Document YG0B0XoXVjX3yEJ2p466xoywK

234 GNTER OBERDRSTER TABLE 1 M easured and Norm alized Lung Talc Burden of Rats at the End o f the 4 -Week Inhalation Study o f Talc Exposure concentrations, mg/m3 0 Control 2 (target) 2.3 (actual) 6 (target) 4.3 (actual) 18 (target) ' 17 (actual) Male n tig talc/lung (from NTP report) fig talc/lung per mg/m3exposure concn (% increase over low concn) Female n tig talc/lung fig talc/lung per mg/m3exposure concn (% increase over low concn) 5 4.3 1.6 -- 5 0.58 0.24 -- 5 81.6 2.1 35.5 4 56.5 1.6 24.6 5 186.0 9.3 43.3 (+22%) 5 127.2 9.3 29.6 (+20%) 5 846.0 + 5.5 49.8 (+40%) 5 546.0 35.2 32.1 (+30%) clearance m echanisms, and evidence has been accum u lating th a t these effects are toxicological im plications of a condition of lung "particle overload" (M orrow, 1988). Since th is concept is now widely accepted and since it has im plications for hum an extrapolation, results from chronic particle inhalation studies in rodents need to be carefully analyzed. T h e purpose of th is article is a c riti cal evaluation of the chronic N T P talc study with re spect to the overload concept. TH E NTP TALC STUDY Four- Week Talc Inhalation Study A subchronic 4-week inhalation study using concen tra tio n s o f 6 an d 18 mg ta lc /m 3 was perform ed prior to in itiating the chronic study (N T P , 1993). T h e purpose of this study apparently was to select appropriate exposure concentrations for the long-term study by avoiding effects associated with excessive particle loading of the lung, or, in other words, avoiding th a t a maximum toler ated dose (M TD ) is exceeded. It has been suggested th a t in chronic carcinogenicity rodent bioassays the highest dose should be at the M TD level with subsequent lower doses at one-half and one-fourth of the M TD (Haseman, 1985). W ith respect to inhalation studies with particles, participants at a recent NTP-sponsored workshop on M aximal Aerosol Exposure Concentrations in Inhala tion Studies (Lewis et ai, 1989) recom m ended the fol lowing: T he chronic study should not be perform ed at the highest technologically feasible concentration; three concentrations should be used of which only the highest should show some interference with lung defense m ech anism s, i.e., clearance im pairm ent; and th e two lower 0.016 S<a UJ 0.012 bd Uz< 0.008 at u< t- jI 0.004 - z<o f-- ^ 0.000 0u*. oo Q Tesi Tooef (recovery) O Tesi Toner (subctu.) OA Test Toner (chronic) X Diesel + TO? (Anatase) TiOj (Rutile) U PVC Cartoon black # AU Ox -- I----------------------------------------- j -- 0.01 o.l 1.0 10.0 100.0 RETAINED DUST VOLUME IN I.UNGS (gib FIG. 1. Pulmonary particle clearance rates and volumetric lung burdens of different retained particles in rats (adapted from Morrow et al., 1991). LUNG PARTICLE OVERLOAD 235 TABLE 2 A verage D aily D eposited D oses o f Inhaled Talc in R ats and M ice Estim ated by U sing Reported D eposition and Breathing Param eters for Normal Anim als and Talc Exposure Param eters Given in the NTP Report Species Exposure concn; mg/m3 Average daily deposition f i g Rat Males Females Mouse Males Females 6 18 6 18 6 18 6 18 28.1 84.3 21.4 63.4 0.98 2.94 0.91 2.74 concentrations should show no interference with clear ance and particle accum ulation. The determ ination of the highest chronic exposure concentration could be based either on model predictions or on results of a sub chronic study in which several exposure levels are tested. A dditionally, M uhle et al. (1990b) suggested to define a maximum functionally tolerated dose (M FTD) for chronic inhalation studies with particles which they a r bitrarily defined as a particulate lung burden causing a two- to fourfold prolongation of lung particle clearance. Thus, using results of a subchronic talc inhalation study for establishing a M T D for th e chronic study is m ost appropriate. Results of the 4-week talc inhalation study in rats showed indeed th a t "the ratio of lung bur den to exposure concentration was somewhat higher at th e 6 a n d 18 m g /m 3 exposure levels" (N T P , 1993, A p pendix F). Also, the N T P report continues: "T he in crease in talc burden with exposure concentration may have been because the maximum ability of the respira tory tra c t to clear p articles was exceeded a t th e 6 a n d 18 m g/m 3exposure levels." Despite this conclusion from the range-finding study, the subsequent chronic study was perform ed a t the two concentrations for which im paired lung clearance was detected in the 4-week study, i.e., the M TD may have been exceeded at both exposure concentrations. Thus, the purpose of the 4-week talc inhalation study remains unclear since the results apparently were not applied to selecting levels for the chronic study in rats. Table 1 shows these results after recalculation from the original data supplied in Appendix F of the N T P report (1993). Recalculation of the data was necessary because calcula tion of the normalized lung burden was done incorrectly in the report, although the right conclusions were drawn about im paired lung clearance. Table 1 shows th a t talc lung burdens in rats norm al ized to th e exposure concentration increased by 20 to 40% at the different exposure levels (the actual exposure concentration in the 4-week study was recalculated based on the inform ation in the N T P report). If lung clearance m echanisms for particles are not impaired, then lung particle accum ulation should be proportional to the exposure concentration and the lung burdens from different exposure concentrations normalized by the exposure concentration should not be-significantly different. Table 1 shows th at these normalized lung bur dens were different, between 20 and 40% more than p re dicted. To achieve the m easured 20% greater th an ex pected normalized lung burden after only 4 weeks of exposure the im pairm ent of lung clearance m ust have been very high, and this result of a 4-week study m ight have been very alarm ing with respect to selecting appro priate exposure concentrations for the long-term study. However, using an exposure duration of only 4 weeks as a range-finding study for a subchronic study makes it very difficult to get an accurate prediction of the accu m ulation kinetics. A longer 13-week study, although more expensive, would be highly preferable since it al lows for a far better evaluation and prediction of the long-term pulm onary accum ulation kinetics of particles. Nevertheless, according to the report, the 4-week talc study identified th e 6 a n d 18 m g /m 3 exposure levels as causing effects on lung clearance m echanism s in rats. T he results in mice from the 4-week study were less clear, only female mice exhibited a more th an 20% in crease in normalized lung talc burden at both the 6 and 18 m g /m 3 levels. Lung clearance will also be im paired from inhalation of more toxic particles such as crystalline silica, but in this case im pairm ent will occur a t much lower lung bur dens th a n those of low-toxicity particles, e.g., T i0 2 or toner. T h e question to be answ ered, therefore, is: Does TABLE 3 A ctual and Predicted Lung Talc Burden in Rats in the Chronic N TP Study (mg/lung) Months (mg/m3) 6 12 18 24 Males Found Predicted Found Predicted 3.15 2.36 12.95 7.08 5.38 2.76 25.74 8.29 Females 12.36 2.82 46.62 8.47 18.45 2.84 42.65 8.51 Found Predicted Found Predicted 2.44 4.52 8.66 9.23 1.78 2.08 2.13 2.14 8.39 13.58 27.49 29.81 5.99 6.23 6.37 6.39 236 GUNTER OBERDORSTER talc deposited in the lung behave like a cytotoxic particle or like a m ore benign particle, i.e., does th e lung overload condition develop w ith talc particles? Results of the pul m onary talc accum ulation of the chronic talc study should provide an answer to this question. For an understanding of these results it will be useful to briefly review findings from other chronic inhalation studies in rats w ith highly insoluble particles of low cy to toxicity. M orrow et al. (1991) evaluated respective studies with different particles on the basis of their re tained volum etric lung burden and their observed lung clearance rates. The use of the correlation between these two param eters is based on the volum etric lung overload concept (M orrow, 1988) which states th a t alveolar m ac rophages (AM) become im paired in their particle clear ance function if a certain fraction of th eir cell volum e is filled by phagocytized particles. T h at m eans, the volu metric rather than the gravimetric particulate lung bur den is of greater relevance to correlate w ith lung clear ance for a range of different particle types. Figure 1 illus trates this correlation between increasing retained volum etric lung burdens of different particle types and decreasing lung clearance rates. The Chronic Talc Inhalation Study The kinetics of talc accum ulation in lungs of both rats and mice during the chronic study deserve special a tte n tion. As discussed above, results of a num ber of studies FIG. 2. Observed and predicted talc accumulation in lungs of male and female rata during 2 years of exposure to 6 mg/ms (top) and 18 mg/m3(bottom). TABLE 4 Average Pulm onary Retention Halftim es and A ver age Clearance Rates for Talc in Rats Estim ated from M easured Pulm onary Talc Burdens in the Chronic N TP Study (mg/m3) T\n, days Clearance rate/day Males Females 6 18 6 18 300 300 250 280 2.31 X 10"3 ' 2.31 X 10"3 2.77 X 1 0'3 2.48 X 10-3 have show n th a t chronic exposure of ra ts to inhaled lowtoxicity particles at high concentrations will exceed the capacity of lung clearance m echanism s to effectively elim inate these particles from the alveolar spaces (e.g., M orrow, 1988; M uhle et a i, 1990a,b). T his overload phe nom enon occurs a t an average volumetric lung burden of retained particles approaching ~ 1 /d /ra t lung (see Fig. 1). In th e following p arag rap h s th is value will be used to evaluate the results of the chronic N T P talc study for retarded particle clearance, possibly indicating lung overload. An evaluation of the lung dosim etry needs to be perform ed for this purpose, including deposition and retention behavior of the inhaled talc particles. Deposition of particles of different aerodynam ic diam eters in rodent lungs has been studied by Raabe et al. (1977, 1988), a n d th e re su lts from th e ir stu d ies indicate th a t the particle sizes used in the N T P chronic talc study (2.7-3.6 ^m ) have a deposition efficiency in the ra t lung of ~10% of the inhaled dose and in mice lungs of ~2.5% of the inhaled dose. Using these values and assum ing an average body weight during the study for male and female rats of ~ 400 and ~ 2 8 0 g, respectively, a n d for m ice o f 34 g (m ale) a n d 31 g (fem ale), th e daily d ep o sited dose for th e tw o in h aled c o n c e n tra tio n s (6 a n d 18 m g /m 3) ca n be e s ti m ated using rat and m ouse specific breathing param eters. R esults of these estim ates are given in T a b le 2. Applying the norm al pulm onary retention halftim e for highly insoluble particles in rat lungs of ~ 7 0 days and assuming th at no impaired clearance of the depos ited particles occurred, the build-up of the talc particles over the exposure period of 24 m onths can be predicted from the daily deposited dose. The respective results are given in T able 3 a n d Fig. 2, together w ith th e actual m ea sured talc burdens in the rats during the chronic study. Under the assum ption th at no impaired particle clear ance occurred, the lung burdens of talc particles are pre dicted to achieve a n equilibrium afte r ab o u t 12 m onths of exposure in both male and female rats. In contrast to this prediction, the actual accumulation of inhaled talc was quite different in the chronic study exceeding the predicted values beyond 6 m onths of exposure and either approaching an equilibrium much later or not at all. This LUNG PARTICLE OVERLOAD 237 is indicative of a prolonged clearance of the deposited talc particles in the rat lungs com pared to normal condi tions. Estim ates of the pulmonary retention halftimes and the respective average pulm onary clearance rates for the talc particles which would best describe the ac tu al observed talc b urdens are p resen ted in T ab le 4. These estim ated pulmonary retention halftim es of the retained talc particles range between 250 and 300 days, which is m arkedly longer th an the norm al retention halftim e for highly insoluble particles in rat lungs of ~ 7 0 days. It can be concluded from this analysis th at at both the high- and low-exposure concentrations the pulmonary retention halftim e for talc particles compared to th at of other particles was increased, which may be indicative of lung overload as discussed earlier. It may be surprising th at the estim ated retention halftim es for both exposure levels are quite sim ilar, although the talc lung burdens between the two dose groups are quite different. How ever, it m ust be kept in m ind th a t the data in Table 8 are estim ates only of the retention halftim es and, unlike data reported for other studies, they are not based on actual m easurem ents of pulm onary particle clearance. Prolonged particle clearance in the lung is not neces sarily an indication of particle overload. For example, cytotoxic particles like S i0 2will significantly retard p ar ticle clearance in the lung at m uch lower lung burdens th a n th o se seen w ith m ore benign particles like T i 0 2. T o analyze this aspect further, the actual lung gravimetric burdens found in the chronic N T P talc study in the lungs of the rats (Table 3) were converted into volum et ric lung burdens of talc (specific density of talc: 2.8 g/ cm 3). T h e resu lts for th e ra t are given in T ab le 5 for th e two different exposure concentrations and sexes. Table 5 shows th a t already a t 6 m onths of exposure even in TABLE 5 Volum etric Lung Burden of Talc in Rats Found in the Chronic N TP Study (/d/lung) Months (mg/m3) 6 12 18 ' 24 " Males Females 6 1.13 1.92 4.41 6.59 18 4.63 9.19 16.65 15.23 6 0.87 1.61 3.09 2.23 18 3.00 4.85 9.82 10.65 the low-dose groups the volum etric lung burden of the re ta in ed talc particles approaches a value of 1 /il p er ra t lung, suggested by M orrow (1988) to indicate the begin ning of lung particle overload. By the end of the exposure, all of the exposed groups are clearly in the vol um etric overload range. It is helpful to view this finding in the context of other reported results. Figure 3 shows the results of several long-term particle inhalation studies summarized by M orrow et al. (1991) w hich were discussed earlier in th is p ap er (Fig. 1). T h e d ata of T ab les 4 an d 5, associated talc clearance rates and retained volumetric talc lung bur dens, and respective results from a crystalline S i0 2study (O berdrster et a i, 1994) are added to this Figure. T he following conclusions can be drawn: One is th a t th e datapoint for highly cytotoxic S i0 2 lies clearly outside the reported correlation curve for low-cytotoxicity particles. T h e o th er is th a t th e values obtained from th e present N T P talc study fit quite nicely into the datapoints of M orrow et al. (1991) in d icatin g th a t, indeed, th ese talc particles behave m uch like th e other particles of low cy totoxicity with respect to volumetric im pairm ent of lung clearance. 0.016 5Q W 0.012 5et w uz < 0.00 el<Jt U J 0 .0 0 4 - Zo-- H < 0.000 eut. --r~ 0.01 O # Oxi o O 0 Tes! Toner (recovery) O Tes) Toner (subchr I O A Teel Toner (chronic) X Diesel + TrO? (A nalase) m f < 0 (Rutila) PVC Carbon black Talc (NTP study) O S IO j (cristobalita) A o ? 0.1 LO 10.0 100.0 RETAINED DUST VOLUME IN LUNGS (pi) FIG . 3. Pulmonary particle clearance rates and volumetric lung burdens of different retained particles in rats including talc and the highly fibrogenic S i0 2(see Fig. 2). ' 238 GUNTER OBERDRSTER TABLE 6 Actual and Predicted Lung Talc Burden in Mice in the Chronic N T P Study (mg/lung) Months (mg/m3) 6 12 18 24 6 Actual Predicted 18 Actual Predicted Males 0.07 0.17 0.07 0.08 0.23 1.41 0.22 0.23 0.10" 0.08 1.91 0.23 0.75 0.08 4.97 0.23 6 Actual Predicted 18 Actual Predicted Females 0.10 0.11 0.07 0.07 0.26 0.97 0.20 0.22 Error in the N TP Report? 0.31 0.07 1.75 0.22 0.74 0.07 5.53 0.22 Sim ilar analyses were applied to the results of the chronic mouse talc study. Normal pulm onary retention data for highly insoluble particles in mouse lungs are docum ented by Kreyling (1990) who reported respective retention halftim es of about 55 days; this m eans th at an equilibrium lung burden during a chronic particle inha lation study would be reached after about 300 days of exposure. Table 6 shows the lung burden data, measured and predicted values, in lungs of the talc-exposed mice. No equilibrium lung burden is reached during the course of the 2-year exposure, very sim ilar to the rat lungs over loaded w ith talc particles (Fig. 2), and the m easured lung doses are considerably higher than would be predicted if lung clearance would not be prolonged. Using these data of pulm onary talc accum ulation in the mice, average pulm onary retention halftim es and av erage clearance rates were calculated and the results are given in T ab le 7. T h e results clearly show th a t mice also exhibited a m arked increase in pulm onary retention halftim e for talc particles with increasing lung burdens, i.e., a severe retard atio n of norm al A M -m ediated p a rti cle clearance, com pared to a norm al retention halftim e in mice lungs of ~ 5 5 days (Kreyling, 1990). It is of in te r est to note th at mice appear to be more sensitive toward overload effects in the talc study as far as im pairm ent of particle clearance is concerned; with respect to fibrotic and tum origenic effects, th e opposite is true, i.e., talcexposed mice did not show these responses. T his is con sistent with findings in mice from other studies with different p articles (i.e., T i 0 2, carbon black, diesel e x haust) in which particle-exposed mice showed a lower pulm onary inflammatory and fibrotic response than rats an d mice did n o t develop lung tum ors (H einrich et al., 1986; M uhle et al., 1990a; M auderly, 1994). SIGNIFICANCE OF CARCINOGENICITY OF INHALED TALC OBSERVED IN RATS The discussion of the chronic N T P study in the pre ceding paragraphs can be summarized by stating th at lung particle clearance in both rats and mice was im paired resulting in altered accum ulation kinetics of talc particles chronically inhaled at concentrations of 6 and 18 m g /m 3. However, th e finding of greatest concern in these studies is the significantly increased incidence of lung tum ors in female rats. An epidemiological study in pottery workers with exposure to talc also showed an in creased lung tum or incidence, i.e., increased sta n d a rd ized m ortality ratio (SM R ) (T hom as a n d S tew art, 1987; Thom as, 1990). However, these results are very difficult to interpret in term s of their biological plausibility: T he SM R for lung tum ors was higher in workers exposed to nonfibrous talc as opposed to those exposed to fibrous talc (containing asbestiform fibers), and the latency pe riod after nonfibrous talc exposure may have been as low as 5 years of exposure. In addition, the predom inant exposure of the pottery workers was to S i0 2 and other m inerals as well as num erous substances including chro mium and other m etals which were also present in the exposure atmosphere. Smoking status was not evalu ated. Thus, no definitive conclusions can be drawn from this study with respect to pulm onary carcinogenicity of talc in exposed hum ans. W ith respect to the experimentally observed in creased lung tum or incidence in female rats, some other studies have also reported a greater pulm onary carcino genic response in female rats than in male rats when ex posed chronically to particles, e.g., Sb20 3 (G roth et al., 1986), volcanic ash (W ehner et al., 1986), or a prevalence of m alignant tum ors in females was found compared to m ales, e.g., T i 0 2 (Lee et al., 1985), although T i 0 2-exposed males also showed a high incidence in adenom as which probably will develop into m alignant tum ors. O ther studies did not find a sex-specific tumorigenic re sponse and lung tum ors were induced in both male and female rats, e.g., diesel soot and carbon black (M auderly TABLE 7 Average Pulm onary Retention H alftim es and Aver age C learance Rates for Talc in Mice Estim ated from Observed Pulm onary Talc Burdens in the Chronic NTP Study (mg/m3) T,n , days Clearance rate/day Males Females 6 18 6 18 380 850 400 1000 1.82 x 10"3 8.15 X 10~4 1.73 X 10-3 6.93 x 10 * Particle Exposure LUNG PARTICLE OVERLOAD AM Activation Acute Inflammation Impaired Clearance Particle Accumulation 239 Tumors- Metaplasia --------- Mutations - -Epith. Cell Proliferation " (Hyperplasia) X . Chronic Inflammation Fibrosis FIG . 4. Hypothetical sequence of pulmonary events in rats induced by chronic exposure to high concentrations of highly insoluble lowtoxicity particles (AM, alveolar macrophages). et al., 1987; H einrich et al., 1992). T h u s, th e tu m o r re sponse to high particulate lung burdens cannot be viewed as a sex-specific phenom enon. Perhaps the prev alence of tum ors in female rats in some studies can be explained by an earlier response in females com pared to males. T here may be only a gradual rather than principal difference between males and females regarding tum or induction, yet further studies are needed for clarifica tion. Hyperplastic and interstitial fibrotic responses oc curred in both sexes of talc-exposed rats to a significant degree, and these responses may be m echanistically linked to tum origenesis (Dungw orth, 1994). A chain of events can be postulated starting with inflammatory processes which develop into chronic inflam m ation m aintained by high burdens of particles accum ulating in the lung due to im paired clearance; inflam m ation-asso ciated target cell proliferation and potentially inflam m atory cell-induced m utational events may result in m etaplasia and the eventual development of tum ors (Fig. 4). It appears th a t these m echanistic linkages are especially prevalent in rats but do not occur to the same degree in other laboratory rodents which respond with a much lower degree of inflam m ation and hyperplasia to inhaled particles (D ungw orth, 1994). Is the tum origenic response in the talc study a conse quence of im paired lung clearance, i.e., lung overload, due to m echanism s suggested above? We should recall th at im paired particle clearance per se may not always be a sym ptom of lung particle overload. For example, as was show n in Fig. 3, exposure to S i0 2 particles clearly leads to a prolongation of particle clearance at much lower lung burdens (either gravim etric or volumetric) than those of other more benign particles. Likewise, S i0 2 has been shown to induce lung tum ors in chronic rat inhalation studies at much lower concentrations (M uhle et al., 1989) th a n th o se inducing tum ors w ith more benign particles, i.e., T i 0 2 or carbon black; thus, with respect to particle-induced lung tum ors we should evaluate not only an effect on particle clearance but also the cytotoxicity of the inhaled particles. W ith respect to a particle effect on lung clearance talc resembles more the nuisance dust T i0 2 and not the cy totoxic S i0 2 (Fig. 3). Obviously, the im pairm ent of p a r ticle clearance is highly im portant since it contributes significantly to the increase of dose in the lung in general and in specific target cells, and the observed tum or-re sponse of talc is likely to be a secondary effect m echa nistically attributable to the chronic inflammatory and cell-proliferative events (Fig. 4). Although the basic u n derlying m echanisms and pathogenic sequence of p arti cle-induced lung tum ors may be quite sim ilar for cyto toxic (such as S i0 2) a n d m ore benign particles (such as T i 0 2)--i.e., chronic inflam m ation, cell p ro liferation, fi brosis, tum ors--one crucial difference lies in th e dose levels which induce the effect. Clearly, crystalline S i0 2 is m ost effective in th is regard, i.e., effects occur a t low lung burdens, w hereas talc m ay be com parable to T i 0 2. Although Fig. 3 im plies th a t talc particles fit very well into the category of low-toxicity particles, the clearance rates for talc derived for this figure are only crude esti m ates based on the pulm onary talc accum ulation data of the N T P report; it cannot be excluded th at the actual clearance rates might have been different. Furtherm ore, the results of the 4-week talc study indicate th at at lung talc burdens as low as 130-180 g significant effects oc curred on lung clearance function. T his would not be consistent with a low-toxicity particle but would be in dicative of a high-toxicity p article-- like crystalline S i0 2. To resolve this uncertainty a further evaluation of the study results is w arranted. A q u antitative com parison of the cell-proliferative and fibrogenic responses observed in the talc study with those found in other chronic p arti cle stu d ies ( T i0 2, to n er, carbon black) would be very u se ful for this purpose. However, such com parison would require a side-by-side evaluation of histological lung sec tions of the different studies. A nother sensitive indicator of a pulm onary response to particles in the alveolar space is the increase of P M N s in bronchoalveolar lavage as a m arker of inflammation to differentiate particles of varying cytotoxicity. R esults from M uhle et al. (1991) an d B ellm ann et al. (1991) from a chronic study in ra ts w ith to n er, T i 0 2, an d crystalline S i0 2 particles are com piled in Fig. 5. Added to th is figure are th e resu lts o f th e chronic N T P talc study on lung lavage PM N s for lung burdens m easured a t 24 m o n th s of exposure. T h e dose 240 GUNTER OBERDORSTER 60- aa ^ 40- za a. 20 o T oiw A no, O sto, X Control 0.01 0.10 1.00 10.00 Retained Pulm onary Dust (nl/lung) 100.00 FIG . 5. Increase of PMN in pulmonary lavage of rats in chronic particle inhalation studies (toner, T i0 2, and S i02: Muhle et al., 1991; Bellmann et aL, 1991; talc: NTP, 1993) as a function of the volumetric lung burdens of different particulate compounds. Data are expressed as percentages of the total lavaged cells rather than as absolute cell numbers since the lavage techniques used in the different studies might have been different. (Line was drawn through the points). p a ra m e te r is th e sam e as th a t used in Fig. 3, i.e., retain ed particle volume in the lungs. Two conclusions are obvi ous from th is graph: One is th a t talc fits extrem ely well into the overall dose-response curve for T i0 2and toner particles. T he other is th a t crystalline S i0 2 is very different from the rest of the particles, providing further evidence for the suggestion th a t indeed talc can be cate gorized sim ilar to T i0 2 and toner as a low-toxicity p ar ticle. F urther com parisons of the effects of talc with diverse particulate m aterial are scanty. Results of an in vitro study on the cytotoxicity of talc for macrophages showed th a t several different talc preparations were slightly m ore toxic to th ese cells th a n m agnetite (Fe30 4, used as surrogate for a nonfibrogenic dust), whereas crystalline silica was significantly more cytotoxic in this study (D a vies et al., 1983). Thus, w ith respect to the classification of talc particles we can conclude th at they have a significantly lower bi ological activity than crystalline silica and may be slightly more active than other low-toxicity particles, such as F e30 4or T i 0 2. A lower T L V for talc com pared to T i0 2or iron oxide appears to be justified (ACGIH, 1994). However, based on reported results of several chronic anim al inhalation studies and on our own study results, the present TLV of 5 m g/m 3 for the former category of nuisance particles (now term ed PNOC) in general ap pears to be too high and a lowering would be justified to avoid potential particle overload effects on lung clear ance. Extrapolation from results of rat studies to hu m ans using rat- and human-specific data for alveolar macrophage m orphom etry and num bers and for particle kinetics shows th a t on the basis of a volum etric overload effect a TLV of about 1 m g/m 3would prevent overloadinduced im paired particle clearance (O berdorster, 1994). T his value is for PNOC of u n it density, for PNOC of higher density the TLV could be higher. For particles of m aterials w ith greater biological activity the TLV should be lower than one would derive from their den sity. Justification for a lower TLV for PNOC is based on the following reasoning: Excess lung tum ors in chronic rat inhalation studies with particles occurred only when lung clearance was im paired and even then only in the higher exposure groups; excess tum ors were never seen when lung clearance rem ained unaffected. Since the rat appears to be more sensitive with respect to tum or re sponses after nonfibrous particle exposure than other species--including hum ans--the setting of occupational exposure limits aimed at preventing lung overload-im paired lung clearance would also protect from any other effects, including any potential tum origenic effect. Both concentrations of talc used in the chronic study resulted in lung overload and exceeded the M TD or M FTD. Since any highly persistent particulate com pound of low cytotoxicity has a carcinogenic potential in rats when chronically inhaled at high enough concentra tions the classification of such particles w ith respect to hum an pulm onary carcinogenicity m ust be carefully considered. A classification as "Animal Carcinogen" ac cording to the ACGIH (1994) categories for carcinoge nicity would be appropriate. Compounds in this category would not be likely to cause cancer in hum ans except under uncom m on or unlikely levels of exposure (AC GIH , 1994). T hus, the conclusion of the N T P report (1993) th a t there was clear evidence of carcinogenicity in female rats needs to be qualified by a statem ent th a t lung tum ors were m ost likely produced secondary to p ar ticle overload and related chronic toxicity. ACKNOWLEDGMENTS The author gratefully acknowledges the critical reading of this pa per and valuable suggestions by Dr. Paul E. Morrow. 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