their catches, and I guess the primary responsibility for that will have to be with the Japanese to inspect and interdict any contaminated seafood."

Maidment agrees that more monitoring will be needed. He also suspects that shellfish living on the seabed around Fukushima might face more contamination than the fish that just happen to migrate through the seas near the stricken plant. But he says "it's too early to draw conclusions," and he emphasizes that the general public needs to put the radiation issue in perspective.

For example, suppose that your drinking water contained 100 becquerels per liter, which is basically 100 Bq/kg. "If that water constituted 10 percent of your dietary intake of food, by weight, and you consumed that exclusively per year, you would increase your background radiation by about 20 percent," Maidment said. "I can double my background radiation just by moving from Philadelphia to Denver. So these are levels of radiation that most of us are not aware of."

I'm betting that you're more aware of the radiation issue than you were 20 days ago, and that you have some thoughts you'd like to share. Feel free to weigh in with your comments below.

Update for 12:55 a.m. ET March 31: One of my Facebook friends, Lynda Williams (the Physics Chanteuse), points out that I have not actually defined what a becquerel is. One becquerel is a unit of radioactivity that's equal to one nuclear decay per second. Here's the way she put it: "It is one atom of a radioactive isotope decaying and emitting radioactive decay. So 5,000 Bq/kg means that 10,000 atoms are decaying per second and shooting off a particle per second in one kilogram. So if the pig has a mass of 100 kilograms, we are talking about a whole body exposure of — do the math, 500,000 particles shooting through its body every second. The 'data' means more if you explain what it really means." Thanks, Lynda!

More angles on radiation:

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Alan Boyle, Science Editor, NBC News