Detecting ionizing radiation does not mean that a disaster has or will occur. Detecting radiation is generally a good thing if the radiation is expected, intended or natural. We and all living things on earth are naturally radioactive. This is due primarily to our requirement to have a healthy level of potassium and carbon in our bodies (and these are both naturally radioactive).
It is not difficult to measure small fractions of the radiation coming from natural sources. This includes alpha, beta and gamma radiation coming from radon in the air and all of its decay products. Similarly, the radium in the earth from the natural decay of uranium and thorium is also fairly easily measured. Radiation dose rates from natural background are readily measured with the right equipment. Dose rates above these values are again not difficult to quantify for gamma and x-ray radiation.
The average radiation dose a US citizen receives from breathing unfiltered air from the environment is over 200 mrem in a year due to natural radon. This is more than twice the limit an adult citizen is allowed to receive if they were to be working at a nuclear facility. A radiation worker can receive up to 5000 mrem in a year legally but they have special training to enable them to keep their doses as low as possible and how to be informed and monitor their doses to support this function. Typical radiation workers in the nuclear industry receive around 100 mrem per year from occupational exposure. Typical annual doses to airline pilots and stewardess are also in this range if they do enough transcontinental flights as the cosmic radiation increases substantially with altitude and flight duration.
Legal exposure levels to members of the public who are not working on a nuclear facility range from 10 mrem to 25 mrem per year depending on the regulatory body having jurisdiction. This is approximately the same level of exposure a person receives from the natural potassium in their body required to stay healthy. Potassium is both essential for life and a high energy beta and gamma emitter.
Scientific organizations specializing in this field report that the minimum radiation dose required to see any medical effects is around 10,000 mrem. This only holds if very large numbers of people in a community receive this or higher doses to be able to see a small but statistically significant increase in the probability of cancer. Legally an emergency responder can receive doses in this range if it is required in the process of saving lives and they are fully informed of the health risks associated with these efforts.
In order to see any medical effects for a single person from a radiation exposure requires receiving a dose of around 100,000 mrem. At levels this high, male fertility will start to decrease and the skin will react with reddening like a sunburn. Doses that are considered comparable to chemical exposures which are immediately dangerous to life and health are greater than 400,000 mrem.
All of these effects are typically attributable to receiving the dose over a short period of time. Many in the field believe the body's healing ability can easily repair from radiation exposures if they are in the range of environmental levels due to the natural healing process. This is consistent with the scientific results
Medical exposures can range as high as 1,000 mrem in some rare cases but on average, the annual dose citizens receive from medical services is similar to the dose we all receive from natural sources.