Despite Health Canada telling citizens that there is nothing to worry about following the Fukushima nuclear disaster, the lasting effects of exposure to that radioactive material still aren’t known. But there are many other sources of radiation that people encounter on a regular basis and whose potentially harmful impact on our health is too often downplayed, according to the former executive director of Physicians for Global Survival.

Dale Dewar, coauthor with Florian Oelck of the newly published book From Hiroshima to Fukushima to You: A Primer on Radiation and Health (Between the Lines), says that although the Fukushima fallout is still a concern, people need to be more vigilant about controlling the things they can when it comes to ionizing radiation.

“We really do not know the extent of the ionizing radiation that’s going to be reaching us [from Fukushima]; we are just watching the West Coast unfold,” Dewar says in a phone interview. “One of the really frightening things about ionizing radiation is that [effects such as] even most rapidly growing cancers, like leukemia and thyroid cancers, are not visible in an exposed population until several years after. Part of the reason we do not know the full extent is that nobody is officially monitoring it, and that’s a very scary thought.

“However, you need to protect yourself against carcinogens before worrying about Fukushima,” says Dewar, an associate professor in the department of family medicine at the University of Saskatchewan. “What are you doing to minimize your exposure to cigarettes or your exposure to the sun? What about radon? How are you adjusting or looking at those exposures? What about the medical tests you agree to have done?”

Diagnostic exams like X-rays, positron emission tomography (PET scans), and computerized (or computed) axial tomography (aka CT or CAT scans) are sources of exposure that people tend to overlook.

“I have patients all the time saying, ‘I need a CT scan of my head because I have headaches all the time,’ ” Dewar says. “The history of the headaches will tell you more than the CT scan. If you feel they [medical exams] are absolutely necessary for your health, then you have to expect the risk. But if you don’t even know that the risk is there, how can you make the choice?”

The amount of ionizing radiation in CT scans is a concern, “as is the ease with which physicians order them”, Dewar and Oelck note in their book: the dose is 100 to 1,000 times greater than from conventional X-rays, depending on the body part being examined. PET scans, meanwhile, have also been shown to increase the risk of cancer. In 2011, a team of researchers associated with the McGill college of medicine reviewed more than 80,000 patient records and found that the risk of cancer went up by three percent over the follow-up period of five years for every 10 millisieverts (mSv) of radiation exposure resulting from the imaging technique.

Although From Hiroshima to Fukushima to You isn’t exactly light reading, the book, as its subtitle implies, goes over everything you’ve ever wanted to know about radiation and then some, covering things like nuclear reactors, uranium mining, nuclear weapons, the transportation of nuclear materials, and the history and science behind radiation. Then there are chapters on the use of radiation in medicine and radiation’s effect on health.

Dewar explains that we’re all exposed to natural background radiation all the time, through the sun and rocks and even our food. But what’s important to remember is that those doses are cumulative over time.

There are two types of ionizing radiation that can affect the human body. External radiation is the kind that comes from X-rays, for instance: it alters molecules along its route through the body but no radiation remains in the body afterward. Internal radiation, meanwhile, occurs when a radioactive agent is ingested, injected, implanted, or absorbed through the skin. Take inhaled radon gas as an example. The radiation continues as long as the element is radioactive.

“Long-term, low-dose exposure usually doesn’t cause any obvious symptoms, but even though no immediate effect is evident, eventually some people will develop systemic illness, autoimmune disorders…cancer, and genetic effects,” Dewar and Oelck write. “Other effects may be even more insidious; some studies have indicated trends, not statistically significant, that deserve more follow-up,” such as high blood pressure and heart disease in people who’ve been exposed to radiation.

That’s why people having multiple medical tests should be aware of their lifetime exposures.

“Although expo-sure for a single dental x-ray is very low, when added to medical and industrial exposure, the subsequent lifetime dose may reach the lifetime occupational limit of 400 mSv,” the two write. “Knowing this, an individual might choose alternatives that do not involve radiation or, in some cases, choose not to have a test at all.”

Dewar adds: “We always say that levels are negligible or permissible, because it’s likely not going to harm us immediately and exposure is not going to show up in measurable statistical levels right away. But there is no safe level of radiation.”