Apoptosis involves a highly organized and programmed series of events aimed at maintaining genomic stability by eliminating defective host cells. The purpose of this study was to determine the threshold doses and environmental UV-A and UV-B exposure times necessary to produce apoptosis and necrosis in the normal cells of a human fibroblast cell line. Environmental UV-A and UV-B doses were measured over a 6 year period with a four-channel UV radiometer. The fibroblasts were irradiated once using an Oriel UV Solar Simulator with six doses of environmentally-based UV. Doses corresponded to 0, 11, 19, 23 and 45 min of average environmental UV-A and UV-B radiation at solar noon in Puerto Rico. The Annexin-V binding method was used to differentiate between normal fibroblasts and apoptotic or necrotic fibroblasts. The threshold dose from apoptosis to necrosis was found between 24–28 kJ/m², which corresponded to 19 and 23 min of environmental UV-A and UV-B exposure. This study provides the first data that specify the environmental threshold doses of UV-A and UV-B at which human fibroblasts undergo apoptosis and necrosis. These results may provide valuable dose–response thresholds for apoptosis and necrosis for future mechanistic studies and baseline data for skin cancer prevention programs.