Special environmental conditions exist for radiocarbon dating when samples originate in the immediate vicinity of sources of fossil carbon. A number of studies have been made for localities where the plumes of volcanic gases or exhaust gases affect plants growing downwind. A case in point is the Clear Lake area in northern California, where a large number of fumaroles enter a large but shallow freshwater lake. A study of shells growing in the lake has indicated that dating errors of greater than 10⁴ years are possible. The potential influence of high levels of dead radiocarbon on lacustrine food resources which were utilized by human populations some 10,000 years ago has been assessed.