Soils producing crop plants that are toxic because of selenium are confined to small areas, but occur throughout the world. Such soils are confined to semiarid regions or areas of impeded drainage. They contribute no significant hazard to human health and only locally to animal health.

Environmental contamination with selenium is increasing, but will probably stay well below a hazardous concentration. Locally, mining and industrial wastes may produce minor hazards. However, the effect of added selenium in the atmosphere and waters in combination with other contaminants is not known and should be studied.