Occupational exposure to chrysotile asbestos and cancer risk: a review of the amphibole hypothesis

Journey to the centre of the lung. The perspective of a mineralogist on the carcinogenic effects of mineral fibres in the lungs

This work reviews the bio-chemical mechanisms leading to adverse effects produced when mineral fibres are inhaled and transported in the lungs from the perspective of a mineralogist. The behaviour of three known carcinogenic mineral fibres (crocidolite, chrysotile, and fibrous-asbestiform erionite) during their journey through the upper respiratory tract, the deep respiratory tract and the pleural cavity is discussed. These three fibres have been selected as they are the most socially and economically relevant mineral fibres representative of the classes of chain silicates (amphiboles), layer silicates (serpentine), and framework silicates (zeolites), respectively. Comparison of the behaviour of these fibres is made according to their specific crystal-chemical assemblages and properties. Known biological and subsequent pathologic effects which lead and contribute to carcinogenesis are critically reviewed under the mineralogical perspective and in relation to recent progress in this multidisciplinary field of research. Special attention is given to the understanding of the cause-effect relationships for lung cancer and malignant mesothelioma. Comparison with interstitial pulmonary fibrosis, or "asbestosis", will also be made here. This overview highlights open issues, data gaps, and conflicts in the literature for these topics, especially as regards relative potencies of the three mineral fibres under consideration for lung cancer and mesothelioma. Finally, an attempt is made to identify future research lines suitable for a general comprehensive model of the carcinogenicity of mineral fibres.

Is asbestos still a problem in the world? A current review

Asbestos has been used by automobile, construction, manufacturing, power, and chemical industries for many years due to its particular properties, i.e. high tensile strength, non-flammable, thermal and electrical resistance and stability, and chemical resistance. However, such a mineral causes harmful effects to human health, including different types of cancer (e.g., mesothelioma). As a result, the use of asbestos has been banned since the 1980s in many countries. Nonetheless, asbestos is still part of the daily life of the population as asbestos-containing materials (ACMs) are still present in many buildings constructed and renovated before the 1990s. This work aims to present a current literature review about asbestos. The literature review was composed mainly of research articles published in international journals from the medical and engineering disciplines to provide an overview of asbestos use effects reported in interdisciplinary areas. The literature review comprised asbestos characteristics and its relationship to the risks of human exposure, countries where asbestos use is permitted or banned, reducing asbestos in the built environment, and environmental impact due to use and disposal of asbestos. The main findings were that ACMs are still responsible for severe human diseases, particularly in areas where there is a lack of coordinated asbestos management plans, reduced awareness about asbestos health risks, or even a delay in the implementation of asbestos-ban. Such issues may be more prevailing in developing countries. The current research in many countries contemplates several methodologies and techniques to process ACMs into inert and recyclable materials. The identification and coordinated management of ACM hazardous waste is a significant challenge to be faced by countries, and its inadequate disposal causes severe risk of exposure to asbestos fibres. Based on this work, it was concluded that banning asbestos is indicated in all countries in the world.

Ongoing downplaying of the carcinogenicity of chrysotile asbestos by vested interests

Industries that mine, manufacture and sell asbestos or asbestos-containing products have a long tradition of promoting the use of asbestos, while placing the burden of economic and health costs on workers and society. This has been successfully done in recent years and decades in spite of the overwhelming evidence that all asbestos types are carcinogenic and cause asbestosis. In doing so, the asbestos industry has undermined the WHO campaign to reach a worldwide ban of asbestos and to eliminate asbestos-related diseases. Even worse, in recent years they succeeded in continuing asbestos mining and consuming in the range of about 1.3 million tons annually. Nowadays, production takes place predominantly in Russia, Kazakhstan and China. Chrysotile is the only asbestos type still sold and represents 95% of asbestos traded over the last century.The asbestos industry, especially its PR agency, the International Chrysotile Association, ICA, financed by asbestos mining companies in Russia, Kazakhstan and Zimbabwe and asbestos industrialists in India and Mexico, continues to be extremely active by using slogans such as chrysotile can be used safely.Another approach of the asbestos industry and of some of its insurance agencies is to broadly defeat liability claims of asbestos victims.In doing so they systematically use inappropriate science produced by their own and/or by industry-affiliated researchers. Some of the latter were also engaged in producing defense material for other industries including the tobacco industry. Frequent examples of distributing such disinformation include questioning or denying established scientific knowledge about adverse health effects of asbestos. False evidence continues to be published in scientific journals and books.The persisting strong influence of vested asbestos-related interests in workers and public health issues including regulations and compensation necessitate ongoing alertness, corrections and appropriate reactions in scientific as well as public media and policy advisory bodies.

Mesothelioma risk among those exposed to chrysotile asbestos only and mixtures that include amphibole: a case-control study in the USA, 1975-1980

OBJECTIVES

Occupational asbestos exposure is causally linked to mesothelioma. However, whether exposure to only chrysotile asbestos is associated with mesothelioma risk, and the heterogeneity in risk by different fibre types/lengths remains unclear. We investigated whether mesothelioma risk differs among workers exposed to only chrysotile asbestos compared with chrysotile and ≥1 amphibole (ie, amosite, tremolite, anthophyllite and crocidolite) over the working lifetime.

METHODS

We analysed next-of-kin interview data including occupational histories for 580 white men (176 cases and 404 controls) from a case-control study of mesothelioma conducted in the USA in 1975-1980. Asbestos exposure was determined by an occupational hygienist using a job-exposure matrix and exposure categories included chrysotile only and nine chrysotile-amphibole mixtures. Logistic regression models were used to estimate the ORs and 95% CIs of mesothelioma, comparing each asbestos category to the unexposed group, adjusted for age at death and data source. Analysis of contrasts was used to assess overall heterogeneity and pair-wise differences in risk.

RESULTS

Exposure to long and short chrysotile only was associated with increased mesothelioma risk compared with the unexposed (OR=3.8 (95% CI 1.3 to 11.2)). The complex mixture of extra-long amosite, short and long chrysotile, tremolite and anthophyllite was associated with the highest risk (OR=12.8 (95% CI 4.1 to 40.2)). There was evidence for overall heterogeneity among the asbestos exposure categories (p heterogeneity=0.02). However, the lower risk observed for exposure to chrysotile only compared with the complex mixture was not significant (p difference=0.10).

CONCLUSIONS

Our findings suggest that policies aimed at regulating asbestos should target both pure chrysotile and mixtures that include amphibole.

Asbestos-related cancers: the 'Hidden Killer' remains a global threat

Introduction: Asbestos, the most frequent cause of occupational cancer, continues to be consumed on a massive scale, with millions of people exposed on a daily basis. This review explains why we have failed in curtailing the silent epidemic of asbestos-related disease and why the numbers of asbestos victims are likely to remain high. Emerging and developed countries have to be reminded that asbestos exposure has yet to become a problem of the past. The worldwide spread of asbestos, followed by the surge of asbestos-related cancers, resembles the lung cancer epidemic caused by smoking and stimulated by manufacturers.Areas covered: Underreporting of malignant mesothelioma and asbestos-induced lung cancer, frequently-used arguments in the amphibole/chrysotile debate and the conclusion from bona-fide research organizations, that all forms of asbestos are carcinogenic, are reviewed. Special attention is paid to the consequences of ubiquitous environmental asbestos and the 'changing face' of malignant mesothelioma in countries with heavy asbestos use in the past.Expert opinion: Experts in oncology, respiratory medicine, occupational and public health, and basic researchers must take responsibility and acknowledge the ongoing silent epidemic of asbestos-related diseases. The call for a world-wide asbestos ban is more urgent than ever.

Mortality and mesothelioma incidence among chrysotile asbestos miners in Balangero, Italy: A cohort study

BACKGROUND

We studied cancer mortality and mesothelioma incidence in 974 male workers employed at least 6 months at the Balangero mine (Italy), the largest chrysotile mine in Western Europe, active from 1917 to 1985.

METHODS

Vital status as of 31 May 2013, causes of deaths and mesothelioma incidence from 1990 were ascertained. Past exposure to asbestos by working area and calendar period was estimated, based on historical data of fibers concentrations. Individual cumulative exposure was assessed by applying estimates to the job history of cohort members. Standardized mortality ratios (SMRs) for selected causes and standardized incidence ratios for malignant mesothelioma (MM) were calculated based on regional reference rates. Poisson regression analysis was used to study MM and lung cancer risk by latency, duration, and cumulative exposure.

RESULTS

Mortality was increased for all causes (SMR = 1.28; 95% confidence interval [CI] = 1.17-1.40), pleural cancer (SMR = 4.30; 95% CI = 1.58-9.37), asbestosis (SMR = 375.06; 95% CI = 262.68-519.23). An increase was also found for lung cancer (SMR = 1.14; 95% CI =  0.81-1.55) and peritoneal cancer (SMR = 3.25; 95% CI = 0.39-11.75). The risk of both pleural and peritoneal cancer mortality and of mesothelioma incidence increased with increasing cumulative exposure, duration, and latency. Poisson regression analyses showed an increase in mesothelioma risk with cumulative asbestos exposure and suggest a similar trend for lung cancer. Asbestosis mortality also increased with cumulative exposure.

CONCLUSIONS

Among Balangero chrysotile miners and millers, the occurrence of malignant and nonmalignant asbestos-related diseases was increased by exposure, with dose-response relation. The study confirms the carcinogenicity of chrysotile asbestos, in particular for pleural mesothelioma.

Does exposure to asbestos cause prostate cancer? A systematic literature review and meta-analysis

OBJECTIVE

The relationship between asbestos and prostate cancer (PCa) is not well understood due to small number of cases. This study aimed to determine the incidence and mortality of PCa among workers or residents exposed to asbestos based on a systematic review and meta-analysis METHODS: All published studies citing the standardized mortality ratio (SMR) or standardized incidence ratio (SIR) of PCa in workers or residents exposed to asbestos were collected by conducting a search on PubMed, EMBASE, Cochrane Library, and Web of Science before April 2018. Standardized mortality rate for PCa with its 95% confidence interval (CI) was pooled using a fixed-/random-effect model in STATA (Version14.0). This study is registered with PROSPERO, number CRD42018095195.

RESULTS

A total of 17 independent studies were included for the analysis. The overall pooled SMR of PCa was 1.22, with a 95% CI of 1.13 to 1.32, with no heterogeneity across the studies (I = 18.8%, P = .234). Subgroup analysis shows that exposure to crocidolite, cement, studies conducted in Europe and Oceania, and long study follow-up (≥25 years) all contribute to significantly higher SMR, and we found no evidence of publication bias (Begg test P value = .592, Egger test P value = .874).

CONCLUSIONS

This meta-analysis suggested that exposed to asbestos might be associated with an increased risk of PCa. High-exposure level of asbestos could contribute to significantly higher risk of PCa mortality.

Adverse health effects of asbestos: solving mysteries regarding asbestos carcinogenicity based on follow-up survey of a Chinese factory

The present review summarizes the results of several follow-up studies assessing an asbestos product manufacturing plant in Chongqing, China, and discusses three controversial issues related to the carcinogenicity of asbestos. The first issue is the amphibole hypothesis, which asserts that the carcinogenicity of asbestos is limited to amphiboles, such as crocidolite, but not serpentines, such as chrysotile. However, considering the possible multiple component of asbestos carcinogenicity in the presence of tobacco smoke or other carcinogens, chrysotile cannot be regarded as non-carcinogenic. Additionally, in a practical sense, it is not possible to assume "pure" chrysotile due to its ubiquitous contamination with tremolite, which is a type of amphibole. Thus, as the International Agency for Research on Cancer (IARC) assessed, all forms of asbestos including chrysotile should be regarded carcinogenic to humans (Group 1). The second issue is the chrysotile/tremolite paradox, which is a phenomenon involving predominant levels of tremolite in the lung tissues of individuals who worked in locations with negligible levels of tremolite due to the exclusive use of chrysotile. Four possible mechanisms to explain this paradox have been proposed but this phenomenon does not support the claim that amphibole is inert. The final issue discussed is the textile mystery, i.e., the higher incidence of cancer in asbestos textile plants compared to asbestos mines where the same asbestos was produced and the exposure levels were comparable. This phenomenon was first reported in North America followed by UK and then in the present observations from China. Previously, levels of fiber exposure were calculated using a universal converting coefficient to estimate the mass concentration versus fiber concentration. However, parallel measurements of fiber and mass concentrations in the workplace and exposed air indicated that there are wide variations in the fiber/mass ratio, which unjustifies the universal conversion. It is possible that contamination by airborne non-fibrous particles in mines with mass fiber conversion led to the overestimation of fiber concentrations and resulted in the textile mystery. Although the use and manufacturing of asbestos has been banned in Japan, more than 10 million tons of asbestos had been imported and the majority remains in existing buildings. Thus, efforts to control asbestos exposure should be continued.

Chrysotile effects on the expression of anti-oncogene P53 and P16 and oncogene C-jun and C-fos in Wistar rats' lung tissues

Chrysotile is the most widely used form of asbestos worldwide. China is the world's largest consumer and second largest producer of chrysotile. The carcinogenicity of chrysotile has been extensively documented, and accumulative evidence has shown that chrysotile is capable of causing lung cancer and other forms of cancer. However, molecular mechanisms underlying the tumorigenic effects of chrysotile remained poorly understood. To explore the carcinogenicity of chrysotile, Wistar rats were administered by intratracheal instillation (by an artificial route of administration) for 0, 0.5, 2, or 8 mg/ml of natural chrysotile (from Mangnai, Qinghai, China) dissolved in saline, repeated once a month for 6 months (a repeated high-dose exposure which may have little bearing on the effects following human exposure). The lung tissues were analyzed for viscera coefficients and histopathological alterations. Expression of P53, P16, C-JUN, and C-FOS was measured by western blotting and qRT-PCR. Our results found that chrysotile exposure leads the body weight to grow slowly and lung viscera coefficients to increase in a dose-dependent manner. General sample showed white nodules, punctiform asbestos spots, and irregular atrophy; moreover, HE staining revealed inflammatory infiltration, damage of alveolar structures, agglomerations, and pulmonary fibrosis. In addition, chrysotile can induce inactivation of the anti-oncogene P53 and P16 and activation of the proto-oncogenes C-JUN and C-FOS both in the messenger RNA and protein level. In conclusion, chrysotile induced an imbalanced expression of cancer-related genes in rats' lung tissue. These results contribute to our understanding of the carcinogenic mechanism of chrysotile.

Dietary Iodine Affected the GSH-Px to Regulate the Thyroid Hormones in Thyroid Gland of Rex Rabbits

Iodine (I) is an essential trace element that can influence animal health and productivity. In this study, we investigated the effects of dietary iodine on the antioxidant indices of organ (liver and thyroid gland) and messenger RNA (mRNA) expression of glutathione peroxidase (GSH-Px) in Rex rabbits. A total of 120 4-month-old Rex rabbits (2235.4 ± 13.04 g BW) were divided into four equal groups, and their diets were supplemented with iodine (0, 0.2, 2, or 4 mg/kg dry matter (DM)). The iodine concentration in basal diet (control group) was 0.36 mg/kg DM. In most of measured parameters, supplemental iodine exerted no significant effect. Growth and slaughter performance and organ weight were not influenced significantly by iodine supplementation. Serum T₃ was significantly lower in 2-mg I group than in 0.2 and 4-mg I groups (P < 0.05). Superoxide dismutase (SOD), GSH-Px, methane dicarboxylic aldehyde (MDA), and thyroperoxidase (TPO) in the serum and liver were not influenced (P > 0.05). Conversely, serum catalase (CAT) was significantly reduced (P < 0.05). In the thyroid, GSH-Px was higher in the 2-mg I group than in the 0.2- and 4-mg I groups (P < 0.05). RT-PCR results showed that the mRNA expression level of GSH-Px in the liver was not significantly influenced (P > 0.05). In the thyroid gland, the mRNA expression level of GSH-Px was higher in the 2-mg I group than in the 4-mg I group (P < 0.05), which agreed with the activity of GSH-Px. In conclusion, iodine supplementation exerted no effect on the performance and antioxidant capacity of the body, but dietary iodine influenced serum T₃ or GSH-Px in the thyroid gland. Thus, on the basis of serum T₃ and GSH-Px levels in the thyroid gland, we hypothesized that GSH-Px secretion was increased by adding dietary iodine in the thyroid, which may inhibit the H₂O₂ generation and further influence the thyroid hormone synthesis.

Short fiber tremolite free chrysotile mesothelioma cohort revealed

In 1995, Dell and Teta published a cohort mortality study of asbestos molding compound workers at a Union Carbide Corporation (UCC) plastics manufacturing plant in Bound Brook, New Jersey. They reported that the factory workers were exposed to "asbestos (mostly chrysotile)," implying that the asbestos used at the Bound Brook plant occasionally contained amphiboles. However, UCC statements and testimony from recent litigation indicate that the Bound Brook plant exclusively used short fiber chrysotile asbestos. These recent documents also point to lower exposures than those reported by Dell and Teta. This chrysotile-only cohort should be included in analyses of chrysotile potency.

Mesothelioma from asbestos exposures: Epidemiologic patterns and impact in the United States

Mesothelioma, a rare tumor, is highly correlated with asbestos exposure. Mesothelioma, similar to all asbestos-related diseases, is dose/intensity dependent to some degree, and studies showed the risk of mesothelioma rises with cumulative exposures. Multiple processes occur in an individual before mesothelioma occurs. The impact of mesothelioma in the United States has been continuous over the last half century, claiming between 2,000 and 3,000 lives each year. Mesothelioma is a preventable tumor that is more frequently reported as associated with asbestos exposure among men than women. However, the rate of asbestos-associated mesothelioma is on the rise among women due to better investigation into their histories of asbestos exposure. It is of interest that investigators detected asbestos-associated cases of mesothelioma in women from nonoccupational sources-that is, bystander, incidental, or take-home exposures. It is postulated that asbestos-associated mesotheliomas, in both men and women, are likely underreported. However, with the implementation of the most recent ICD-10 coding system, the correlation of mesothelioma with asbestos exposure is expected to rise to approximately 80% in the United States. This study examined the demographic and etiological nature of asbestos-related mesothelioma.

Size- and type-specific exposure assessment of an asbestos products factory in China

This study describes fibre size and type-specific airborne asbestos exposures in an asbestos product factory. Forty-four membrane filter samples were analysed by scanning electron microscopy to determine the size distribution of asbestos fibres, by workshop. Fibre frequencies of bivariate (length by width) categories were calculated and differences between workshops were tested by analysis of variance. Data were recorded for 13,435 chrysotile and 1075 tremolite fibres. The proportions between size metrics traditionally measured and potentially biologically important size metrics were found to vary in this study from proportions reported in other cohort studies. One, common size distribution was generated for each asbestos type over the entire factory because statistically significant differences in frequency between workshops were not detected. This study provides new information on asbestos fibre size and type distributions in an asbestos factory. The extent to which biologically relevant fibre size indices were captured or overlooked between studies can potentially reconcile currently unexplained differences in asbestos-related disease (ARD) risk between cohorts. The fibre distributions presented here, when combined with similar data from other sites, will contribute to the development of quantitative models for predicting risk and our understanding of the effects of fibre characteristics in the development of ARD.

Toxicological and epidemiological studies on effects of airborne fibers: coherence and public [corrected] health implications

Airborne fibers, when sufficiently biopersistent, can cause chronic pleural diseases, as well as excess pulmonary fibrosis and lung cancers. Mesothelioma and pleural plaques are caused by biopersistent fibers thinner than ∼0.1 μm and longer than ∼5 μm. Excess lung cancer and pulmonary fibrosis are caused by biopersistent fibers that are longer than ∼20 μm. While biopersistence varies with fiber type, all amphibole and erionite fibers are sufficiently biopersistent to cause pathogenic effects, while the greater in vivo solubility of chrysotile fibers makes them somewhat less causal for the lung diseases, and much less causal for the pleural diseases. Most synthetic vitreous fibers are more soluble in vivo than chrysotile, and pose little, if any, health pulmonary or pleural health risk, but some specialty SVFs were sufficiently biopersistent to cause pathogenic effects in animal studies. My conclusions are based on the following: 1) epidemiologic studies that specified the origin of the fibers by type, and especially those that identified their fiber length and diameter distributions; 2) laboratory-based toxicologic studies involving fiber size characterization and/or dissolution rates and long-term observation of biological responses; and 3) the largely coherent findings of the epidemiology and the toxicology. The strong dependence of effects on fiber diameter, length, and biopersistence makes reliable routine quantitative exposure and risk assessment impractical in some cases, since it would require transmission electronic microscopic examination, of representative membrane filter samples, for determining statistically sufficient numbers of fibers longer than 5 and 20 μm, and those thinner than 0.1 μm, based on the fiber types.

Exposure to airborne asbestos in thermal power plants in Mongolia

BACKGROUND

Coal-fired thermal power plants (TPPs) in Mongolia use various types of asbestos-containing materials (ACMs) in thermal insulation of piping systems, furnaces, and other products.

OBJECTIVE

To investigate the occupational exposure of insulation workers to airborne asbestos in Mongolian power plants.

METHODS

Forty-seven air samples were collected from four power plants in Mongolia during the progress of insulation work. The samples were analyzed by phase contrast microscopy (PCM) and transmission electron microscopy (TEM).

RESULTS

The average phase contrast microscopy equivalent (PCME) asbestos fiber concentration was 0·93 f/cm(3). Sixteen of the 41 personal and one of the area samples exceeded the United States Occupational Safety and Health Administration (US OSHA) short-term exposure limit of 1·0 f/cm(3). If it is assumed that the short-term samples collected are representative of full-shift exposure, then the exposures are approximately 10 times higher than the US OSHA 8-hour permissible exposure limit of 0·1 f/cm(3).

CONCLUSION

Power plant insulation workers are exposed to airborne asbestos at concentrations that exceed the US OSHA Permissible Exposure Limit. Action to mitigate the risks should be taken in Mongolia.

Mouse models of mesothelioma: strengths, limitations and clinical translation

SUMMARY 

Mouse models of cancer are invaluable for obtaining detailed knowledge about tumor development and for screening therapeutic and preventive approaches. Mesothelioma is an unusual cancer because the same carcinogen, asbestos, causes a similar disease in both humans and animals. Unlike most other cancers, murine mesothelioma can therefore be regarded as a disease homolog, rather than a model as such. However, because asbestos-induced cancer has low penetrance and a long lag time, most translational studies have utilized more efficient models such as tumor transplantation. In consequence, many promising results have not translated into positive findings in patients. Here, we describe the widely used murine mesothelioma models and critically discuss their relative advantages and disadvantages. We emphasize the use of the appropriate model for the specific research question and the need to use multiple models in order to obtain robust and translatable data.

Stress, oxidative injury and disease

The living system on earth is largely using oxygen for burning metabolic fuel for energy. The toxicity of oxygen is largely due to the formation of free radicals in living systems. Stress is also responsible for the generation of free radicals. The evidence for the involvement of free radicals and oxidative injury in producing metabolic disturbance, maladjustment and many diseases has been accumulating since long. It is largely believed that the root cause of many chronic diseases is stress induced free radicals and resultant oxidative injury.

Exposure-specific lung cancer risks in Chinese chrysotile textile workers and mining workers

OBJECTIVE

Whether there is a difference in the exposure-response slope for lung cancer between mining workers and textile workers exposed to chrysotile has not been well documented. This study was carried out to evaluate exposure-specific lung cancer risks in Chinese chrysotile textile workers and mining workers.

SUBJECTS AND METHODS

A chrysotile mining worker cohort and a chrysotile textile worker cohort were observed concurrently for 26 years. Information on workers' vital status, occupational history and smoking habits were collected, and causes and dates of deaths were verified from death registries. Individual cumulative fiber exposures were estimated based on periodic dust/fiber measurements from different workshops, job title and duration, and categorized into four levels (Q1-Q4). Standardized mortality ratios (SMRs) for lung cancer were calculated and stratified by industry and job title with reference of the national rates. Cox proportional hazard models were fit to estimate the exposure-specific lung cancer risks upon adjustment for age and smoking, in which an external control cohort consisting of industrial workers without asbestos exposure was used as reference group for both textile and mining workers.

RESULTS

SMRs were almost consistent with exposure levels in terms of job titles and workshops. A clear exposure-response relationship between lung cancer mortality and exposure levels was observed in both cohorts. At low exposure levels (Q1 and Q2), textile workers displayed higher death risks of lung cancer than mining workers. However, similarly considerably high risks were observed at higher exposure levels, with hazard ratios of over 8 and 11 at Q3 and Q4, respectively, for both textile and mining workers, after both age and smoking were adjusted.

CONCLUSION

The chrysotile textile workers appeared to have a higher risk of lung cancer than the mining workers at a relatively low exposure level, but no difference was observed at a high exposure level, where both cohorts displayed a considerably high risk.

Occupational asbestos exposure and lung cancer--a systematic review of the literature

The objective of this study was to evaluate the scientific literature concerning asbestos and lung cancer, emphasizing low-level exposure. A literature search in PubMed and Embase resulted in 5,864 citations. Information from included studies was extracted using SIGN. Twenty-one statements were evidence graded. The results show that histology and location are not helpful in differentiating asbestos-related lung cancer. Pleural plaques, asbestos bodies, or asbestos fibers are useful as markers of asbestos exposure. The interaction between asbestos and smoking regarding lung cancer risk is between additive and multiplicative. The findings indicate that the association between asbestos exposure and lung cancer risk is basically linear, but may level off at very high exposures. The relative risk for lung cancer increases between 1% and 4% per fiber-year (f-y)/mL, corresponding to a doubling of risk at 25-100 f-y/mL. However, one high-quality case-control study showed a doubling at 4 f-y/mL.

Asbestos Exposure Causes Mesothelioma, But Not This Asbestos Exposure: An Amicus Brief to the Michigan Supreme Court

Manufacturers of asbestos brakes, supported by many manufacturing and insurance industry amicus curie, requested the Michigan Supreme Court to dismiss testimony of an expert regarding the ability of asbestos dust from brakes to cause mesothelioma as "junk science". Scientists are concerned with the sweeping and unequivocal claims that any conclusion that asbestos from brakes caused a signature asbestos-related disease in a particular person must be "junk science". The manufacturers' sweeping pronouncements are what veer from accepted, reliable mainstream scientific methods and conclusions. This article outlines the evidence supporting the conclusion that asbestos from brakes can and does cause mesothelioma, and describes the defendants' attempts to fabricate doubt about this conclusion.

Occupational exposure and lung cancer

Lung cancer is the leading cause of cancer death for male and the second most usual cancer for women after breast cancer. Currently there are available several non-specific cytotoxic agents and several targeted agents for lung cancer therapy. However; early stage diagnosis is still unavailable and several efforts are being made towards this direction. Novel biomarkers are being investigated along with new biopsy techniques. The occupational and environmental exposure to carcinogenic agents is an everyday phenomenon. Therefore until efficient early diagnosis is available, avoidance of exposure to carcinogenic agents is necessary. In the current mini-review occupational and environmental carcinogenic agents will be presented.

Cancer mortality in Chinese chrysotile asbestos miners: exposure-response relationships

Objective

This study was conducted to assess the relationship of mortality from lung cancer and other selected causes to asbestos exposure levels.

Methods

A cohort of 1539 male workers from a chrysotile mine in China was followed for 26 years. Data on vital status, occupation and smoking were collected from the mine records and individual contacts. Causes and dates of death were further verified from the local death registry. Individual cumulative fibre exposures (f-yr/ml) were estimated based on converted dust measurements and working years at specific workshops. Standardized mortality ratios (SMRs) for lung cancer, gastrointestinal (GI) cancer, all cancers and nonmalignant respiratory diseases (NMRD) stratified by employment years, estimated cumulative fibre exposures, and smoking, were calculated. Poisson models were fitted to determine exposure-response relationships between estimated fibre exposures and cause-specific mortality, adjusting for age and smoking.

Results

SMRs for lung cancer increased with employment years at entry to the study, by 3.5-fold in ≥10 years and 5.3-fold in ≥20 years compared with <10 years. A similar trend was seen for NMRD. Smokers had greater mortality from all causes than nonsmokers, but the latter also had slightly increased SMR for lung cancer. No excess lung cancer mortality was observed in cumulative exposures of <20 f-yrs/ml. However, significantly increased mortality was observed in smokers at the levels of ≥20 f-yrs/ml and above, and in nonsmokers at ≥100 f-yrs/ml and above. A similarly clear gradient was also displayed for NMRD. The exposure-response relationships with lung cancer and NMRD persisted in multivariate analysis. Moreover, a clear gradient was shown in GI cancer mortality when age and smoking were adjusted for.

Conclusion

There were clear exposure-response relationships in this cohort, which imply a causal link between chrysotile asbestos exposure and lung cancer and nonmalignant respiratory diseases, and possibly to gastrointestinal cancer, at least for smokers.

Abuse of Epidemiology: Automobile Manufacturers Manufacture a Defense to Asbestos Liability

Much of the "debate" about the relationship between asbestos exposure from automobile brake work and asbestos-induced cancer has been fueled by studies that have been funded by corporations with billions at stake in tort litigation. The authors explore how asbestos-lined brake manufacturers have corrupted medical literature to escape liability, analyzing studies funded by these companies to enable them to claim that work with asbestos brake linings never causes mesothelioma. They reveal how the companies have redefined scientific criteria for the determination of cause-effect relationships and manipulated scientific data to give the impression of an absence of effect. But the absence of evidence is not evidence of the absence of an effect.

Occupational exposure to asbestos and lung cancer in men: evidence from a population-based case-control study in eight Canadian provinces

Background

Asbestos is classified as a human carcinogen, and studies have consistently demonstrated that workplace exposure to it increases the risk of developing lung cancer. Few studies have evaluated risks in population-based settings where there is a greater variety in the types of occupations, and exposures.

Methods

This was a population based case–control study with 1,681 incident cases of lung cancer, and 2,053 controls recruited from 8 Canadian provinces between 1994 and 1997. Self-reported questionnaires were used to elicit a lifetime occupational history, including general tasks, and information for other risk factors. Occupational hygienists, who were blinded to case–control status, assigned asbestos exposures to each job on the basis of (i) concentration (low, medium, high), (ii) frequency (<5%, 5-30%, and >30% of the time in a normal work week), and (iii) reliability (possible, probable, definite). Logistic regression was used to estimate odds ratios (ORs) and their corresponding 95% confidence intervals (CI).

Results

Those occupationally exposed to (i) low, and (ii) medium or high concentrations of asbestos had ORs for lung cancer of 1.17 (95% CI=0.92 – 1.50) and 2.16 (95% CI=1.21-3.88), respectively, relative to those who were unexposed. Medium or high exposure to asbestos roughly doubled the risk for lung cancer across all three smoking pack-year categories. The joint relationship between smoking and asbestos was consistent with a multiplicative risk model.

Conclusions

Our findings provide further evidence that exposure to asbestos has contributed to an increased risk of lung cancer in Canadian workplaces, and suggests that nearly 3% of lung cancers among Canadian men are caused by occupational exposure to asbestos.

Occupational and environmental causes of lung cancer

Because tobacco smoking is a potent carcinogen, secondary causes of lung cancer are often diminished in perceived importance. The goal of this review is to describe the occurrence and recent findings of the 27 agents currently listed by the International Agency for Research on Cancer (IARC) as lung carcinogens. The IARC's updated assessments of lung carcinogens provide a long-overdue resource for consensus opinions on the carcinogenic potential of various agents. Supplementary new information, with a focus on analytic epidemiologic studies that has become available since IARC's most recent evaluation, are also discussed.

Lung cancer risk at low cumulative asbestos exposure: meta-regression of the exposure-response relationship

PURPOSE

Existing estimated lung cancer risks per unit of asbestos exposure are mainly based on, and applicable to, high exposure levels. To assess the risk at low cumulative asbestos exposure, we provide new evidence by fitting flexible meta-regression models, a notably new and more robust method.

METHODS

Studies were selected if lung cancer risk per cumulative asbestos exposure in at least two exposure categories was reported. From these studies (n = 19), we extracted 104 risk estimates over a cumulative exposure range of 0.11-4,710 f-y/ml. We fitted linear and natural spline meta-regression models to these risk estimates. A natural spline allows risks to vary nonlinearly with exposure, such that estimates at low exposure are less affected by estimates in the upper exposure categories. Associated relative risks (RRs) were calculated for several low cumulative asbestos exposures.

RESULTS

A natural spline model fitted our data best. With this model, the relative lung cancer risk for cumulative exposure levels of 4 and 40 f-y/ml was estimated between 1.013 and 1.027, and 1.13 and 1.30, respectively. After stratification by fiber type, a non-significant three- to fourfold difference in RRs between chrysotile and amphibole fibers was found for exposures below 40 f-y/ml. Fiber-type-specific risk estimates were strongly influenced by a few studies.

CONCLUSIONS

The natural spline regression model indicates that at lower asbestos exposure levels, the increase in RR of lung cancer due to asbestos exposure may be larger than expected from previous meta-analyses. Observed potency differences between different fiber types are lower than the generally held consensus. Low-exposed industrial or population-based cohorts with quantitative estimates of asbestos exposure a required to substantiate the risk estimates at low exposure levels from our new, flexible meta-regression.

Cause-specific mortality in relation to chrysotile-asbestos exposure in a Chinese cohort

INTRODUCTION

The carcinogenic potency of chrysotile asbestos remains a contentious topic, and more data are needed to address this issue. We examine cause-specific mortality, especially lung cancer, and its association with chrysotile-asbestos exposure in a Chinese cohort.

METHODS

A cohort of 577 workers from a chrysotile-textile plant was followed prospectively from 1972 to 2008. Occupational history, exposure information, and smoking data were obtained from company records and personal interviews; vital status and causes of death were ascertained from death registries and hospitals. Workers were classified into three exposure levels on the basis of exposure assessments of different workshops. Standardized Mortality Ratios (SMRs) were calculated in terms of exposure levels and other indices.

RESULTS

Among 259 identified deaths, 53 died from lung cancer, with an SMR of 4.08 (95% confidence interval 3.12, 5.33), and 96 from all cancers with an SMR of 2.09 (1.71, 2.55). In addition, two deaths from mesothelioma were observed. Increased mortality from respiratory diseases was also observed (SMR 3.38, 95% confidence interval 2.72, 4.21). Asbestos-exposure levels, exposure years, and birth cohorts showed a clear trend of risk for lung cancer and respiratory diseases.

CONCLUSION

The current analysis indicated that exposure to chrysotile asbestos was closely associated with excess mortality from lung cancer and respiratory diseases.

The ticking time-bomb of asbestos: its insidious role in the development of malignant mesothelioma

The relationship between asbestos exposure and malignant mesothelioma (MM) has been well established. Despite bans on asbestos use in an increasing number of nations, the prolonged latency from exposure to diagnosis, and the ongoing presence and use of these dangerous fibres, have led to the increasing prevalence of this deadly disease worldwide. Whilst occupational contact has been implicated in the bulk of diagnosed cases over the past 50 years, a significant proportion of disease has been linked to para-occupational, domestic and environmental exposure. In this review, we will provide an update on the impact of historical and ongoing asbestos contact in both occupational and non-occupational settings. Furthermore, we will address the unresolved controversies surrounding the use of chrysotile asbestos, the effect of gender and genetics on development of this disease, childhood mesothelioma and co-aetiological factors including SV40 exposure.

Oxidative stress: a new risk factor for thyroid cancer

Oxidative stress (OS) is a state of excessive free radicals and reactive metabolites among which the most important class is reactive oxygen species (ROS) - radicals derived from oxygen - as represented by the superoxide anion radical (O2(·-)) and its reactive metabolites, hydroxyl radical (·OH) and hydrogen peroxide (H(2)O(2)). In essence, OS represents an imbalance between the production of oxidants - ROS - and their elimination by antioxidative systems in the body. Many studies have linked OS to thyroid cancer by showing its association with abnormally regulated oxidative or antioxidative molecules. The study by Wang et al. in the December 2011 issue of Endocrine-Related Cancer (18, 773-782) further supports this relationship by demonstrating a high total oxidant status and OS index in thyroid cancer patients. The origin of ROS in thyroid cancer patients has not been defined, but thyroid cancer itself can be one since inflammation, a major event in it, is a classical source of ROS. ROS may in turn enhance the mitogen-activated protein (MAP) kinase and phosphatidylinositol-3-kinase (PI3K) pathways, forming a vicious cycle propelling thyroid tumorigenesis. Regardless of the mechanism, the clinical implication of the association of OS with thyroid cancer is severalfold: one, OS is a new risk factor for thyroid cancer; two, OS confers thyroid cancer patients an increased risk for cardiovascular diseases, degenerative neurological disorders, and other cancers that are classically associated with OS; and three, interference with OS may reduce this risk and be therapeutically beneficial to thyroid cancer itself in thyroid cancer patients. These interesting possibilities deserve further studies.

A meta-analysis of asbestos and lung cancer: is better quality exposure assessment associated with steeper slopes of the exposure-response relationships?

BACKGROUND

Asbestos is a well-recognized cause of lung cancer, but there is considerable between-study heterogeneity in the slope of the exposure-response relationship.

OBJECTIVE

We considered the role of quality of the exposure assessment to potentially explain heterogeneity in exposure-response slope estimates.

DATA SOURCES

We searched PubMed MEDLINE (1950-2009) for studies with quantitative estimates of cumulative asbestos exposure and lung cancer mortality and identified 19 original epidemiological studies. One was a population-based case-control study, and the others were industry-based cohort studies.

DATA EXTRACTION

Cumulative exposure categories and corresponding risks were abstracted. Exposure-response slopes [KL (lung cancer potency factor of asbestos)] were calculated using linear relative risk regression models.

DATA SYNTHESIS

We assessed the quality of five exposure assessment aspects of each study and conducted random effects univariate and multivariate meta-regressions. Heterogeneity in exposure-response relationships was greater than expected by chance (I2 = 64%). Stratification by exposure assessment characteristics revealed that studies with well-documented exposure assessment, larger contrast in exposure, greater coverage of the exposure history by exposure measurement data, and more complete job histories had higher meta-KL values than did studies without these characteristics. The latter two covariates were most strongly associated with the KL value. Meta-KL values increased when we incrementally restricted analyses to higher-quality studies.

CONCLUSIONS

This meta-analysis indicates that studies with higher-quality asbestos exposure assessment yield higher meta-estimates of the lung cancer risk per unit of exposure. Potency differences for predominantly chrysotile versus amphibole asbestos-exposed cohorts become difficult to ascertain when meta-analyses are restricted to studies with fewer exposure assessment limitations.

Occupational exposure to asbestos and ovarian cancer: a meta-analysis

OBJECTIVE

A recent Monographs Working Group of the International Agency for Research on Cancer (IARC) concluded that there is sufficient evidence for a causal association between exposure to asbestos and ovarian cancer. We performed a meta-analysis to quantitatively evaluate this association.

DATA SOURCES

Searches of PubMed and unpublished data yielded a total of 18 cohort studies of women occupationally exposed to asbestos.

DATA EXTRACTION

Two authors independently abstracted data; any disagreement was resolved by consulting a third reviewer.

DATA SYNTHESIS

All but one study reported standardized mortality ratios (SMRs) comparing observed numbers of deaths with expected numbers for the general population; the exception was a study that reported standardized incidence ratios. For simplicity, we refer to all effect estimates as SMRs. The overall pooled SMR estimate for ovarian cancer was 1.77 (95% confidence interval, 1.37-2.28), with a moderate degree of heterogeneity among the studies (I2 = 35.3%, p = 0.061). Effect estimates were stronger for cohorts compensated for asbestosis, cohorts with estimated lung cancer SMRs > 2.0, and studies conducted in Europe compared with other geographic regions. Effect estimates were similar for studies with and without pathologic confirmation, and we found no evidence of publication bias (Egger's test p-value = 0.162).

CONCLUSIONS

Our study supports the IARC conclusion that exposure to asbestos is associated with increased risk of ovarian cancer.

Cancer mortality among Chinese chrysotile asbestos textile workers

To determine mortality associated with exposure to chrysotile asbestos, a cohort of asbestos workers from an asbestos textile factory in China was followed prospectively from 1972 to 2008. A total 577 workers were successfully followed, achieving a follow-up rate of 98.5% over 37 years. Employment data and smoking information were obtained from factory and individual workers. Vital status was ascertained from factory personnel records and the municipal death registry. Workers were categorized into high, medium and low exposure groups in terms of their job titles and workshops. Follow-up generated 17,508 person-years, with 259 deaths from all causes, 96 all cancers and 53 lung cancers and 2 mesotheliomas. The highest cancer mortality was observed in the high exposure group, with 1.5-fold age-adjusted mortality from all cancers and 2-fold from lung cancer compared to the low exposure group. Age and smoking adjusted hazard ratio in the high exposure group was 2.99 (95%CI, 1.30, 6.91) for lung cancer and 2.04 (1.12, 3.71) for all cancers. Both smokers and nonsmokers at the high exposure level had a high death risk of lung cancer, with a clearer exposure-response trend seen in smokers. This study confirmed increased mortality from lung cancer and all cancers in asbestos workers, and the cancer mortality was associated with exposure level.

Evaluation of exposure to the airborne asbestos in an asbestos cement sheet manufacturing industry in Iran

Iran imports nearly 55,000 tons of Chrysotile asbestos per year and asbestos cement (AC) plants contribute nearly 94% of the total national usage. In the present study, airborne asbestos concentrations during AC sheet manufacturing were measured. The fiber type and its chemical composition were also evaluated by scanning electron microscopy (SEM), with energy-dispersive X-ray analysis. Airborne total fiber concentrations of 45 personal samples were analyzed by phase contrast microscopy. The results have highlighted that 15.5% of samples exceed the threshold limit value (TLV) established the American Conference of Governmental Industrial Hygienists, which is 0.1 fiber per milliliter (f/ml). Personal monitoring of asbestos fiber levels indicated a ranged from 0.02 ± 0.01 to 0.16 ± 0.03 f/ml. The geometrical mean was 0.05 ± 1.36 f/ml, which is considerably lower than the TLV. SEM data demonstrate that the fibrous particles consisted, approximately, of Chrysotile (55.89%) and amphiboles (44.11%). We conclude that the industrial consumption of imported Chrysotile asbestos is responsible for the high airborne amphibole asbestos levels in the AC sheet industry. More research is needed to improve characterization of occupational exposures by fiber size and concentration in a variety of industries.

Advances in metal-induced oxidative stress and human disease

Detailed studies in the past two decades have shown that redox active metals like iron (Fe), copper (Cu), chromium (Cr), cobalt (Co) and other metals undergo redox cycling reactions and possess the ability to produce reactive radicals such as superoxide anion radical and nitric oxide in biological systems. Disruption of metal ion homeostasis may lead to oxidative stress, a state where increased formation of reactive oxygen species (ROS) overwhelms body antioxidant protection and subsequently induces DNA damage, lipid peroxidation, protein modification and other effects, all symptomatic for numerous diseases, involving cancer, cardiovascular disease, diabetes, atherosclerosis, neurological disorders (Alzheimer's disease, Parkinson's disease), chronic inflammation and others. The underlying mechanism of action for all these metals involves formation of the superoxide radical, hydroxyl radical (mainly via Fenton reaction) and other ROS, finally producing mutagenic and carcinogenic malondialdehyde (MDA), 4-hydroxynonenal (HNE) and other exocyclic DNA adducts. On the other hand, the redox inactive metals, such as cadmium (Cd), arsenic (As) and lead (Pb) show their toxic effects via bonding to sulphydryl groups of proteins and depletion of glutathione. Interestingly, for arsenic an alternative mechanism of action based on the formation of hydrogen peroxide under physiological conditions has been proposed. A special position among metals is occupied by the redox inert metal zinc (Zn). Zn is an essential component of numerous proteins involved in the defense against oxidative stress. It has been shown, that depletion of Zn may enhance DNA damage via impairments of DNA repair mechanisms. In addition, Zn has an impact on the immune system and possesses neuroprotective properties. The mechanism of metal-induced formation of free radicals is tightly influenced by the action of cellular antioxidants. Many low-molecular weight antioxidants (ascorbic acid (vitamin C), alpha-tocopherol (vitamin E), glutathione (GSH), carotenoids, flavonoids, and other antioxidants) are capable of chelating metal ions reducing thus their catalytic activity to form ROS. A novel therapeutic approach to suppress oxidative stress is based on the development of dual function antioxidants comprising not only chelating, but also scavenging components. Parodoxically, two major antioxidant enzymes, superoxide dismutase (SOD) and catalase contain as an integral part of their active sites metal ions to battle against toxic effects of metal-induced free radicals. The aim of this review is to provide an overview of redox and non-redox metal-induced formation of free radicals and the role of oxidative stress in toxic action of metals.

Lung cancer mortality from exposure to chrysotile asbestos and smoking: a case-control study within a cohort in China

Objective

To confirm the association between exposure to chrysotile asbestos and lung cancer risk and to demonstrate the combined effect of smoking and asbestos exposure.

Methods

A case–control study of 1139 asbestos workers identified 41 male lung cancer cases in 2001; each case was matched by age (±5 years) with five controls. Workers in seven workshops were categorised into high-, medium- and low-exposure subgroups, and conditional logistic regression was applied to estimate the odds ratios for lung cancer risk associated with the different exposure levels. Smoking, age at first exposure, and exposure duration were considered as covariates/confounding factors. A joint effect of asbestos exposure and smoking on lung cancer risk was analysed using a conditional logistical model.

Results

54% of cases had high exposure and 24% low exposure, while 24% of controls had high exposure and 44% low exposure. Smoking was more common in cases (90%) than in controls (73%). The adjusted OR for lung cancer was 3.66 (95% CI 1.61 to 8.29) for high exposure and was elevated slightly for medium exposure (1.25; 95% CI 0.47 to 3.31). Smoking was related to lung cancer risk (OR 3.33; 95% CI 1.10 to 10.08). In comparison with the low-exposure non-smoking group, the OR for the high-exposure smoking group was 10.39 (1.34 to 82.45), in contrast to 5.23 (0.50 to 54.58) for high-exposure non-smoking workers.

Conclusions

These results confirm the strong association between exposure to chrysotile asbestos and lung cancer risk, and support an interactive effect of asbestos exposure and smoking which is more than additive.