The terms maximum allowable concentration (M.A.C.) and threshold limit value (T.L.V.) differ in their respective meanings. The M.A.C. is a ceiling concentration whereas the T.L.V. is a time-weighted average of the concentration of the hazardous agent in the atmosphere. The value attributed to M.A.C. or T.L.V. will vary with the criteria decided upon for the response selected. This response may be a clinical or biochemical change in human subjects resulting from the hazardous agent.
An investigation of lead workers is reported, and from the data obtained from them and from their working environment recommendations have been made. The survey was carried out in printing works and storage battery factories; the order of development of abnormal biochemical findings was reconfirmed. The T.L.V. was determined for each stage of lead absorption by a graphic method. T.L.V.s for an eight to 10-hour working day were determined as 0·10 mg./m.3 to produce a urinary lead of 0·15 mg./l., 0·12 mg./m.3 for a coproporphyrinuria of about 50 μg./l., 0·14 mg./m.3 for basophilic stippling at the 0·3 per thousand level, and most likely 0·14 to 0·15 mg./m.3 for lead anaemia. The biochemical function chosen was an increased excretion of coproporphyrin, and to keep this below 50 μg./l. we would suggest that the T.L.V. should be about 0·12 mg./m.3 or a little more in the case of a 40-hour week.
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