We can’t feel when we are being exposed. Radiation is invisible.
Radiation has no color, taste, or smell. Therefore, people without measuring equipment are unable even to feel the exposure.
It harms human body from inside and out.
Radiation harms cells. When a body is exposed to a high amount of radiation, it can cause burns.
It is difficult to avoid penetrating radiation.
Gamma and neutron rays can penetrate almost everything. To block them you need a thick wall of concrete or lead.
If people are exposed to 4 to 7 Sievert (Sv) of radiation in an instant, a half of those people will die within 30 days. This is equivalent to the amount received by people within 1100 meters of the hypocenter in Hiroshima.
As the dose of exposure increases, DNA in cells suffers damages and elements in the body are more frequently replaced with radioisotopes. Even if the acute symptoms subside, the long-term risk of cancer and other diseases increases.
In Hiroshima, many people survived the immediate aftermath of bombing but succumbed to radiation effects after more than 10 years.