When it comes to medical applications, there don't seem to be specific guidelines that tell us how much radiation is too much. Further, a definition of "too much" could vary from patient to patient.
For example, the patient who receives accurately targeted radiation therapy to destroy a tumor will get much higher doses than someone who gets a CT scan. That amount of exposure is just right for the cancer patient at that moment, but if a healthy person was exposed to that much radiation in general, it could be too much. At an extreme, medical overdoses are called radiation poisoning or acute radiation syndrome.
A combination of the amount and frequency of exposure, part of the body, and period of time are the factors that help define whether there has been overexposure. So, for example, several dozen mammograms over a lifetime may not be problematic, while a dozen scans in a year could overexpose a patient.
In some cases, "too much" is a result of errors. Those high doses being used for radiation oncology purposes can be harmful if the beam isn't accurately targeted. Patients may be overdosed when radiation testing equipment has not been correctly calibrated, or when a human being makes a mistake inputting dosage settings.
Money may be a factor, too. In profit-driven medical systems, or where defensive medicine is practiced, studies show that patients are given more of these radiation-based tests than in areas where those aren't factors. Those extra tests could cause overexposure to radiation.
What Happens If We Are Exposed to Too Much Radiation?
Too much exposure to radiation causes both short- and long-term problems.
When the body is over-radiated, then healthy cells and tissues are destroyed. There may be symptoms that appear within a short time after overexposure (hours or days) such as nausea, vomiting, diarrhea, fainting, hair loss, skin burns or flaking skin, hair loss and others.
Too much exposure to radiation over a period of time can lead to chronic illnesses such as cancer, eventually resulting in death. Once study showed that 15,000 Americans die each year from radiation exposure during their lifetimes from natural and other sources. That same study showed that most doctors, including radiologists, do not understand the dangers of overexposure to radiation.
Extreme overexposure all at one time (which can happen when machinery malfunctions) can cause death.