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Low doses of radiation are frequently inferred to mean doses of the order of 10-100 mGy, regardless of the end point or the nature of the irradiation, as if the energy were deposited uniformly through all targets. Yet, the fluence in a charged particle beam at which there is an average of 1 charged particle transit per target leaves 37% of the targets untouched. At an average of 3 transits per target, 5% of the targets experience no particle transits. Only at an average of 5 transits per target do 99% of the targets achieve one or more transits. One might consider this to be the lowest meaningful fluence for 1-hit detectors. For 2-hit detectors an average of 7 transits per target are required to achieve 2 or more hits in 99% of the targets. These conclusions arise from the cumulative Poisson distribution. To translate these numbers into the lowest meaningful dose requires consideration of target size and particle LET.