The old saying goes where there's smoke, there's fire, but steam is a different story, even in the case of a nuclear power plant that suffered multiple meltdowns. Despite fresh worries about a new meltdown at the Fukushima Daiichi complex in Japan, the steam that set off this concern is merely a result of atmospheric conditions—and a reactor that is still hot from having melted down in 2011.

Think of it as seeing your breath in cold weather. The damaged reactors at Fukushima are still hot, nearly three years after the disaster, thanks to the ongoing radioactive decay of the damaged nuclear fuel. This is why used nuclear fuel sits in cooling pools of waters for years after time spent fissioning in a reactor. The radioactive detritus at Fukushima is still throwing off roughly one million watts worth of heat, according to Fairewinds Energy, a nuclear safety advocacy group based in Burlington, Vt. That heat turns water into steam—and when the air is cold enough, as it is in winter in Japan, that steam is visible. "This also happened last year at this time, and periodically since the tsunami in 2011," notes David McIntyre, a spokesman for the U.S. Nuclear Regulatory Commission (NRC). "We are in touch with the Japanese regulator and TEPCO [the utility responsible for Fukushima], and from what we've seen and heard there is no reason to suspect that this steam is an indicator of anything bad happening."

Nor is this plume of steam—sometimes visible, sometimes not—only apparent in winter. When the atmospheric conditions are right, with relatively low temperature and high humidity, the steam is visible even in summer, as happened in July 2013. It is fortunate that physics suggests such steam is nothing to worry about, because it is impossible to check firsthand. Due to the meltdown in that reactor, radiation levels are too high for any human to enter without receiving an unacceptable dose.

What about the fishes?
Another perennial concern is that the water contaminated with radioactive particles still leaking from the stricken nuclear power plant site is poisoning Pacific Ocean fish and other seafood. There is no doubt that ingesting radionuclides is one of the worst forms of radiation exposure, because it continues for a long period of time. But, with the exception of bottom-feeding fish and sessile (immobile) filter feeders caught in the immediate vicinity, any radionuclides from Fukushima have been diluted by the vastness of the Pacific to insignificant quantities. The extra radionuclides from Fukushima are simply not enough to create a dose large enough to cause any human health effects outside the immediate vicinity of the stricken nuclear power plant.

Nor is the radioactive contamination from Fukushima the cause of changes to Pacific sea-bottom life observed in recent years off the U.S. west coast, as the marine scientists at Deep Sea News recently noted. Those shifts most likely stem from the copious quantities of carbon dioxide spewed by fossil fuel–fired power plants that are changing the climate and, thus, the tiny plants known as phytoplankton that serve as the base of the oceanic food chain.

When it comes to radiation, the nuclear weapons testing conducted from the 1940s to the 1980s contributed orders of magnitude more radioactivity to the oceans than Fukushima (even when combined with Chernobyl, a much larger nuclear catastrophe). There is also an estimated 37 x 10^18 becquerels worth of radioactivity in the oceans from naturally dissolved uranium in seawater anyway, which some view as a future nuclear fuel source but is not generally considered a health risk. (A becquerel measures the rate of radiation emission.) And there are other naturally occurring radioactive elements in seawater as well, such as polonium.

That means the tuna caught in the Pacific have always been naturally radioactive (and pose less risk than dental x-rays, as the Woods Hole Oceanographic Institution notes). Or as marine scientist Ken Buesseler of Woods Hole put it in a scientific paper on the subject published in 2012, "though [cesium] isotopes are elevated 10 to 1,000 [times] over prior levels in waters off Japan, radiation risks due to these radionuclides are below those generally considered harmful to marine animals and human consumers, and even below those from naturally occurring radionuclides."

Marine scientists have calculated that, based on all the radioactive particles released (or leaking) from Fukushima, a dose due to this most recent nuclear accident would add up to a total of roughly one microsievert (a unit of radiation exposure) of extra radiation—roughly one tenth the average daily dose most Americans experience, one fortieth the amount from a cross–North America flight and one one-hundredth the exposure from a dental x-ray. This also means that no one in the U.S. should be taking potassium iodide pills, especially because there has been no radioactive iodine issuing from Fukushima for several years now. (Radioactive iodine has a half-life of just eight days, meaning that all of it was gone within three months of the March 2011 nuclear accident in Japan.)

Likewise, the debris from Fukushima that has begun to arrive on U.S. shores is also relatively benign. In fact, any radiation from the flotsam is likely to have far less an impact than the novel species it may carry with it across the Pacific, which could potentially spark a biological invasion.

What to really worry about
None of this is to say that radiation from Fukushima does not pose any dangers or that its spread should not be monitored. But the ongoing serious causes for concern revolve around the dangerous and delicate work of cleaning the nuclear disaster site—and the real health risks are being faced by the people in the immediate vicinity of the stricken nuclear reactors.

There is radioactive rubble to contain or dispose of, undamaged fuel rods to be safely removed (and discarded), and an unknown amount of melted nuclear fuel to contain. Contaminated soil in regions surrounding the nuclear power plant must either be removed or the area should be turned into a de facto nuclear “park,” much like Chernobyl’s exclusion zone in Ukraine and Belarus or the Rocky Flats National Wildlife Refuge in Colorado, which is the former home of a nuclear-bomb making facility. Then there's the more than 100,000 metric tons of cooling water and groundwater contaminated with radioactive particles being stored in tanks at the site, some of which is either leaking, reaches the sea naturally or is periodically released intentionally to prevent a flood.

In addition, if the melted nuclear fuel proves bad enough—like Chernobyl's lethal mass of molten core known as the "elephant's foot"—it will have to be entombed for a number of years rather than removed, because of radiation risk from what is essentially a cooled shell of ceramic armor surrounding a highly radioactive core that remains hot and is still undergoing radioactive decay.

Bottom line: until Fukushima has a sarcophagus entombing it or all the nuclear fuel has been carted away expect periodic reports of steam for years to come. But don’t worry about it reaching the U.S. As the NRC's McIntyre notes: "Advice for people on the west coast to buy radiation suits because of this steam is simply irresponsible."