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J Res Health Sci. 22(2):e00550. doi: 10.34172/jrhs.2022.85

Original Article

Risk Assessment of Silicosis and Lung Cancer Mortality associated with Occupational Exposure to Crystalline Silica in Iran

Nafiseh Nasirzadeh 1, Zahra Soltanpour 2, Yousef Mohammadian 2, *, Farough Mohammadian 3
1Department of Occupational Health Engineering, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran
2Department of Occupational Health Engineering, Faculty of Health, Tabriz University of Medical Science, Tabriz, Iran
3Department of Occupational Health Engineering, Environmental Health Research Center, Research Institute for Health Development, Kurdistan University of Medical Sciences, Sanandaj, Iran
*Corresponding author: Yousef Mohammadian (PhD) Tel:+98 09141243406 Email: Mohammadian_yosef@yahoo.com

Abstract

Background: Exposure to crystalline silica has long been identified to be associated with lung diseases. Therefore, the present study aimed to assess the risk of silicosis and lung cancer associated with occupational exposure to crystalline silica in Iran.

Study Design: It is a systematic review study.

Methods: Different databases were searched, and the Cochrane method was used for the systematic review. Thereafter, cumulative exposure to crystalline silica (mg/m3 -y) was calculated in every industry. The relative risk of death from silicosis was performed using Mannetje’s method. Based on the geometric mean of exposure, the lung cancer risk of exposure to crystalline silica was also calculated.

Results: As evidenced by the results, worker’s exposure to silica ranged from a geometric mean of 0.0212- 0.2689 mg/m3 (Recommended standard by the American Conference of Governmental Industrial Hygienists (ACGIH) was 0.025 mg/m3 ), which is generally higher than the occupational exposure limit recommended by National Institute for Occupational Safety and Health (NIOSH), ACGIH, and occupational exposure limits. The relative risk of silicosis was in the range of 1 to 14 per 1000 people, and the risk of lung cancer in workers ranged from 13-137 per 1000 people.

Conclusion: Since workers are at considerable risk of cancer due to exposure to silica in Iran, exposure control programs need to be implemented in workplaces to decrease the concentration of silica.

Keywords: Crystalline silica, Exposure, Lung cancer risk Systematic review

Copyright

© 2022 The Author(s); Published by Hamadan University of Medical Sciences.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Background

Crystalline silica is the second most common mineral, existing in more than 90% of the earth’s crust1; therefore, sand, rock, and soil have the most abundant crystalline silica.2 Workers are exposed to crystalline silica in mining, smelting, sandblasting, building, and glass industries.3 Previously published studies demonstrated that masons, plasters, and miners had the highest exposure to crystalline silica.4,5 The respiratory system is known as the primary pathway of exposure to crystalline silica dust.6 Exposure to crystalline silica has long been identified to be associated with lung diseases,7 such as silicosis which is a well-known lung disease.8

Exposure to higher concentrations of crystalline silica can cause “acute silicosis” which has a high mortality.9 “Chronic silicosis” is the most common form of pulmonary fibrosis among crystalline silica-exposed workers.2 The studies have suggested that pulmonary fibrosis can elevate the risk of lung cancer.10 International Agency for Research on Cancer (IARC) has introduced crystalline silica in the form of quartz or cristobalite as a human carcinogen.11 Workers are exposed to a low concentration of crystalline silica in workplaces; nonetheless, they are faced with a significant risk of cancer.12

There are an estimated 23 million crystalline silica-exposed workers in China13; moreover, over three million in India14 and over two million employees in the United States15 are exposed to crystalline silica. Annually, almost 800 workers die from lung cancer as a result of inhaling crystalline silica in Britain.16 The published studies have reported that workers in developing countries are exposed to crystalline silica in workplaces.17

Recently, risk assessment has become one of the most important aspects in the management of occupational diseases.18 There is a dearth of research on the assessment of the risk of lung cancer due to exposure to crystalline silica.18-21 Some studies have collected quantitative exposure data for the estimation of risk, for instance, Mannetje et al in IARC used a quantitative method for estimating the rate of silicosis mortality in six cohort studies and reported that the rate of silicosis mortality was above the risk of 1 per 1000 typically deemed acceptable by the Occupational Safety and Health Administration (OSHA).22 In the other study, Steenland et al examined lung cancer in 10 silica-exposed cohorts and indicated that the estimated excess lifetime risk of lung cancer for a worker exposed from age 20 to 65 at 0.1 mg/m3 crystalline silica (the permissible level in many countries) was 1.1%-1.7% (The background lifetime risk of death from lung cancer is 3%-6%.).23 In Iran, Azari et al assessed the relative risk of death from silicosis and lung cancer in traditional brick production and reported that this risk was in the range of 1-63.6 per 1000 people, and the risk of lung cancer was 124.08 per 1000 people.24

There is not any organized and comprehensive study on the status of exposure to crystalline silica and its health risk in Iranian workplaces. Therefore, the present study aimed to provide a systematic review of exposure to crystalline silica in all silica-related industries, as well as the estimation of the risk of silicosis and lung cancer due to exposure to crystalline silica.


Methods

Search strategy

All of the available studies in the field of occupational exposure to crystalline silica, including case-control and cohort studies, were provided.The literature search strategy was conducted using the following keywords: «Silica», “Crystalline silica», «Exposure”, «Occupational exposure», “Industrial», «Workplace», «Factory”, and “Iran”. All of the articles that reported the concentration of crystalline silica in air samples, as well as those published in English and Persian languages, were selected for the study. Due to the numerous applications of silica in recent decades, the query was carried out from 2000-2021. The Cochrane review method was used as a guideline for the systematic review.25,26 According to this method, PECO (Participants, Exposure, Comparators, and Outcomes) statement is as follows:

  • Participants: Humans, who had occupational exposure to crystalline silica

  • Exposure: Exposure to crystalline silica in silica-related industries

  • Comparators: People exposed to crystalline silica and other people

  • Outcomes: Increasing the concentration of crystalline silica in environmental or individual samples

Web of Science (WOS), Scopus, PubMed, Google Scholar, and SID (Scientific Information Database) were selected to implement the search strategy. In addition, the manual inspection of reference lists was used in order to gain access to more articles and reduce bias.

Screening of articles

The screening of articles was performed by title, abstract, and full text of the articles, separately. The inclusion criteria were all articles performed on occupational exposure to crystalline silica in Iran. On the other hand, the exclusion criterion entailed the articles on the biomonitoring of individuals. Moreover, abstracts (without their full-text available online), review and mini-review articles, conference papers, meta-analyses, modeling studies, books, and unpublished studies were excluded.

We used EndNote X9® (Thomson Reuters, Toronto, Canada) software27 to prepare the list of the articles and finally downloaded the full text of the screened articles. In order to reduce the error, search strategies were used by two researchers in this study separately. When there were disagreements, a third researcher was involved.

Data extraction

As illustrated in Table 1, data extraction was performed based on year, monitoring station number, mean and standard deviation concentration of crystalline silica, method of detection, city, occupation, and industrial activity.

Table 1. Basic characteristics of the included studies
Year Monitoring
station number
Mean±SD Method of detection City Occupation Industrial activity Ref
2000-2.62 ± 0.00XRDSemnanFerrosiliconFurnace 28
2003-0.49 ± 0.105NIOSH7500ArakLead metal miningThe mines 29
2004220.057 ± 0.016NIOSH7500KashmarMiningThe mines 30
2007400.86 ± 1.04NIOSH7500HamadanCrushingStone deformation operations 31
2007750.008 ± 0.004SpectrophotometryGolestanWheat flour producingFood industry 32
2008240.01 ± 0.005NIOSH7500KhafIron stone- HammeringStone deformation operations 33
2008241.48 ± 0.39NIOSH7500KhafIron stone- ExcavationMines 33
2009100.275 ± NMNIOSH7602East-TehranStone cutting and millingStone deformation operations 3
2009100.343 ± NMNIOSH7602East-TehranFoundry workFoundry 3
2009100.132 ± NMNIOSH7602East-TehranGlass manufacturingGlass manufacturing 3
2009100.267 ± NMNIOSH7602East-TehranAsphaltingAsphalt manufacturing 3
2009100.193 ± NMNIOSH7602East-TehranConstructionConstruction 3
2009100.261 ± NMNIOSH7602East-TehranSand and gravel miningSand and gravel production 3
2009100.272 ± NMNIOSH7602East-TehranSandblastingSandblast 3
2009100.328 ± NMNIOSH7602East-TehranCeramic manufacturingTile and ceramic industry 3
2009100.160 ± NMNIOSH7602East-TehranBricks manufacturingBricks manufacturing 3
2009100.220 ± NMNIOSH7602East-TehranCement manufacturingCement manufacturing 3
2011500.29 ± 0.039NIOSH 7601TehranMetro excavatingExcavations 34
2011250.164 ± 0.112NIOSH7601-Glass sandblastingSandblast 35
2011515.5 ± 0.00NIOSH7501MashhadIron stoneMines 36
2012480.34 ± 0.11NIOSH7602MazandaranFoundryFoundry 37
2012480.19 ± 0.13NIOSH7602MazandaranBrick industryBrick industry 37
2012480.28 ± 0.10NIOSH7602MazandaranSand and gravel productionSand and gravel production 37
2012480.24 ± 0.17NIOSH7602MazandaranAsphaltingAsphalt industry 37
201340.04 ± 0.02NIOSH7601PakdashtFoundryFoundry 38
201480.21 ± 0.19NIOSH7500-Tile industryTile and ceramic industry 39
2014600.19 ± 0.138NIOSH7601TehranDemolition of buildingsConstruction 40
2015220.088 ± 0.055NIOSH7601PakdashtFoundryFoundry 41
2015120.589 ± 3.04NIOSH7500KhuzestanCement CompanyCement production 42
2016550.246 ± 0.047NIOSH7602-MiningMines 43
2016600.25 ± 0.13NIOSH7601SaveInsulatorInsulator industry 20
201650.13 ± 0.019NIOSH7602-ConstructionConstruction 44
2016440.17 ± 0.79NIOSH7602DorudSand washingSand and gravel production 45
2016960.313 ± 0.180NIOSH7602MazandaranSandblastingSandblasting 21
2016960.169 ± 0.065NIOSH7602MazandaranCeramic manufacturingTile and ceramic industry 21
2016960.282 ± 0.095NIOSH7602MazandaranSanding and gravelingSand and gravel production 21
2016960.194 ± 0.130NIOSH7602MazandaranBrick producingBrick production 21
2016960.239 ± 0.171NIOSH7602MazandaranAsphalt manufacturingAsphalt manufacturing 21
2016960.338 ± 0.110NIOSH7602MazandaranFoundryFoundry 21
2016960.125 ± 0.093NIOSH7602MazandaranGlass manufacturingGlass manufacturing 21
2016960.318 ± 0.120NIOSH7602MazandaranStone cutting & millingStone deformation operations 21
2016550.27 ± 0.11NIOSH7601-Insulator manufacturingInsulator industry 46
20161141.02 ± 0.17NIOSH7601East-IranMinesMines 47
2017400.297 ± 272NIOSH7602TehranMachine brick producingBricks manufacturing 24
201750.045 ± 0.03NIOSH7601SaveInsulatorInsulator industry 20
2017110.052 ± 0.025NIOSH7601SaveInsulatorInsulator industry 20
2017140.041 ± 0.014NIOSH7601SaveInsulatorInsulator industry 20
201770.024 ± 0.008NIOSH7601SaveInsulatorInsulator industry 20
2017120.039 ± 0.02NIOSH7601SaveInsulatorInsulator industry 20
20171270.507 ± 0.23NIOSH7601SaveInsulatorInsulator industry 48
2017300.507 ± 0.23NIOSH7601-InsulatorInsulator industry 48
2017550.25 ± 0.05NIOSH7602-FoundryFoundry 49
201760.17 ± 0.02NIOSH7602-Automobile manufacturingMachine production 19
2018360.12 ± 0.3NIOSH7500KhorasanCement manufacturingCement industry 50
2018550.27 ± 0.05NIOSH7601-InsulatorInsulator factories 51
201850.223 ± 0.051NIOSH7601-FurnacesFurnace 52
201850.218 ± 0.00NIOSH7601-FurnaceFurnace 52
2019400.034 ± 0.037NIOSH7601KermanshahCement manufacturingCement industry 53
2020720.027 ± 0.008NIOSH7602NishaburConcretingConcrete Workers 54
2020300.651 ± 0.69NIOSH7602TehranTraditional brick producingBricks manufacturing 24

Risk assessment

Prior to conducting risk assessment, we investigated the homogeneity of data; moreover, in order to detect and remove outliers, we used the box plot at a 95% confidence level. Mean and geometric standard deviation were calculated for every industrial activity. Thereafter, cumulative exposure to crystalline silica (mg/m3-y) (Mean of concentration × Years of exposure) was calculated for risk assessment. The relative risk of death from silicosis was determined using Mannetje’s method.23 In this method, the exposure history and crystalline silica concentration are two main factors. In addition, the exposures of all industrial workers in different studies were classified according to the Mannetje category for cumulative exposure.23Table 2 displays the exposure-related mortality rates and mortality rate ratios from silicosis in Mannetje’s method.

Table 2. Relative risk of silicosis- related mortality in exposed workers according to their cumulative exposure (mg/m3-year) to crystalline silica in Mannetje’s method
Cumulative exposure to crystalline silica (mg/m 3 -y) Relative risk (95% CI)
0-0.991.00
0.99- 1.973.39 (1.42, 8.08)
1.97- 2.876.22 (2.56, 15.12)
2.87- 4.339.40 (3.71, 23.80)
4.33-7.1213.69 (5.04, 37.18)
7.12- 9.5822.64 (7.88, 65.10)
9.58- 13.2123.97 (8.05, 71.32)
13.21- 15.8940.25 (13.25, 122.3)
15.89-28.1025.11 (8.09, 77.91)
 > 28.1063.63 (19.87, 203.8)

The lung cancer risk of crystalline silica was calculated according to the model of Rice et al12 using formula 1. This model is based on the geometric mean of exposure to crystalline silica and 45 years of exposure. In this formula, (A) is the risk of death from lung cancer in workers, and (GM) denotes the geometric mean of exposure to crystalline silica.

A = 0.77 + 373.69 × GM (1)


Results

Based on the research reports of the databases, a total of 72 articles were published from September 2000 to September 2020 [PubMed (n = 28), Scopus (n = 8), WOS (n = 5), SID (n = 24), and other databases (n = 7)]. Due to duplication, 15 articles were ruled out. Finally, 36 papers were selected for the study and analyzed by the Preferred Reporting Items for Overviews of Reviews (PRIOR) method (). A total of 24 articles were published in Persian. A number of 1, 421 measuring stations in various industries were investigated in 36 studies conducted in the field of worker exposure in Iran. As presented in Table 1, the studies were conducted in eight provinces, including Markazi, Razavi Khorasan, Hamadan, Golestan, Tehran, Mazandaran, Khuzestan, and Lorestan, as well as 13 cities, namely Arak, Save, Kashmar, Khaf, Nishabur, Hamadan, a city in Golestan, Tehran, Pakdasht, a city in Mazandaran, a city in Khorasan, Khuzestan, Dorud. The majority of studies were managed in Razavi Khorasan province (n = 4).

jrhs-22-e00550-g001
Figure 1. Preferred Reporting Items for Overviews of Reviews displaying selected literature reviews.

The studies were performed in 17 industrial activities, such as cement manufacturing, mining, construction, foundry, furnace, stone deformation, glass manufacturing, asphalt manufacturing, sand and gravel production, sandblast, tile, and ceramic industry, brick production, insulator industry, excavation, concrete, food industry, and machine industry. Most studies were performed in mines and foundries (n = 6). One study was carried out in the food industry and another in machine industry. The box plot demonstrated that four studies encompassed outliers. As depicted in Table 1, these data were highlighted by Italic font.28,33,36,47 Outliers were deleted and not used in risk assessment; therefore, 2, 036 measuring stations in various industries were selected from 32 studies for risk assessment. In most studies, laborers worked six days a week and their working hours were more than 8 hours.

The geometric mean concentration for workers’ exposure to crystalline silica ranged from 0.0212-0.2689 mg/m3 (Table 3). In addition, the mean concentration of crystalline silica was obtained at 0.1476 ± 0.1628 in Iranian industries, which is higher than recommended exposure standard limit by the American Conference of Governmental Industrial Hygienists (ACGIH), Iran’s national occupational exposure limits (0.025 mg/m3),55 NIOSH (0.05 mg/m3),56 as well as OSHA (0.01 mg/m3).57

Table 3. Mean concentrations of exposure to crystalline silica in various industrial activities in Iran
Industrial activity Number of samples Mean and arithmetic
standard deviation
Mean and geometric
standard deviation
Cement manufacturing980.2408 ± 0.24430.1516 ± 0.2116
The mines(77 + NM)a0.2643 ± 0.21710.1901 ± 0.1772
Construction 750.1710 ± 0.03560.1683 ± 0.0290
Foundry2350.2332 ± 0.13650.1806 ± 0.1246
Furnace 100.2205 ± 0.00350.2205 ± 0.0025
Stone deformation operations1700.3658 ± 0.35650.1656 ± 0.3087
Glass manufacturing1060.1285 ± 0.00490.1285 ± 0.0035
Asphalt manufacturing1540.2487 ± 0.01590.2483 ± 0.0129
Sand and gravel production1980.2483 ± 0.0530 0.2433 ± 0.0459
Sandblast1310.2496 ± 0.07690.2408 ± 0.0628
Tile and ceramic industry1140.2266 ± 0.06740.2357 ± 0.0825
Bricks production1890.2512 ± 0.2159 0.1605 ± 0.1970
Insulator industry2760.2005 ± 0.19120.1155 ± 0.1814
Excavations500.2948 ± 0.11690.2689 ± 0.1046
Concrete Workers720.0335 ± 0.03110.0212 ± 0.0291
Others 810.0890 ± 0.11460.0368 ± 0.0810
Total1421 0.1476 ± 0.1628

aThe number of measuring stations was not mentioned in one of the studies that measured the concentration of silica in the mines.

As presented in Tables 1 and 2, workers in various industries were exposed to different grades of crystalline silica. In this study, we observed that iron-stone miners were exposed to the highest amount of crystalline silica (Table 1)33; nonetheless, to maintain homogeneity, the data related to the study by Naghizadeh et al on iron-stone miners were removed from the risk assessment process. It seems that health risk for iron-stone miners can be at the highest level; therefore, this issue needs more assiduous attention in the future. Furthermore, according to Table 3, workers in excavations are exposed to the highest concentration of crystalline silica (0.2689 ± 0.1046 mg/m3). The geometric mean concentration of crystalline silica was the lowest in the food industry (0.008 ± 0.004 mg/m3).


Discussion

Scarselli et al studied some workers potentially at risk of silica exposure selected from the Italian database of workplaces. They reported that the most involved sectors at high risk of silica exposure were construction, mining and quarrying, metalworking, and manufacturing of non-metallic products. In addition, they reported that workers in the manufacturing and construction industries were exposed to the highest level of crystalline silica.58 On the contrary, among the 16 industrial activities classified in this study, the manufacturing and construction industries were the eighth industries.

In 2015, according to OSHA compliance data from 1979 to 2015, Doney et al reported that workers in the poured concrete foundation had the highest exposure to crystalline silica. Moreover, out of 100 000 workers, 99.7% of cases were potentially exposed to crystalline silica at higher than the occupational exposure limit recommended by NIOSH in 2014.59 Concrete workers in Iran had the lowest mean airborne silica exposure levels (0.0212 ± 0.0291), which is lower than the occupational exposure limit recommended by NIOSH, ACGIH, and occupational exposure limits.

The relative risk of silicosis-related mortality based on Mannetje’s method and cumulative exposure categories was estimated to be in the range of 1-24 per 1000 people, ranging in the cumulative exposure categories of 0-0.99 to 9.58-13.21 in Mannetje’s method. In general, the mean rate of silicosis mortality in Iranian industries was 14 per 1000 people. These rates are above the risk of 1 per 1000 usually deemed acceptable by the US OSHA.3

According to Table 4, the risk of lung cancer due to exposure to crystalline silica based on the Rice model was in the range of 13-137 per 1000 people. In a 44-year cohort study on 34 018 workers, Liu et al reported the risk of lung cancer mortality as 128 per 1000 when the mean cumulative concentration (using a 25-year lag) was 0.01 to 1.12 mg/m3-y.60 In present study, most investigations were exposed-nonexposed studies. Since risk assessment is calculated by considering the history of exposure to silica, retrospective cohort studies may demonstrate more accurate estimations than other studies. The results of present study demonstrated that the risk of lung cancer was at the highest level among the stone deformation operations (1-137 per 1000). Inconsistent with this finding, Poinen-Rughooputh et al reported that miners were exposed to the highest risk of lung cancer mortality (in the range of 1-104 per 1000).61 Moreover, in present study, the lowest risk of lung cancer mortality was estimated in concrete workers in the range of 1-13 per 1000 people.

Table 4. Risk of lung cancer mortality in exposed workers in different industries were calculated according to Rice et al. model
Industrial activity Mean±(SD) (mg/m 3 ) Estimated excess lifetime risks of mortality from lung cancer
Cement manufacturing0.2408 ± 0.244391
The mines0.2643 ± 0.2171100
Construction 0.171 ± 0.035665
Foundry0.2332 ± 0.136588
Furnace 0.2205 ± 0.003583
Stone deformation operations0.3658 ± 0.3565137
glass manufacturing0.1285 ± 0.004949
Asphalt manufacturing0.2487 ± 0.015994
sand and gravel production0.2483 ± 0.0530 94
Sand blast0.2496 ± 0.076994
Tile and ceramic industry0.2266 ± 0.067485
Bricks production0.2512 ± 0.2159 94
Insulator industry0.2005 ± 0.191276
Excavations0.2948 ± 0.1169111
Concrete Workers0.0335 ± 0.031113
Others 0.0368 ± 0.08115

In 2016, in a meta-analysis study, based on worldwide studies up to April 2016, the highest pooled concentration mortality ratio of exposure to crystalline silica was estimated at 6.03 (95% CI: 5.29-6.77) in mixed industries of Japan. Moreover, Italy had the highest number of observed lung cancer deaths (798 cases) before 2006.61 In their study, although the estimated health risk was high in Asian countries after Canada, the studies in Iran have been neglected. According to the results of the current research, the risk of both silicosis and lung cancer mortality is high in Iranian industries, and even numerous studies were conducted before 2016 in Iran.

Among the notable limitations of this study, we can refer to incomplete information on workers of all industries; therefore, we could not estimate the risk of mortality based on the percentage of exposed workers in industries. The expression of mortality based on the percentage of exposed cases can provide a better understanding of the hazard. Due to the high risk of silicosis and lung cancer mortality, it seems that the prevalent occupational health engineering strategies are not sufficient to protect workers; therefore, workers’ exposure to crystalline silica dust should be controlled in Iranian workplaces.


Conclusion

The authors provided a lung cancer risk assessment of occupational exposure to crystalline silica in Iranian industrials based on the collected quantitative exposure data. As evidenced by the obtained results, occupational exposure to crystalline silica was higher than occupational exposure limits. Furthermore, the relative risk of death from silicosis was in the range of 1-24 per 1000 people, and the risk of lung cancer ranged from 13-137 per 1000 people. It seems that the prevalent occupational health engineering strategies are not sufficient to protect workers; therefore, workers’ exposure to crystalline silica dust should be controlled in Iranian workplaces.

Highlights
  • The present study estimated silicosis and lung cancer caused by crystalline silica.

  • Workers’ exposure to crystalline silica ranged from 0.0212-0.2689 mg/m3.

  • This range was higher than the recommended standard limit by ACGIH (0.025 mg/m3).

  • The relative risk of silicosis mortality was in the range of 1-14 per 1000.


Acknowledgments

We would like to express our special appreciation for Dr. Mohammad Asghari Jafarabadi, Professor of Biostatistics, Tabriz University of Medical Sciences, Iran, who helped to analyze data in this study. Their willingness to give their time so kindly is very much appreciated.


Conflicts of Interest

The authors declare that they have no conflict of interest.


Ethical Approval

The study was based on the literature search and was not an experimental study; therefore, there was no need for ethics committee approval.


Funding

No funding was received.


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Submitted: 05 Apr 2022
Revised: 12 Jul 2022
First published online: 30 Jun 2022
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