How much radiation is dangerous? Radiation is measured using the unit sievert, which quantifies the amount of radiation absorbed by human tissues. One sievert is 1,000 millisieverts (mSv). In the U.S., the average person is exposed to about 6.2 millisieverts a year, mostly from background radiation and medical tests. Some facts about radiation exposure: - A person would need to be exposed to at least 100 mSv a year to have an increase in cancer risk. Exposure to 1,000 mSv during a year would probably cause a fatal cancer many years later in five out of every 100 people. - A total body CT scan exposes a person to about 10 mSv. - A mammogram exposes a woman to about 0.7 mSv. - CT colonography is about 5 to 8 mSv. - A CT heart scan is about 12 mSv. - A typical chest X-ray involves exposure of about 0.02 mSv - A dental X-ray can be 0.01 mSv. - Coast-to-coast airplane flight exposes a person to about .03 mSv. Airline crews flying the New York-Tokyo polar route are exposed to 9 mSv a year. As we watch the nuclear crisis escalate in Japan, many of us have questions on radiation sickness — what are the symptoms and what are the long-term health effects? And should those of us in the U.S. be taking any extra precautions? NBC's chief medical editor Dr. Nancy Snyderman explains. Q: What are the main symptoms of radiation sickness? Radiation sickness (acute radiation syndrome, or ARS) occurs when the body is exposed to a high dose of penetrating radiation within a short period of time. The first symptoms of ARS typically are fatigue, hair loss, nausea, vomiting and diarrhea, as well as skin changes such as swelling, redness, itching and radiation burns. Symptoms may present within a few minutes to days after the exposure, and may come and go. This seriously ill stage may last from a few hours up to several months. Q: How much radiation does there have to be, and how long do you have to be exposed, to cause health problems? Most studies on radiation and cancer risk have looked at people exposed to very high doses of radiation. Consequently, it is harder to measure the much smaller increase in cancer risk that might come from much lower levels of radiation exposure. Some scientists and regulatory agencies believe that even small doses of ionizing radiation may increase cancer risk, although by a very small amount. The rule of thumb is that the risk of cancer from radiation exposure increases as the dose of radiation increases. Since everyone’s body is different, there is no definitive threshold dose at which you can be totally safe. Q: Is soap and water really enough to get radiation off you? Any person with contamination on their clothing or body should remove their clothes and shower. Soap and water can go a long way toward minimizing absorption through the skin and keeping local contamination from spreading. Q: What are the long-term health consequences of radiation exposure? Cancer is the most common long-term consequence of radiation exposure. Your bone marrow and your thyroid gland are especially sensitive to radiation. Research shows that some cancers types are more strongly linked to radiation exposure. These cancer types include leukemia, thyroid cancer, multiple myeloma, in addition to skin, lung, stomach and breast cancer. Young children are very vulnerable to the long-term effect of radiation because their cells are actively dividing as part of normal growth and development. Fetuses are also particularly susceptible to the effects of radiation, and mutations can occur if the radiation exposure happens during the early pregnancy. History shows us that genetic mutations can occur in adults exposed to nuclear radiation. These mutations can sometimes be passed from parent to child. Q: If the Japanese power plant continues to spew radiation and the wind blows it out to sea, how concerned should we be in the U.S. for our own safety? Not concerned because the amount of radiation is low and any residual will be dissipated by the jet stream before it hits the coast of California. Q: Iodine tablets are selling out in America; should we be stocking up? Are there any other steps we should take in the U.S.? There is absolutely no need to panic and right now and there are no special steps that experts are recommending. After a nuclear event, local public health or emergency management officials will tell the public if they should be taking potassium iodide or if other protective actions are needed. Remember, iodine prophylaxis is not one size fits all. If you have a seafood or shellfish allergy, a thyroid condition or certain skin disorders, you should not take potassium iodide before consulting with your doctor. More What are the symptoms of radiation sickness?