Under what condition does radiation become harmful?

Radiation becomes harmful primarily when it leads to ionization of atoms in biological tissues. Ionization refers to the process where radiation interacts with atoms, causing them to lose or gain electrons and thus become charged ions. This can disrupt molecular bonds in cells and lead to cellular damage, which may result in mutations, cancer, or other biological effects. The extent of harm depends on the type of radiation, the dose, the duration of exposure, and the sensitivity of the biological tissues involved.

The other options do not accurately represent the conditions under which radiation is harmful. Radiation emitted from non-radioactive sources does not involve ionization; instead, it is typically less energetic and not as biologically damaging. Low levels of radiation exposure do not necessarily cause harm; in many cases, the body can tolerate low doses without significant risk, implying that risk is more about the quality and quantity of exposure rather than just the presence of radiation. Protective materials are designed to absorb radiation and minimize exposure; therefore, their absorption of radiation is a protective measure rather than a harmful effect on biological tissues.