Silicosis Caused by Rice Husk Ashes

Shuchun LIU1, Ning LIU2 and Jie LI3

1School of Public Health, Harbin Medical Univeraity, 2Institute of Industrial Health, Anshan Iron and Steel Complex and 3Jingdezhen Institute of Occupational Medicine

Abstract: Silicosis Caused by Rice Husk Ashes: hardly studied at all in China and other countries. Shuchun LIU, et al. School of Public Health, Harbin In this connection, we performed epidemiological, Medical Univeraity-The processing of rough rice experimental and etiological studies in order to pro (paddy) can produce rice husk which is about 20% of vide a scientific basis for preventing the adverse paddy. Rice husk is utilized as an energy resource in effects of RHA. many countries. Rice husk ash is used to a great extent as an industrial raw material in the ceramic Materials and Methods and metallurgical industries. A number of workers Epidemiological investigation are exposed to rice husk ash dust. Whether rice

husk ashes can cause pneumoconiosis and other ad We studied the RHA-exposed workers in a ce verse effects is still a problem awaiting solution. In ramic factory who were receiving a health examina this connection, we performed epidemiological, ex tion and chest X-ray2). The dust concentration, the

perimental and aetiological studies. Rice husk ash dust dispersion and the amount of free silica in the dust can cause silicosis characterized by diffuse in air in workshop were measured by the filter mem terstitial fibrosis and nodules. The free silica content brane method and the gravimetric method of Na increased as the treatment temperature for rice husk tional Criteria in P. R. of China3), respectively. rose, and so did cristobalite and tridymite content. When rice husk was treated at 1,300•Ž, free silica Experimental pathology study content amounted to 93.8%, of which cristobalite Exeperimental dust: The quartz dust, which con was 38.0% and tridymite 6.1%. These products are tains 98% free silica and 98% of which is less than the main agents in the development of diffuse pul

monary fibrosis and silicotic nodules. 5 gm in size, was provided by the Institute of Occu

(J Occup Health 1996; 38: 57-62) pational Medicine, Chinese Academy of Preventive Medicine. The rice husk dust, RHA at 350 °C, 650 °C and 1 Key words: Rice husk ash, Aetiology, Pneumoconi ,300 °C incinerated was ground in an agate osis, Free silica mortar into particles 95% of which were less than 5 gm in size. The dust was sterilized at 120 °C at high The processing of rough rice can produce rice pressure and diluted to dust suspensions with husk which is about 20% of the rice. It is utilized normal saline. as energy resource and its ashes are used as an in The method of administration : Seventy-two 3-4 dustrial raw material in China, Japan, India and month-old healthy male Wistar rats weighing 150 Thailand etc, especially in ceramic and metallur 180 g were randomly divided into the exposed gical industries'). Workers are therefore exposed to groups and the control group. The exposed groups rice husk ash dust. In general, it is considered that consisted of the quartz group, 350 °C group, 650 °C rice husk ash (RHA) is vegetable dust and safe so group and 1,300'C group. Each group was com that the exposed workers are not protected from posed of 12 rats. Every rat in the exposed groups RHA when they are working. Whether RHA can was intratracheally given 50 mg of the experimental cause pneumoconiosis and other adverse effects is dust once, and each one in the control group was still a problem awaiting solution. The problem is given 1 ml of normal saline. Three rats in each Received Jan 26, 1995; Accepted Nov 20, 1995 group were killed after the 1st month, 3rd month, 6 Correspondence to: S. Liu, School of Public Health, Harbin Medi th month and 12th month of the experiment for cal Univeraity, 199 Dazhi Street, Harbin, Heilongjiang 150001, pathological examination. People's Republic of China Gross and histopathological observation : We noted the general condition and body weight of each rat, were determined by atomic absorption spectro and the gross figures of lung and lymph nodes, the metry. lung volume and wet lung weight. Microscopic and electron microscopical examinations wre performed. Statistical Analysis The pathological specimens were stained with hema Two tailed Student's t-test and F test were used toxylin-eosin, van Gieson and Foof. Pulmonary fi for statistical evaluation. brosis was graded by King's pathological grading of Results lung fibrosis4). Epidemiological investigation Cytotoxicity study We investigated 16 workers in a ceramic factory The hemolytic toxicity test of erythrocytes for the who have been engaged in packing RHA into a rats5' 6) and the cyotoxicity test of alveolar macro saggar and screening RHA. All of them are RHA phages for the guinea pig7) were performed with the exposed workers whose occupational history, dust rice husk dust, 350'C-carbonized rice husk, 650'C-, exposure history and standing were obtained 850 C-, 1,1000C and 1,300 °C incinerated rice through interviews. Their average age is 54.7 and husk respectively. The dust suspension (2 ml) and thier maximal age is 56 year. All of them are male. red cell suspension (4 ml, RCS) were put into each We took chest X-rays and obtained thier previous test tube of dust ; RCS (4 ml) and saline (2 ml) chest X-ray films. According to the "Roentgen were put into the control tube for autolysis ; RCS Diagnostic Criteria and Principles of Management (4 ml) and Saponin (2 ml) were put into the con of Pneumoconiosis," 7 cases in pneumoconiosis cat trol solution tube. Each tube was incubated with egory I (3 in category I+), 3 cases in category II (2 shaking at medium temperature in an agitator for in II+) and 6 cases in category 0+ were found. 1 h and then was centrifuged at 2,000 rpm for 10 The average length of employment up to the occur min. The supernate (0.5 ml) from each tube to rence of pneumoconiosis was 21.3 years, but in one which saline (4.5 ml) was added was measured for case only 4 years. Roentgenographic abnormalities hemolysis optical density at a wavelength of 460 were mainly small round shadows in the early stage. mm by spectrophotometry. The percentage of he Then the small shadows increased, and became big molysis and the hemolytic index were calculated. and widespread, and developed into clusters. The The cell suspension was diluted with culture fluid X-ray in each case showed typical pneumosilicosis 199, Corp. of Pharmceutic Manufacture, Japan to 5 changes. (Figs. 1-2) X 105 cells/ml, which was counted in a hemocyto When the saggars were being cleaned, the mean meter and then put into the culture dish (1.5 ml/ concentration (geometric mean) of dust was 4.9 mg ml). After cells adhered to the bottom of the cul /m3 ; when RHA was being screened, the mean con ture dish, 1.5 ml of the dust suspension (500 ,ug/ centration (geometric mean) of dust was 19.3 mg/ ml) was put into the culture dish while the quartz m3 ; and when RHA was being blended, the concen or the medium alone was put into another culture tration was 12.8 mg/m3. The dust diameter was dish, as a positive control and the negative control, respectively. The morphologic changes in the cells were examined and the death rate for alveolar mac rophages was calculated.

Chemical analysis Rice husk dust was washed with distilled water three times and dried, and was ground into particles 95% of which were less than 5,um in diameter. Preparation of RHA : Lots of washed and dried rice husk were placed into a high temperature stove, and heated separately to 350'C (350'C group), 650 C (650'C group), 850'C (8500C group), 1,100'C (1,100'C group) and 1,300'C (1,300'C group). Measurement of free silica : The free silica content and the differences between the amounts of silica in the groups were measured by a pyrophosphate method 3) and with an X-ray diffractometer. The chemical components in the 1,100'C group RHA group and 350'C group were all significantly higher than that of the 1,300'C group (P<0.01). Lung organ factors: Organ factors for the lung were 0.85 in the saline group, 1.42 in the rice husk group, 1.27 in the 3500C group, 1.14 in the 650'C group, 1.70 in the 1,300'C group and 2.27 in the quartz group. All results for the exposed groups were significantly greater than that for the saline group. There was no significant deifference between the 1,300'C group and the quartz group. Gross observation : At an early stage, the surface of the lung in all exposed groups was smooth and soft. At the 6th month after administration, both the hardness and volume of lung were increased, and this was most obvious in the 650'C group and 1,300'C group. The pulmonary hilar lymph nodes at the 12th month were as large as green grams in Fig. 2. Pneumoconiosis category (II). the rice husk group and 350'C group, as large as soybeans in the 650'C group and larger than soy less than 5 mm in 80%. The percentage of free beans in the 1,300'C group. silica in the sedimentary dust was 72.3%. (Fig. 3) Microscopic examination : At early stage, intersti tial pneumonia and histiocyte proliferation were the Animal experiment main changes in each exposed group. We observed Wet lung weight: The wet lung weight means for dust foci, dust-cell foci and foreign body multi each group was 3.80•}1.03 g in the rice husk dust, nuclear giant cells in the rice husk and 350'C 3.56•}0.99 g in the 3500C group, 4.57•}1.60 g in groups, and found cell nodules with proliferated re the 650'C group, 5.44•}1.96 g in the 1,300'C ticular fibers in the 6500C and 1,300'C groups. At group, 2.28•}0.61 g in the saline group and 7.19•} a later stage, rice husk group revealed foreign body 2.45 g in the quartz group. The wet lung weight granuloma with the proliferation of reticular fibers. for all exposed groups was higher than that for the In the 350'C group, dust focus formation was the saline group. The wet lung weight for the rice husk main change, and lymph nodes at different stages from the 3rd month to the 12th month after ad minstration had dust foci with proliferated reticular fibers. In the 650"C group, proliferated and widen ed collagen fibers were noted. Compared with this group, extensive lung fibrosis and silicotic nodules could be seen in the 1,300'C group. Following King's pathological grading of lung fibrosis, the rice husk group and 350'C group were grade I at the stage of the 12th month after administration, the 650 C group was grade I at the 3rd to the 6th month and grade II at the 12th month. The 1,300 C group was grade I at the 3rd month, grade II at the 6th month and grade III at the 12th month. (Figs. 4-8) Electron microscopic examination : At an early stage, vacuolar degeneration of the lamellar body of Type II pulmonary alveolus epithelium and pro liferated Type II epithelial cells could be seen. At a later stage, hyperlastic fibroblasts were noted. In the 1,300'C group at the 3rd month after adminis tration and in the 650 C group at the 6th month after administration, we could see proliferated and collagen fibers. In the 1,300'C group at the 12th Fig. 3. An ash sieve. month after administration, obviously proliferated Fig. 4. At the 12th month, there were foreign body granuloma Fig. 7. At the 12th month, cell nodules with proliferated in the rice husk group. reticular fibers were noted in 650 C group.

Fig. 5. At the 12th month, the saline group was normal. Fig. 8. At the 12th month, extensive lung fibrosis and silicotic nodules could be seen in 1,300 C group.

Table 1. Hemolytic index of RHA

as the treatment temperature rose. Fig. 6. At the 12th month, 350 C group presented dust foci Cytotoxicity of alveolar macrophages : The obser with proliferated reticular fibers. vation of in vitro cytotoxicity of alveolar macro phages in guinea pigs gave different death rates, i. fibroblasts could be seen and collagen fibrosis was e., 7.6% in the rice husk group, 9.8% in the 350'C widely formed. group, 12.8% in the 650'C group, 21.2% in the 850 Cytotoxicity examination : The results are shown C group, 24.4% in the 1,100'C group, 26.7% in in Table 1. These suggest that the percentage of the 1,300'C group and 56.2% in the quartz group. erythrocytolysis and the hemolytic index increased Cvtomorpholocic chances : In the 1,300'C group, there were different sized cells with an unclear niosis induced by RHA were apparent as well as se fringe, and the cytoplasm and the nucleus of most rious. The results of pathological studies also cells were dissolved. The changes in the cytoplasm showed that the RHA heated at 1,300'C could and morphologic changes are major features in cause diffuse interstitial fibrosis and nodules. The other groups. When the reatment temperature is hemolytic toxicity and the cytotoxicity of RHA in decreased, few changes in cells could be seen. creased as the treatment temperature rose, and was similar to the quartz group. In brief, the epidemiol Chemical analysis ogical investigation, animal experiment and cytolog The chemical components of RHA in 1,100'C ical experiment suggest that RHA causes pulmonary are showed in Table 2. fibrosis. The free silica content in RHA at different tem It is considered that RHA is vegetable dust and peratures is shown in Table 3. harmless to the human body. Why did RHA have The amount of different crystalline states of silica such serious pathopoiesis? What components of in RHA under varying temperatures is showed in RHA caused such as changes? According to the Table 4. component analysis, the main component of RHA We found that the amount of both free silica and was silica at 92.1%. An other components were different crystalline states of silica had changed less than 0.5% and could not cause pulmonary fi after the rice husk was heated. The higher the tem brosis. In what form does the silicon of rice husk perature was, the greater the amount of free silica, exist? Can it induce silicosis? The component anal cristobalite and tridymite there was. ysis of rice husk showed that the silicon in rice husk exists in the form of polysilicic acid10). The rice Discussion husk which contained a great deal of polysilicic acid We found that there were patients with pneumo only induced local dust cells, and foreign body gran coniosis category I, category II and 0+ among uloma of the rats which were exposed to rice husk RHA-exposed workers. The changes in pneumoco dust for twelve months. No change in pulmonary fibrosis was found in the exposed rats. Therefore, Table 2. Chemical components of RHA the form of silica was closely related to pulmonary fibrosis8' 9). The experiments proved that when pol ysilicic acid in rice husk was heated, both the crys talline silica and the crystalline form of silica in if were changed. The polysilicic acid in rice husk was non-crystalline silica. As the treatment temperature rose, the non-crystalline silica became crystalline and the amount of crystalline silica gradually in creased, and in the crystalline state of silica, quartz Table 3. Free silica in RHA at different temperatures became tridymite and cristobalite. When rice husk was treated at 1,300'C, the free silica content was 93.8% in which cristobalite was 38.0% and tri dymite 6.1%. RHA at 1,300 °C became the inorgan ic dust which mainly consisted of crystalline silica, and crystalline silica caoused diffuse lung fibrosis and silicotic nodules.

We sincerely thank Enzhi Cang, Ruiguang Lin and Zhuoyang Lu for their cooperation and help. Thanks are Table 4. Crystalline states of silica in RHA also due to Beichen Liu and Cheng Wang whose help in cluded English writing.

References 1) Biger EC. Changing rice husk into power. Beijing: China Translating Publications Corporation 1984: 185-201 (in Chinese). 2) National Criteria in P. R. of China: Roentgenodiag nostic criteria and principles of management of not found. pneumoconioses, GB 5906-86, 1986. 3) National Criteria in P. R. of China: Methods for air Simesek J. Laboratory testing of biological activity of borne dust measurement in workplace, GB 5748-85, oremine dust : fibrogenicity, cytotoxicity, and he 1986. molytic activity. Environ Res 1981; 24: 140-151. 4) Belt T, King EJ. 1945 Spec Rept Ser Med Res Coun. 8) Gardend LU. Etiology of pneumoconiosis. JAMA Lond 1945; 250: 29. 1983;111:1925-1936. 5) Gou X. Hemolytic activity and cytotoxicity of white 9) King EJ. The action of different forms of pure silica carbon black. Occupational Medicine 1986; 13: 56 on the lungs of rats. Br J Ind Med 1953; 10: 9-17. 58 (in Chinese). - 10) Zou B. The method of diagonsis, analysis and meas 6) Song Z. Experimental study on the anti-hemolysis uring for nurishment of cultivated plant. Beijing: effect of Chinese medicine anti-silica tablet. Occupa Agricultural Publishing House 1976: 201-202 (in tional Medicine 1988; 15: 10-12 (in Chinese). Chinese). 7) David A. Hurych J, Effenbergerora E, Holusa R,