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N h. A September, 1956 cuntu. both could have been due easily to chance alone. A final word is in order concerning* the methods for developing data by which the environment is described--the conditions of exposure. As much variability can be, and is, encountered with this function as with the methods for laboratory experimentation and in-plant clinical examinations. The ac curacy and reliability of the analytical tech niques, the relation of the time and site of sampling to the true exposure, the presence of other agents which might modify the single Action of the toxin under study are some of the many factors which must be critically examined in the light of their usefulness for correlation with a given set of experimental or clinical findings. It is obvious that there is no single method or pattern of methods which can satisfy the varied requirements for estab lishing threshold limits. It is equally obvious that even though rigid standards were de scribed which would satisfy a discriminat ing jury of scientists, the available data for establishing threshold limits for all but a very few substances would fail to satisfy such limits. The real use for threshold lim its, as a guide for industry in the control of exposures and as a measure for action by governmental agencies, demands a con tinuing improvement in the quality and quantity of the methods- by which truly valid criteria may be achieved. Prepared Discussion. their needed contributions (which was far from Dr. Sterner's intention) or depress the more experienced contributors, it should be immediately pointed out that there are at least two practical and very helpful means of overcoming certain deficiencies inherent in the basic data. One is the safety factor, the other, the periodic re-evaluation of the threshold limit values. The safety factor has been built into most of the values in the threshold limits list.* The factors 2, 5, 10 or even greater have been applied to some values. There are, of course, some notable exceptions, such as the present value for trichloroethylene, for which the threshold limit is the absolute ceiling, but generally such instances are rare. In general, the greater the uncertainty in the data's appli cability to human industrial exposure, the larger the factor applied. This lowering of the limit value by an arbitrary safety factor may t times provoke some controversy, be cause now the value becomes one of opinion, not fact. He that as it may, the safety factor incorporated in the air standards gives in creased assurance of safety to many doubt ful values. The re-examination of the listed values by the committee provides annually for read justment of all values, whatever their sanc tity, upon submission to the committee of new and experimentally supported findings. Repeated scrutiny and re-appraisal of this sort can lead finally only to assignment of safe values on which complete reliance can be placed. The committee welcomes all such information.** HERBERT E. STOKINGER. Ph.D. Need for more Data Chief, Toxicological Services Public Health Service ,, Cincinnati. Ohio TN Tins connection another point implied x in Dr. Sterner's discussion should be strengthened--namely, the need for more data substantiating the choice of safe ex posure levels based on industrial experience. Dr. STERNER has discussed the great diffi Much useful information is undoubtedly in culties attendant on developing ade the files of many plants. Indication of this quate data for threshold limits, the manywas the fine evidence on six industrial sub- Imponderables in their interpretation, and their unsatisfactory and necessarily always The iMsrjwntAl safety factor in the UtmkoU limit value*. although ai|dd at Urns because of uncertainty incomplete nature. All these considerations should certainly be thoughtfully considered and strongly stressed, because errors in in tit* value u rckud to human exposure. often actually provide* an appreciable margin of safety, For this Mason th correctness of tba term "thrahaU limit" may b* uueetloned^lt mldit more properly bo replaced with "*l* judgment cannot be afforded. But lest these many considerations seem so formidable to many potential investigators as to prevent hygiene standard/* ` ""Allan Coleman. Chairman Threshold Limits Commit tee, ACCIH. Connecticut State Department of Health. Hartford l. Connecticut. . 4 Industrial Hituiruc Quarterly 285 stanch derived from many years of plant experience that enme to light last year at these meeting* from the presentation of Table I. Basis for Choice of Threshold Limit Value* Herbert J. Weber.1 Others should have similar material that should be brought to the attention l the Threshold Limits Com Study Tyre No. ( Llninn Percent Total I.Ulinf* 1233 } mittee. Like Webers material, nil of it need Auintnl 91 43 mt be ituvel r presented to show need for InduHiry 2l 21 changing existing limits; equal))* valuable Aoimul A Industry Man is 10 11 are data confirming existing limits. As never 'Krtiiratrd Cuwi" il S before, interest in the value of the control Anintal A Man < of industrial environments is being shared Source unr.truin 3t by management generally. Greater numbers ' in part on C*XK. W. A., lud. Mtd, J 4! 046, of industrial hygienists than ever before are nd from mtltritl of ThmlttU Limit* Com- * . being engaged by industry. Is it too much to duct A.C.C.I.K. lKvms. hope that meetings such as these will orient of other interesting facts: (1) that most of the thinking of properly placed industrial the values have some sort of scientific basis; hygienists to secure much needed plant in (2) that each level has been documented formation to aid in the choice of safe lim either by Warren A. Cook,4 or by the Com its of human exposure? mittee on Threshold Limits; (3) that the The type of information needed may be values based on animal experiments account listed as follows: il) Air concentrations for the largest number, 42^; but (4) that should be determined for the substances values having some industrial basis account under study through a complete cycle of for 1,3 of the total. plant operations and with reasonable regu The values ascribed to the "man" cate * larity iu order to ubtain a true picture of the gory arise from two sources--that of Nel range and lluctuations of exposure. (2) son, et al* and those more recent publica The data should have good accuracy. (3) tions of the Dow Chemical workers, Irish, The observations should be carried out over Rowe, Spencer, Adams et al. a reasonable period of time--a minimum of five years. (4) The air concentration data The "Educated Guess" should be correlated with a good medical A FEW words should be said in defense of program, A pattern Tor such work is that the "educated guess." A review of the of Dv. Steiner's 10-year study of workers values described as guesses indicates in the exposure to butyl alcohol,- instances in which sound information has There is a real need for more data based later become available that the "guess" was on industrial experience. The often-heard remarkably good. Two prominent examples statement "the threshold limits arc nothing only will suffice--hydrogen fluoride and but educated guesses" unquestionably re uranium. A safe exposure level for hydro flects the wish at legist that more data be gen fluoride was set at 3 ppm ou the very firmly based on industrial experience to sub limited evidence supplied by a study in ani stantiate the choice of limits. As a member mals by Ronzani in 19<W.' Last yenr a re of'the Threshold Limits Committee, 1 was port"1 culminating many years of study of concerned over the statement and took the fluoride exposure in the aluminum industry, trouble to review each substance In the involving thousands of air .and urine analy threshold limit list for 1955 as to the basis sis for fluoride and studies of roentgeno- for choice of the level. The results are shown graphic changes In bone, showed without in Table I. It is possible that everyone would question that air levels double the accepted * not arrive nt precisely the same figures, but limit gave rise to perceptible changes In I believe that their magnitude would not be bone in only a few individuals, and only much altered. Although the table shows that afte^ many years of exposure, thus vali the educated guesses account for & relative dating the wisdom of this "educated guess/1 ly small number, it does confirm the often In the ease of uranium, an engineering expressed feeling of the need for more bench-mark had to be "guessed" at early in solidly based levels. Table I shows a number the days of the Manhattan Project. After a 286 September, 1966 review ot the quite limited animal data on uranium then available, Dr. Stafford Warren suggested that the "safe** exposure level for uranium be the same as that for lead, 0.15 mg/cu.m. After $500,000 and many years had been spent in research, the safe levels of exposure to uranium compounds were found to bracket this value very closely. Levels for Cancerigcns "THERE is still one group of substances for which some method should be devised for establishing safe air standards--the indus trial cancerigens. How shall we establish the limits for this type of substance? Thus far the question has been sidestepped com pletely. As a result, with one exception, nickel carbonyl, limits taking into consider ation potential cancerigenicity have not been assigned. Several industrial substances are known or suspected Cancerigens; many more are suspect on the basis of animal ex periments. As a suggested method of ap proach, the following is offered: To the level judged safe for other types of systemic in jury add a safely factor for carcinogenicity. The magnitude of the safety factor is sug gested to be from 100 to 500. This provides at least a second power of 10, which, from the well-known dosage-response hypothesis, provides at least a fourfold longer Interval before effects may be expected to occur, or conversely at least a response with Vi the intensity. This manner of approach has been used for nickel carbonyl. A tentatively safe level for systemic effects from repeated dally exposure has been set at 0.1 ppm; one- hundredth this level, or 0.001 ppm was set for nickel carbonyl on the basis that nickel poisoning gives rise to a substantial in crease in the incidence of lung cancer. It is realized that unfortunately the safe lim its for all industrial cancerigens cannot be so readily resolved. This is especially true of dye intermediates, such as benzidine and naphthyl amine.-? whose major route of entry is not commonly via the lungs but through the skin and gastrointestinal tract. These nrc luundn' and protective equipment prob lems not solvable by air control. There are undoubtedly substances to which the suggested procedure may not strictly apply, but imperfect as it may be, the suggested method is felt to be a step in the right direction and serves better to curb exposures to industrial carcinogens than considering the problem too difficult to cope with at the present time. References ]. Wrxeit. H. J.: ThrelK-W Limit*, A fanal Dfocua- .WK/1 2. tiTKX.VKK. J. II.. C*nl!t`lt. H. C,, UkihKMVKC, H. F.. Cl'UCK, XI.: A Tun-Voar Study dI llutyl Akohol Ex- [voiMir*. AtttA Quart., 1949. t. Cuak. W. A.: Maximum Allowable CoiCntrationx of Industrial Aimonphwle Contaminants. 1*4. Med., UM5, . 4. Ncuoik. K. W,, Krk. J. Rost. M., WoooxaK, L. f... Sii.vkhmak. L.: Sensory Rntsunc* To Certain Indus trial Solvent*. J. Hug. A Tfixie.. 25:283, J J4X 5. RoNXAn'i. F-*. Ulm 4tr Einltua* dcr Eiiwtmunfien von raitendon Csaen der Industrial) xiif die KchuUkraff* dor OrennUmus uafcenUbar dcr Infcktiven Krankheitea. Arrh. f. ihv.. 78:217. JS00. <j. Ikww, D. A.: Clinical Finding* Which Can be An- ticipnlcd After Lory-Continued Exposure to Fluorides. Prenantni before Symposium on Fluorine. Kcttarinc Cincinnati. Ohio, May. ItH Engineering and Chemical Application of Standards *' ALIEN D. BRANDT, Sc.D. Bethlehem Steel Company, Bethlehem. Pennsylvania he why and how of threshold limits for To engineers engaged in industrial hy Tair contaminants have been discussed by giene, threshold limits provide the reference the preceding speakers. The logical nextline or bench mark upon which all considera and final consideration is the use or applica tions and calculations for the control of air tion of these limits in preventing occupa contamination are based. It is characteristic tional diseases, in avoiding complaints from of engineers and of others in the physical exposed employees, in maintaining employee sciences to want to reduce to numbers the efficiency, and in promoting good house problems with which they deal,, because only keeping. - then can they be attacked in a precise man-