What amount of radiation is dangerous?

Worried about nuclear radiation from Japan's Fukushima nuclear plant spreading to your part of the world? Don't be.

Even in nearby Tokyo, the danger is still very low, experts say.

Despite the official reassurances, residents and tourists have been leaving the capital in droves. Ironically, many are leaving by plane – even though the typical flight exposes passengers to elevated levels of radiation.

The explosions and fires at the Fukushima nuclear plant on Tuesday night caused radiation in Tokyo to rise 10 times above average. But even at 10 times the normal level, it amounted to less than 1 microsievert.

Flying at 40,000 feet, meanwhile, exposes one to radiation of between 3 and 9 microsieverts per hour – and remember that it takes about 9 hours to fly from Tokyo to Vancouver.

(The sievert unit quantifies the amount of radiation absorbed by human tissues. One sievert is 1,000 millisieverts (mSv). One millisievert is 1,000 microsieverts.)

One microsievert is even less radiation than what one would receive from a typical dental x-ray, which emits about 10 microsieverts.

And even if a person were exposed to that level of radiation in Tokyo all year round, that's still about one-third of the radiation they'd receive from a single organ CT scan.

Radiation is everywhere and people are constantly exposed to it. Generally, in any given year, people are exposed to about 1 to 5 millisieverts of radiation. That's millisieverts, not microsieverts. Much of that exposure comes from the sun, from cosmic radiation from space, and from radon gas in our soil.

As a comparison, one X-ray gives off 400 to 600 microsieverts of radiation. A whole body CT scan gives a much higher radiation dose – about 15 to 20 millisieverts, while a single organ CT involves a dose of about 10 millisieverts.

Right inside the Fukushima plant, the radiation story is quite a bit different.

Radiation levels there reached 10 millisieverts an hour before falling to around 3 millisieverts, Kyodo News Service quoted Japan's nuclear safety agency as saying.

And early on Tuesday, the level peaked briefly at 400 millisieverts an hour -- 20 times the annual exposure for some nuclear-industry employees and uranium miners, who are advised to limit their exposure to 20 mSv a year. (The 400 mSv/ housr would be like getting 230 chest x-rays at once.)

In order to develop radiation sickness, one would have to receive a one-time dose of at least 1 to 2 Sieverts. That's 1,000 to 2,000 millisievert (mSv). Nausea and vomiting would generally occur within 24–48 hours after such an exposure.

A severe dose of radiation would be anything over 3,500 mSv, and would bring on vomiting within an hour, as well as perhaps diarrhea, bloody vomit, high fever and eventual hair loss. Severe exposure is fatal within a month about 50 per cent of the time.

In terms of cumulative exposure, 100 millisieverts a year is the lowest level at which any increase in cancer risk is clearly evident. A cumulative 1,000 mSv over a lifetime would be expected to cause a fatal cancer many years later in five out of every 100 persons with that kind of exposure.