tae a specific concern with plutonium, and, to a lesser extent, other transuranics. Under a number of circumstances plutonium forms aerosols. The physical Character of these aerosols is such that on inhalation by humans they are teferentially depcsited in respiratory tissue. Because of slow clearance and becauseof their insoluble character, particles may experience long residence times in tissue. An aporeciable. mass fraction of the aerosol is usually associated with particles sufficiently large that smal] but physiologically significant volumes of tissue will be exposed to intense (i.e., organism lethal or greater) radiation doses within a meaningful Physiological time. Studies of the effects of intense local radiation to skin and kidney tissue indicate that despite the near mitotic sterilization of the involved tissue, an enhanced carcinogenic response may occur, in the conse that enercy dissipated in a limited volume may be far marecarcinogenic than if the same type of radiation were to ‘dissipate its energy over a much larger tissue mass. The question is then: do particulates of plutonium *zad to exposures that have enhanced carcinogenic potential? If they do, then present standards can be in error by orders of magnitude. = Notice that the emphasis here is on the anonialous hazard associated with a single particle; and that if any threshold is relevant, it is not a dese threshold since local exposures are large, but rather a possible volumetric threshold that must beexceeded by the physical extent of the exposure. Plutonium, as an insoluble aerosol-forming, long-lived alpha-emitter, constitutes a very special case of the low exposure problein. In conclusion, it is indefensible to base estimates of cancer risk on the method of dose averaging over fictitiously large volumes. Similarly, estimates based on non conservative interpretaticns of the Hanford beagle results are highly suspect.