In the case of relatively long irradiation times in the range of minutes, as well as low radiation intensities (< 50 mW/cm²), photochemical effects may predominantly be triggered in the tissue. Here, the energy of the incident optical radiation is converted to chemical reaction energy. These effects can be seen to dominate in the UV and short-wavelength visible spectral ranges. In this respect, certain biological molecules absorb the incident optical radiation, are thereby stimulated, and transfer their energy to oxygen molecules. This creates a highly reactive form of oxygen (singlet oxygen). This oxygen attacks the surrounding tissue and creates free radicals that are also highly reactive and can damage the surrounding cellular molecules, such as proteins or the genetic material deoxyribonucleic acid (DNA). Optical radiation in the short-wavelength UV spectral range also has sufficient photon energy to cause direct damage to the DNA by splitting chemical bonds and linking DNA components differently. Damaging the DNA in this way can cause cancer.
Some chemical compounds and drugs may sensitise the biological tissue for the photochemical effect of optical radiation. This may cause severe biological reactions which are known as "phototoxic" reactions.