Document ZBkZm47xqpoJYnqyBzyxV1BZL

Ionizing Radiation FlC 44-9 -- Deposition of materials inhaled into the bronchial tree, according to particle size m microns Adapted from Atomic Radiation, 1957 by RCA Ser vice Co, Inc 44-9) Fortunately, the lung dynamics are such that most of the particles of size greater than l/i are not retained to any degree, so that the probability of this occurring is low From the standpoint of measuring the concentra tion in air, the particle size does not enter into the problem because the air sample is only counted for total activity In terms of defining or assessing the hazard though, the particle size is important Calculations The degree of the nsk of the inhalation hazard may be estimated from a calculation of the concentration of the radio nuclide together with a knowledge of breath ing rates It can be assumed that a man breathes 10,000 liters (10 cu m) of air in eight hours In general, a man breathes 8 liters per minute at rest, 16 while seated, and 24 while walking A single breath equals about one-half liter One rough estimate of the hazard may be made on the basis that the material released has been uniformly mixed with the air in the room or building An alternate calculation may be made on the assumption that the re leased matenal has been limited to a small number of liters of air which the exposed person has then breathed The radioactivity concentration guides (RCG's) are based upon an average concen tration over a continuous exposure time (a lifetime of 40 hr per week or working time) The concentration at any one location vanes over a time penod as does the exposure time m the particular concentranon for employees whose work takes them into many different locations To find the average concentration, it is best to calculate a time-weighted average concentration The concentration which has been measured existed for the penod of elapsed time represented by the air-sampling time The time-weighted average concen tration is the sum of the products (each con centration found multiplied by the associated lapsed bme), divided by the total elapsed time of all of the sampling time penods Similarly, the time-weighted average con centration of a particular employee is the sum of the products (each concentration in which he worked multiplied by the associated time so worked) divided by the total elapsed time of the penod m question Thus, a single high exposure for a few minutes may average out to be far below the RCG if most of the re mainder of the time penod in question had a zero concentration In such calculations, the decay rate of the radionuclide is an important correction factor if there is a long time in terms of half-life penod between sample collection and analysis or if there is a very short half-life (See, for example, NBS Handbook 51, p 27, item 27 4) Guide doses External sources For whole-body expo sure at any age, a person's total accumulated absorption of ionizing radiation affecting cntical organs (m rems) should not exceed five times the number of vears beyond age 18, that is 5x(age-18), provided that no 13-week increment exceeds 3 rems Doses of ionizing radiation should not exceed those values given in Tables 44-C and-D The Atomic Energy Commission has adopted "Standards of Radiation Protection" which are published in their Manual, Chapter 1460