Health hazards due to the inhalation of amorphous silica

Arch Toxicol (2002) 75: 625±634 DOI 10.1007/s002040100266 R EV IE W A RT I C L E R. Merget á T. Bauer á H. U. KuÈpper á S. Philippou H. D. Bauer á R. Breitstadt á T. Bruening Health hazards due to the inhalation of amorphous silica Received: 31 May 2001 / Accepted: 21 June 2001 / Published online: 29 November 2001 Ó Springer-Verlag 2001 Abstract Occupational exposure to crystalline silica dust is associated with an increased risk for pulmonary diseases such as silicosis, tuberculosis, chronic bronchitis, chronic obstructive pulmonary disease (COPD) and lung cancer. This review summarizes the current knowledge about the health e€ects of amorphous (noncrystalline) forms of silica. The major problem in the assessment of health e€ects of amorphous silica is its contamination with crystalline silica. This applies particularly to well-documented pneumoconiosis among diatomaceous earth workers. Intentionally manufactured synthetic amorphous silicas are without contamination of crystalline silica. These synthetic forms may be classi®ed as (1) wet process silica, (2) pyrogenic (``thermal'' or ``fumed'') silica, and (3) chemically or physically modi®ed silica. According to the di€erent physicochemical properties, the major classes of synthetic amorphous silica are used in a variety of products, e.g. as ®llers in the rubber industry, in tyre compounds, as free-¯ow and anti-caking agents in powder materials, and as liquid carriers, particularly in the manufacture of animal feed and agrochemicals; other uses are found in toothpaste additives, paints, silicon rubber, insulation material, liquid systems in coatings, adhesives, printing inks, plastisol car undercoats, and cosmetics. Animal inhalation studies with intentionally manufactured synthetic amorphous silica showed at least partially reversible in¯ammation, granuloma formation and emphysema, but no progressive ®brosis of the lungs. Epidemiological studies do not support the hypothesis that amorphous silicas have any relevant potential to induce ®brosis in workers with high occupational exposure to these substances, although one study disclosed four cases with silicosis among subjects exposed to apparently non-contaminated amorphous silica. Since the data have been limited, a risk of chronic bronchitis, COPD or emphysema cannot be excluded. There is no study that allows the classi®cation of amorphous silica with regard to its carcinogenicity in humans. Further work is necessary in order to de®ne the e€ects of amorphous silica on morbidity and mortality of workers with exposure to these substances. R. Merget (&) á T. Bruening Research Institute for Occupational Medicine (BGFA), Buerkle-de-la-Camp-Platz 1, 44789 Bochum, Germany E-mail: merget@bgfa.de Tel.: +49-23-43074546 Fax: +49-23-43074505 Keywords Non-crystalline á Amorphous á Silica, Silicosis á Bronchitis á Emphysema á Airway disease á Carcinoma T. Bauer Bergmannsheil, University Hospital, Department of Internal Medicine, Division of Pneumonology, Allergology and Sleep Medicine, Bochum, Germany Introduction H. U. KuÈpper á R. Breitstadt Degussa-HuÈls Corporation, Wesseling and Frankfurt am Main, Germany S. Philippou Department of Pathology, Augusta Krankenanstalten, Bochum, Germany H. D. Bauer Research Institute for Hazardous Substances (IGF), Bochum, Germany Recently, the American Thoracic Society has reviewed a great number of studies on the adverse health e€ects of crystalline silica (American Thoracic Society 1997). The most prominent e€ects of exposure to crystalline silica are silicosis, tuberculosis, chronic bronchitis/chronic obstructive pulmonary disease (COPD) and lung cancer. A review of the health e€ects of amorphous silica with particular reference to cancer has been published recently (McLaughlin et al. 1997). The authors concluded that epidemiological investigations for any potential cancer risk were not informative because the e€ects of 626 crystalline and amorphous silica have not been separated. In the same year, amorphous silicas were considered not classi®able with regard to their carcinogenicity in humans by the International Agency for Research on Cancer (1997). Both reviews focused on carcinogenicity. The present review concentrates on the de®nition, classi®cation, uses and pulmonary e€ects of amorphous silica and describes in more detail the data on synthetic amorphous silica not contaminated with crystalline silica. De®nition and use of amorphous silica Silica is the common name for silicon dioxide (SiO2). Silica may have a crystalline or a non-crystalline (amorphous) structure. In crystalline silica, the silicon and oxygen atoms are arranged in a ®xed geometric pattern. In contrast, in amorphous silica no spatial ordering of the atoms is present. The most common form of crystalline silica is quartz, but cristobalite, tridymite and others also have crystalline structures. Amorphous silica may be divided into (1) naturally occurring silica, (2) silica obtained under uncontrolled conditions, and (3) intentionally manufactured synthetic silica. 1. The most important naturally occurring amorphous silica is diatomaceous earth whose particles are the fossil skeletons of microscopic marine plants known as diatoms. Dust from uncalcined diatomaceous earth was reported to contain between 0.1 and 4% crystalline silica, whereas processing (particularly calcining) leads to contamination with crystalline silica such as cristobalite up to 60% (International Agency for Research on Cancer 1997; Hughes et al. 1998). Exposure to other naturally occurring biogenic (originating in living matter) amorphous silicas has been described in farmers during harvesting, crop burning or incineration (Rabovsky 1995). 2. ``Fused'' silica is silica heated to a liquid phase and cooled down without allowing it to crystallize (silica glass). The processing of these silicas leads to exposure to crystalline forms of silica. Contamination with crystalline silica occurs also in ¯y-ashes from power stations or silica fumes due to metallurgical processes such as the production of ferrosilicon. 3. The group of amorphous silica produced under controlled conditions may be classi®ed as: Table 1 Properties of synthetic amorphous silica. Pyrogenic and precipitated silicas are wet process manufactured. Tables 1±3 were adapted with minor modi®cations from Ferch and Toussaint (1996) (with permission) Property Speci®c surface area (m2/g) pH Primary particle size (nm) Aggregate size (lm) Agglomerate size (lm) Pore size (nm) i) Wet process silica, i.e. precipitated silica and silica gels ii) Pyrogenic (``thermal'' or ``fumed'') silica iii) After-treated silica, e.g. chemically modi®ed, surface-coated or physically treated silica. None of these intentionally manufactured synthetic amorphous silicas contain crystalline silica. The wet manufacturing process carried out in aqueous solution or dispersion (alkali metal silicate solution) may provide two di€erent kinds of synthetic amorphous silicas, namely precipitated silica and silica gels. Pyrogenic silicas are obtained by decomposition of a precursor from a vapour or gas phase at elevated temperature (Legrand 1998). All kinds of synthetic amorphous silicas can be after-treated either physically, chemically, or by surface modi®cation. The methods of after-treatment are various and depend on the product application (Ferch and Toussaint 1996). Depending on the manufacturing process, amorphous silicas have a wide range of physico-chemical properties (Table 1). The major applications depend upon the silica type (Table 2). Approximately 60% of precipitated silicas are used as ®llers in the rubber industry. Increasing amounts are used in tyre compounds for reduced rolling resistance and better wet-grip ``green'' tyres. They are used as free-¯ow and anti-caking agents for powder materials and as carriers of liquids which are transformed into free-¯owing powders, particularly in the manufacture of animal feed and agrochemicals. Toothpaste, paints, and silicon rubber represent further important applications. More than half of the worldwide pyrogenic silica production is used as reinforcing ®ller for silicon rubber, a particularly high and low temperature resistant elastomer with major applications in wires, cables and automotive components. High performance thermal insulation materials utilize the low heat conductivity of pyrogenic silica. These substances are also used as thickening and anti-setting agents in liquid systems of coatings, adhesives, printing inks, plastisol car undercoats, cosmetics and many other systems. The high purity makes pyrogenic silica a preferred carrier and free-¯ow agent for many pharmaceutical and food applications, for toners or ®re extinguisher powders. The estimated 1995 production of amorphous silica was about one million tons (Table 3). The table includes by-products generated in more or less uncontrolled procedures. About 2,400 subjects worldwide are exposed Form of amorphous silica Pyrogenic Precipitated Gels 50±400 3.6±4.3 7±50 <1 1±100 ± 30±800 5±9 5±100 1±40 3±100 >30 250±1000 3±8 3±20 1±20 not applicable 2±20 627 to intentionally produced amorphous silica at work (European Chemical Industry Council 1996). The number of users exposed to these substances is not known, but it is obviously large. Human data Only few studies have evaluated the e€ects of synthetic amorphous silica with respect to airway or lung diseases. Workplace concentrations were assessed in quite a number of studies, among them a few older ones. The ranges of the median total dust concentrations were reported to be <1 to about 10 mg/m3 (European Chemical Industry Council 1996; International Agency for Research on Cancer 1997). The following health e€ects of amorphous silica in humans are discussed in the literature: pneumoconiosis, chronic bronchitis and COPD, bronchiolitis obliterans (BO), and carcinoma. Pneumoconiosis Many older studies reported high numbers of workers with pneumoconiosis in the diatomite industry (Table 4). None of these studies can de®nitely di€erentiate between crystalline and amorphous silica. Recent studies in diatomaceous earth workers showed a low prevalence of radiographic abnormalities (Harber et al. 1998; Hughes et al. 1998). In one study, 5% of the subjects had profusion scores ³1/0 according to the classi®cation of the International Labour Oce of 1980, and the authors concluded that the lower prevalence of pneumoconiosis compared to (=nowadays lower prev.) earlier studies was due to modern dust control measures (Harber et al. 1998). The hypothesis that contamination with crystalline silica is causative for pneumoconiosis in diatomite-exposed workers is strengthened by the ®nding that exposure to natural diatomite (little contamination) was associated with simple ®brosis while exposure to calcined diatomite (high contamination) was associated with progressive pulmonary ®brosis (Smart and Andersen 1952; Caldwell 1958; Dutra 1965; Beskow 1978; Omura et al. 1978; Brambilla et al. 1980). In the few epidemiological studies on workers with long-term exposure to intentionally manufactured synthetic silica (precipitated or pyrogenic), no silicosis was found (Volk 1960; Plunkett and De Witt 1962; Wilson et al. 1979; Ferch et al. 1987a; Choudat et al. 1990). In one study, silicosis caused by amorphous silica obviously not contaminated with quartz was found in 4 of 28 workers (Mohrmann and Kann 1985). However, the authors cannot exclude contamination by small amounts of cristobalite, and detailed information about the amorphous silica origin is not included. In a further study, histological examination of lung biopsies of two subjects with exposure to amorphous silica and a clinical diagnosis of lung ®brosis disclosed non-birefringent material in the vicinity of ®brotic lesions, and birefringent particles were found to a much lesser degree (Philippou et al. 1992). One worker was also exposed to 1±3% of crystalline silica, and no exposure data were provided for the second worker. Chronic bronchitis and COPD Information about exposure to amorphous silica and the diagnosis of bronchitis or COPD is sparse. Ferch et al. (1987b) found obstructive and/or restrictive lung Table 2 Major applications of amorphous silica Form of amorphous silica Application Important properties Pyrogenic Silicone rubber reinforcement Heat insulation Rheology control (numerous liquid systems) Rubber reinforcement Free ¯ow, anti-caking Toothpaste: cleaning, rheology control Paints: matting Desiccant, adsorbent Paints: matting Toothpaste: cleaning, rheology control Surface area, purity, structure Aggregate size, purity Surface chemistry, aggregate/agglomerate size Particle size, surface area, structure Particle size, spherical form Aggregate/agglomerate size, particle size, structure Particle size, structure Porosity Particle size, pore volume Particle size, pore volume, hardness Precipitated Gels Table 3 Worldwide shares of amorphous silica products (estimation for 1995) Form of amorphous silica Production in tons ´103 Synthetic, produced under controlled conditions Pyrogenic 110 Precipitated 900 Gels 90 By-products of technical processes Silica fume and ¯y ashes 2000 Percentage of total volume Percentage of total value 10 82 8 35 50 15 628 Table 4 Epidemiological studies and case reports on occupational respiratory morbidity in workers exposed to amorphous silica (A.S.). Studies on subjects with exposure to amorphous silica not contaminated with crystalline silica are indicated. Only studies reporting the number of the total workforce and those examined were considered cross-sectional. (BAL bronchoalveolar lavage, PC pneumoconiosis, N.R. not reported, SMR standardized mortality ratio) Reference Study type Silica type Study population Exposure (duration)/ further exposure data Results Legge and Rosencrantz 1932 Bruce 1937 Case series Diatomite 108 Miners PC in 75% Cross-sectional Ferrosilicon alloy Vigliani and Mottura 1948 Case series Diatomite (production of ®lter-candles) 38 (plant 1) 26 (plant 2) 20 Workers in 2 factories Ebina et al. 1952 Cross-sectional Diatomite 106 Workers Detail of exposure N.R. 4±8 years 9±22 years Exposure duration N.R.; 400±500 particles/cm3, particle size 0.5±2 lm 24 Workers >10 years Smart and Andersen 1952 Motley et al. 1956 Case reports Case series Diatomite Diatomite 6 Workers 50 Workers Variable N.R. Caldwell 1958 Cooper and Cralley 1958 Case reports Cross-sectional Diatomite Diatomite 8 Workers 869 Workers 1±25 years 251 Workers >5 years Volk 1960a Case series Pyrogenic A.S. not contaminated with crystalline silica 215 Workers Motley 1960 Cross-sectional (selected) Case series Diatomite 98 Workers Precipitated A.S. not contaminated with crystalline silica Diatomite Ferrosilicon alloy 78 Workers Exposure duration N.R.; total dust: ®lling nozzle 15±100 mg/m3, bagging room 2±6 mg/m3, production room 3±7 mg/m3 Exposure duration N.R. 4.7 (1±16) years 1 Worker 10 Workers with PC 20 years in mill Variable Metallurgical plant 40 Workers 11±18 years ``at the previously acceptable atmospheric TLV'' for A.S. Plunkett and De Witt 1962a Dutra 1965 Swensson et al. 1971 Vitums et al. 1977 Case report Case series, follow-up of the 1937 Bruce cohort Cross-sectional PC in 24% PC in 19% PC in 65% PC in 11%; severe forms (I and II) occurred in 4 subjects exposed >15 years PC No correlation between lung function and radiographic appearance PC No PC in workers exposed to raw diatomite; PC in 48% with exposure to calcined diatomite No PC in 720 X-rays of 215 workers Severe changes of lung function in 6%, moderate changes in 14% No PC Severe PC PC validated in 1/10 cases X-ray ®ndings of 1937; in 9/10 cases, transient and due to other diseases PC in 11/40 cases; of 3/11 studied in detail, 2 had impaired lung function, 2 had biopsies showing ®brosis Cooper and Jacobson 1977 Diatomite Beskow 1978 Omura et al. 1978 Follow-up of the 1953±54 cohort Case reports Case series Wilson et al. 1979a Case series Brambilla et al. 1980 Cooper and Sargent 1984 Case reports Follow-up of the 1953±1954 cohort Precipitated A.S. not contaminated with crystalline silica A.S. in a silicon factory Diatomite Diatomite Diatomite 428 of 617 Workers with >5 yrs exposure 6 Workers 162 Workers 81 Controls 165 Workers 6 Workers 473 Workers Robalo-Cordeiro et al. 1985 Case series Ferrosilicon alloy 14 Workers Mohrmann and Kann 1985a Cross-sectional 28 Workers Ferch et al. 1987aa Case series Puntoni et al. 1988 Cohort mortality A.S. not speci®ed but not contaminated with crystalline silica Pyrogenic A.S. not contaminated with crystalline silica Refractory brick Choudat et al. 1990a Cross-sectional (selected) Precipitated A.S. not contaminated with crystalline silica 41 Workers, 90 controls Philippou et al. 1992 Case reports A.S. not speci®ed Checkoway et al. 1993 Cohort mortality Diatomite 2 Workers with lung ®brosis 2570 Workers Corhay et al. 1995 Blast-furnace 47 Workers 45 controls Spain et al. 1995 Cross-sectional (only 26.8% participated) Case report 1 Worker Harber et al. 1998 Case series Animal feed industry; A.S. not speci®ed Diatomite Hughes et al. 1998 Case series Diatomite 1809 Workers a 143 Workers 231 Workers 492 Workers Exposure groups: 30% >20 years PC in 4.7% with profusion 1/1 or more 3±20 years Exposure duration N.R. Mean 8.6 years; total dust <1 mg/m3±10 mg/m3 9±36 yrs All workers >5 years PC Mild PC in 18% No PC in 143 workers with serial X-rays; lung function/symptoms not associated with exposure PC PC in 2.3% with profusion 1/1 or more; ;PC not occurring before 20 years of service Mean 15 years 9/14 with dyspnoe; ®brosis in lung biopsies; BAL: lymphocytic alveolitis Mean 9 years; mean respirable PC in 4 workers dust: 1979, 1.23 mg/m3; 1984, 1.05 mg/m3 1±34 years No correlation between symptoms and exposure; no PC; impaired lung function due to smoking N.R. Excess of bronchial carcinoma; among silicotics excess of death, respiratory tract cancer (larynx), cardiovascular diseases, non-malignant respiratory diseases 8 (1±28) years; inhalable dust Questionnaire, blood gas analyses, 3 0±10.5 mg/m , respirable dust X-rays comparable; reduced 0±3.4 mg/m3 expiratory ¯ows not associated with exposure 12 and 15 years Histological investigation: ®brosis due to A.S. 4 (1±46) years Increased SMR for non-malignant respiratory diseases and lung cancer All >15 years Higher number of non®brous particles in BAL N.R. Bronchiolitis obliterans; silica in lung biopsy 14.4‹10.2 years 5% PC with profusion 1/0 or more; lung function not associated with exposure Complex exposure assessment Dose-response relationship for crystalline silica (PC) This reference did not consider A.S contaminated by crystalline silica 629 630 Table 5 Animal studies on inhaled amorphous silica (A.S.). Note that, in contrast to the human data in Table 4, only studies with amorphous silica probably not contaminated with crystalline silica were included. (N.R. not reported, n.s. not further speci®ed, BAL bronchoalveolar lavage, MMAD mass median aerodynamic diameter) Reference Animals Exposure Species n (controls) GaÈrtner 1952 Rabbits KlosterkoÈtter 1953 a E€ects Silica type Concentration Particle Size Maximal duration Bronchitis/ (mg/m3) (lm) of exposure Emphysema 50 (0) Aerosil (n.s.) N.R. 0.01±0.05 1100 days (5 days/week, 5 h/day) Rats 6 (0) Aerosil (n.s.) N.R. N.R. 300 days (7 days/week, 2±3 h/day) Schepers et al. 1957a Rabbits 10 (50) Pyrogenic silica About 53 About 0.02b 12 months (5 days/week, 8 h/day) Schepers et al. 1957b Guinea-pigs 50 (0) Pyrogenic silica About 53 About 0.02b 24 months (5 days/week, 8 h/day) Schepers et al. 1957c Rats 65 (0) Pyrogenic silica About 53 About 0.02b 24 months (5 days/week, 8 h/day) Schepers 1959 Rabbits 65 Precipitated A.S. 28 (controls n.s.) 135 364 about 0.02 24 months (5 days/week, 8 h/day) Schepers 1962 Monkeys 4 5 5 (15) Quartz 245 Fibre-glass 164 Precipitated A.S. 15 3 8 0.02 KlosterkoÈtter 1965 Rats 235 (0) 120 (0) Aerosil R 972 80 Standard Aerosil 45 About 0.02 0.01±0.05 27 months 8 months 12 months (5 days/week, 8 h/day) 12 months (4 h/day) Interstitial lung disease Macrophage Desquamative catarrh, signi®cant granulomas; no ®brosis emphysema, bronchiolitis obliterans (some) Small subpleural Low grade areas of atelectasis perivascular ®brosis in 3 animals Mural cellular Emphysema, in®ltration; some peribronchial deposition of cellular catarrh collagen; no radiographic PC Others/Comments One animal with purulent hilar lymph node Regression after discontinuation of exposure, but persistent minor focal alveolar mural collagen; high fatality rate not due to pulmonary e€ects No lymphoid tissue reaction; no disability of the animals Emphysema; Reversible bronchiolar periductal and and ductal stenosis peribronchiolar intra-alveolar accumulation of giant cells; some cellular in®ltration High fatality rate Complex tissue Reversal of emphysema due to emphysema in®ltration; some after discontinuation without bronchitis perivascular of exposure or bronchiolitis granulomas; some reticulum deposition Emphysema Radiographs showed E€ects dose-related, mottled shadows cardiac function suppressed also with which disappeared after discontinuation lowest concentration of exposure Emphysema Alveolar wall Lymph node enlargesclerosis; in contrast ment; vascular occlusion; pleural to quartz no PC adhesions; cor pulmonale Granulomas with Desquamative Lymph node catarrh, perifocal small number of enlargement; e€ects emphysema ®broblasts; some of standard Aerosil collagen formation; greater than with regression Aerosil R972 post-exposure Schepers and Dunnom 1981 Rats Total 270 Guinea-pigs (226) Rabbits Precipitated A.S. (Hi-Sil 233) Groth et al. 1981 Rats s. gel Per group: 80 (80) Guinea-pigs Per group: 20 (20) Monkeys Per group: 10 (10) 126 15 Precipitated A.S. Fumed silica 0.0225± 0.035b 24 months (8 h/day) Transient alveolar hyperin¯ation especially in rats 0.27 18 months 0.38 (5 days/week, 6 h/day) Cell aggregates in respiratory bronchioles 0.17 Macrophage accumulation in various tissues especially in guinea-pigs and rabbits; some reticulum deposition in interstitial tissues disappeared on cessation of exposure Macrophage and mononuclear cell aggregates mainly in monkeys; collagen ®bres mainly in monkeys, almost exclusively with fumed silica; early nodular ®brosis (rats and monkeys) Granulomas, macrophage aggregates; increase in ®brotic tissue Reuzel et al. 1991 Rats Per group: 140 (140) Aerosil 200 Aerosil R 974 Sipernat 22S Quartz Up to 31.0 34.7 34.9 58.5 0.012b 0.012b 0.018b 8b 13 weeks (5 days/week, 6 h/day) N.R. Lee and Kelly 1993 Rats Per group: 25 (25) Ludox 10.1 50.5 154 3.7 (MMAD)4 weeks 3.3 (MMAD)(5 days/week, 2.9 (MMAD) 6 h/day) N.R. Lewinson et al. 1994 Warheit et al. 1995 Rats 10 (0) Aerosil R 972 Up to 477 2.9 (MMAD)4 h N.R. 1/10 animals and 3/10 animals in 50 and 150 mg/m3 group showed silicotic nodules; no to minimal collagen ®ber deposition N.R. Rats Per group: 24 (0) Cristobalite 10, 100 3.4±3.6 N.R. N.R. Quartz 100 (Min-U-Sil) A.S. (Zeofree 80) 10, 100 Ludox 10, 50, 150 a 3.3±3.5 Several studies up to 4 weeks (5 days/week, 6 h/day) No radiographic signs of lung disease Lung function a€ected to variable degrees; more alterations in monkeys; fumed silica more potent Changes with A.S. reversed, but not with quartz; silicotic nodules only with quartz With the exception of few particle-laden macrophages no adverse e€ects with 10 mg/m3 Only gross pathology In¯ammatory markers in BAL less pronounced and transient with A.S. 2.4±3.4 2.9±3.7 (all MMAD) The number indicates the total number of exposed animals, with number of control animals in parentheses Additional information of the aggregate size was provided b 631 632 function impairment associated with confounding factors (smoking) but not with exposure to Aerosil, a pyrogenic amorphous silica (Ferch et al. 1987b). Choudat et al. (1990) reported a reduction of forced expiratory ¯ow in the group exposed to precipitated amorphous silica compared to a control group, but there was no correlation between the extent of exposure and pulmonary function. The authors concluded that smoking and exposure to amorphous silica have synergistic e€ects on the development of small airway diseases. Wilson et al. (1979) failed to show a signi®cant association between the degree of exposure to precipitated amorphous silica and the annual change in lung function. Bronchiolitis obliterans (BO) Recently, a case report of BO was published by Spain et al. (1995). An animal feed worker was exposed to a large number of agents (microorganisms, proteolytic enzymes, various organic substances) including possibly amorphous silica. No information about exposure to crystalline silica was provided. The authors suspected amorphous silica as the cause of BO because silica was found in an open lung biopsy. However, it is not mentioned whether the silica in the lung tissue was of crystalline or amorphous origin. Carcinoma Two cohort mortality studies in the diatomaceous earth industry (Checkoway et al. 1993) and a refractory brick factory (Puntoni et al. 1988) found an increased risk of bronchial carcinoma. However, neither study examined mortality by the type of silica (amorphous or crystalline) or by the exposure level, thus an independent e€ect of amorphous silica cannot be determined. Regulations The regulatory issue of silica exposure has been reviewed by the IARC (International Agency for Research on Cancer 1997). There is a tendency to set separate limits for the various kinds of amorphous silicas. For intentionally produced synthetic amorphous silicas, the exposure limits in di€erent countries vary between 4 mg/ m3 (Germany) and 10 mg/m3 (USA, France). The threshold limit value (TLV) for amorphous silica has been set to 10 mg/m3 of total dust in the USA, a value also assigned to nuisance dust (American Conference of Governmental Industrial Hygienists 1987). In Germany, two limit values for amorphous silica were stipulated at the end of the 1980s. The ®rst one of 0.3 mg/m3 for respirable dust applies to silica fume, calcined diatomaceous earth and silica produced under uncontrolled conditions (Deutsche Forschungsgemeinschaft 1989). For intentionally manufactured amorphous silica and uncalcined diatomaceous earth, the MAK (Maximale Arbeitsplatz-Konzentration, maximum workplace concentration) was set to 4 mg/m3 for the inhalable dust fraction. The TLV for the avoidance of skin and eye irritation has been set to even lower values (0.2 mg/m3, twice the value for quartz) (Ratney 1988). Whether these concentrations imply a health risk has to be shown by further epidemiological studies that are currently being performed in Germany. Animal experiments The health e€ects of amorphous silica with regard to carcinogenicity were reviewed recently (Lewinson et al. 1994; International Agency for Research on Cancer 1997; McLaughlin et al. 1997). The present review on animal experiments is therefore restricted to the adverse non-malignant e€ects of inhaled amorphous silica on the lung (Table 5). Short-term inhalation studies on rats with amorphous silica showed transient pulmonary in¯ammatory responses at 24 h, but not 8 days after exposure (Warheit et al. 1995). This was in contrast to crystalline silica that produced persistent neutrophil recruitment and cytotoxic e€ects. Long-term animal inhalation experiments performed with amorphous silica showed some di€erences between species. Inhalation studies in monkeys, rats and guineapigs with di€erent amorphous silicas for about 1 year at concentrations of 15 mg/m3 showed particle-laden macrophage and mononuclear cell in®ltrates together with collagen formation in monkeys, but to a much lower extent in rats or guinea-pigs (Groth et al. 1981). Di€erences between animal species were con®rmed in another study showing less macrophage reaction in rats than in guinea-pigs and rabbits, whereas guinea-pigs showed less alveolar hyperin¯ation (Schepers and Dunnom 1981). In addition, the location of macrophage accumulation di€ered between species, rabbits showing a more perivascular in®ltrate and guinea-pigs a more peribronchial pattern (Schepers and Dunnom 1981). Few studies compared the e€ects of di€erent amorphous silicas, but all found di€erences between substances (KlosterkoÈtter 1965; Groth et al. 1981; Reuzel et al. 1991). However, no speci®c product properties were de®ned that may predict adverse e€ects. There are two important consistent ®ndings. Firstly, emphysema or alveolar hyperin¯ation was present in many animal studies, and especially in rats, this was the cause of a high mortality. Interestingly, this process was partially reversible after discontinuation of exposure (Table 5). Secondly, in¯ammation and ®brogenic e€ects were less pronounced than following quartz inhalation (Schepers 1962; Reuzel et al. 1991; Warheit et al. 1995), and persistent or progressing silicotic nodules were not found after the discontinuation of exposure. Regression of granuloma and connective tissue formation after 633 discontinuation of exposure was found in all animals species. Lung clearance is probably an important factor that determines the occurrence of silicosis. In contrast to quartz, a number of amorphous silica products have been shown to be almost completely eliminated from the lungs of various animal species after discontinuation of exposure within months. Also, the ®nding of amorphous silica accumulation in the lymph nodes (KlosterkoÈtter and Einbrodt 1965; Reuzel et al. 1991) was at least partially reversible. The di€erence in clearance between crystalline and amorphous silica is not yet fully understood. Alveolar macrophages transport phagocytosed material from the alveoli to the lymph nodes (Lee and Kelly 1993; Lehnert et al. 1986). Amorphous silica have a surface area 10±1000 times larger than quartz and can therefore be expected to dissolve faster. This might explain that while amorphous silica is accumulated in macrophages and lymph nodes, it is eliminated much faster than crystalline silica (Pratt 1983; this study was performed with fused silica). Conclusions In summary, with the exception of few case reports with poorly described exposure quality, there is no evidence for a ®brogenic e€ect of intentionally manufactured synthetic amorphous silica to the human lung. Animal studies show no persistent silicotic nodules even in longterm inhalation experiments with high concentrations of amorphous silicas that are probably not encountered in workplaces (reported values in workplaces do not exceed 10 mg/m3). This contrasts with inhalation experiments using crystalline silica which clearly demonstrated such e€ects. Although some collagen formation has been described in animals exposed to amorphous silica, this is at least partially reversible after discontinuation of exposure. However, some studies describe a minor persistent interstitial collagen deposition. As the available information in humans is not sucient to de®nitely exclude a ®brogenic e€ect of amorphous silica in exposed workers, further epidemiological evidence should be obtained. Bronchitis, airway obstruction and emphysema were considered by few studies as outcome variables. Such e€ects in workers exposed to amorphous silica have been described, but the importance of confounders cannot be quanti®ed suciently in these studies. In¯ammatory responses and emphysema have been described in a number of animal studies, especially in rats and monkeys. Thus, parameters assessing bronchitis, airways obstruction and emphysema have to be considered in further epidemiological studies as primary outcome variables. Acknowledgements We thank Professor W. Mohrmann (Klinik fuÈr Berufskrankheiten der Berufsgenossenschaft der keramischen und Glas-Industrie, Bad Reichenhall), Dr. R. Schmoll, Dr. H. Ferch, and Dr. H.E. Toussaint (Degussa Corporation, Germany) for comments and help with technical questions. References American Conference of Governmental Industrial Hygienists (1987) Documentation of the threshold limit values and biological exposure indices. ACGIH, Cincinnati American Thoracic Society (1997) Adverse e€ects of crystalline silica exposure. 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