free radical is an especially reactive atom or group of atoms that has one or more unpaired electrons; especially : one that is produced in the body by natural biological processes or introduced from outside (as in tobacco smoke, toxins, or pollutants) and that can damage cells, proteins, and DNA by altering their chemical structure. Free radicals cause lipid peroxidation, membrane damage, dysregulation of cellular processes, and mutations of the genome. In fact, reactive oxygen species can damage virtually any cellular component. Cell damage causes aberrations in ion homeostasis, cell signaling, and gene expression.