Annals of Occupational Hygiene Advance Access published online on July 11, 2007
Annals of Occupational Hygiene, doi:10.1093/annhyg/mem030
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Evaluation of a National Register on Occupational Exposure to Carcinogens: Effectiveness in the Prevention of Occupational Cancer, and Cancer Risks among the Exposed Workers
1 Finnish Institute of Occupational Health, Topeliuksenkatu 41 a A, 00250 Helsinki, Finland
2 Finnish Cancer Register, Liisankatu 21 B, 00170 Helsinki, Finland
* Author to whom correspondence should be addressed. Tel: +358-30-4741; fax: +358-9-2414634; e-mail: timo.kauppinen{at}ttl.fi
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Objectives: The objective of this study was to evaluate the performance and effectiveness of a register of employees exposed to carcinogens (the ASA Register) which has been in operation in Finland since 1979, and to study cancer risks among the notified workers. Methods: The impact of ASA at workplaces was studied by questionnaires mailed to 1448 work departments, which were notified to ASA in 1996, and to 1033 departments, which departed ASA in 1991–1996. The mailing was responded by 69% of departments. The cancer incidence of 35 138 workers notified to ASA in 1979–1988 was followed up through the files of the Finnish Cancer Register for the period 1980–2003. Results: Changes eliminating or substantially reducing exposure to carcinogens were reported by 73% of departments notified to ASA in 1996. The ASA notification process had directly prompted measures to reduce exposure (8% of cases) or contributed to them (24% of cases). Estimations based on responses of the workplaces suggested that the ASA registration had decreased exposure of 600 workers year–1 (out of
15 000 notified workers, which is <1% of the employed in Finland), preventing thereby an unknown number of occupational cancers. Other benefits of ASA included the saving of the treatment costs of prevented cancers, the prevention of other health outcomes of carcinogens, improved safety behaviour of exposed workers and avoidance of human suffering among cancer patients and their families. The labour safety authorities had better possibilities to direct their activities against carcinogen exposure. These benefits should be considered against the annual costs, mainly due to 7–8 person-years of work required by tasks related to ASA. The results of the cancer incidence study among notified workers were based on a relatively short follow-up (on average 19 years). The incidence of mesothelioma was significantly increased in the ASA cohort, probably due to exposure to asbestos. Conclusions: These results suggest that a national exposure register may stimulate preventive measures at workplaces. Partially based on the results of the present study the Finnish Ministry of Social Affairs and Health continues ASA registration.
cancer epidemiology • exposure • health surveillance
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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The International Labour Office (ILO) recommended in 1977 recording systems to be constructed for the monitoring of occupational exposure to carcinogens (ILO, 1988). This recommendation prompted the Finnish Register of Employees Exposed to Carcinogens (ASA Register; Finnish abbreviation) in 1979. Employers were obliged to provide data on use of carcinogens and to notify exposed workers annually to labour safety authorities [to Finnish Institute of Occupational Health (FIOH) from 2006] to be entered into a database maintained by FIOH. The ultimate aim of registration was to promote prevention of occupational cancer in Finland. Obligatory registration was expected to stimulate identification, assessment and elimination of carcinogenic exposures at workplaces resulting consequently in decreased risk of occupational cancer among notified workers. Knowledge on exposures was also thought to increase the effectiveness of inspection activities of labour safety authorities by enabling focused guidance and improved control at workplaces where significant exposure occurred. In addition, ASA Register was designed as surveillance system which can be used to follow up cancer risks of exposed workers at the national level by linking its data with cancer data from the Finnish Cancer Register (FCR) (Alho et al., 1988).
About 15 000 workers from 1500 work departments were notified annually to ASA Register from the mid-1980s to 2000. The addition of environmental tobacco smoke (ETS) in the list of carcinogens in 2001 increased the number of annual notified workers to >25 000 (1% of the employed labour force) (Table 1). Exact figures on the completeness of ASA registration are not available. Although registration is obligatory, it is probable that temporary and low exposures, as well as exposures in small workplaces are not fully covered. The total number of workers exposed to carcinogens in Finland was estimated to be 500 000 (24% of the employed) in the international CAREX database (Kauppinen et al., 2000). The reason for the large difference between CAREX estimates and ASA data is that CAREX addresses carcinogens evaluated by International Agency for Research on Cancer (IARC), whereas ASA is based on an administrative list of carcinogens decided by the Finnish Ministry of Social Affairs and Health. This list is more limited than the IARC list because the registration efforts would increase manyfold, if all IARC carcinogens were covered. According to CAREX, the most common carcinogen exposures at work in Finland are solar (ultraviolet) radiation, ETS, silica dust, wood dust, radon and diesel exhaust. Out of these carcinogens, only ETS (from 2001) and selected wood dusts (oak and beech from 2004) are covered by ASA. The most common exposures in ASA are chromium (VI) compounds, nickel and its inorganic compounds, asbestos, benzene, polycyclic aromatic hydrocarbons (PAHs) and chloroform (Saalo et al., 2006). ASA does not include any information on the level of exposure, smoking or other lifestyle factors of the notified workers. The number of exposed workers notified to ASA have been reported in more detail elsewhere (see e.g. Heikkilä and Kauppinen, 1992).
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ASA Register has not been systematically evaluated with the exception of a small questionnaire-based survey on causes of exit among departments which left ASA Register in 1980–1983 (Alho et al., 1988). Cancer incidence among notified workers has not been studied in detail, because the pilot studies have not revealed any significant cancer risks, and because the follow-up becomes valid only after 10–20 years since the onset of exposure. In the late 1990s, the Finnish Ministry of Social Affairs and Health, social partners and researchers decided to evaluate the performance and effectiveness of ASA. The evaluation was based on a study carried out by FIOH and FCR. The study included two parts. The first sub-study was a questionnaire-based survey on effects of ASA at workplaces. It aimed at quantifying changes in exposure and other preventive measures related to carcinogens prompted by ASA. The second part of the project was an epidemiological study on cancer morbidity among workers notified to ASA. The questionnaire study was originally carried out in 1998 using ASA data from 1996. The first study report (in Finnish) was written in 1999 but the follow-up period of the ASA cohort was considered too short at that time. The cancer morbidity study was therefore updated in 2005 to allow for a longer and more valid follow-up. The results of the questionnaire study and the updated morbidity study are reported in the present article.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Study on preventive measures taken
A questionnaire was mailed to all workplaces (departments) which had appeared in the ASA Register during 1990–1996 and to all 11 regional offices of labour safety inspectors. There were three versions of the questionnaire: (i) Long version was sent to workplaces that appeared in the register in 1996. This version included questions on the impacts of registration, time required for compliance with registration requirements and opinions on ASA registration and dissemination of exposure information (1448 workplaces). (ii) Short version was sent to those workplaces that had appeared in ASA, but had withdrawn from it 1991–1996 (1033 workplaces). This version included only questions on effects of registration and reasons for removal. (iii) The questionnaire sent to labour safety inspectors focused on time spent in ASA-related activities and opinions on the future of the register.
In total, 2481 questionnaires were mailed to workplaces and 1700 (69%) were returned after one reminder. We received 1340 (54%) responses identifying the workplace and 250 (10%) responses from which the responder had removed the sheet identifying the workplace. Another 110 (4%) questionnaires were returned by post office (address unknown). The responses without identifying information were excluded from the statistical analysis, because they could not be linked with company information in ASA (e.g. industrial class, carcinogens notified, size of the workplace).
Since the questionnaires also touched upon opinions about ASA, we asked them to be filled jointly by the employer and employees (e.g. shop steward). Most respondents had done this (62%), but it was more common in those departments that were registered in 1996 (67%) than in withdrawn departments (57%). In another 28% of the cases, the employer alone was responsible for the answers (22% of remaining and 34% of withdrawn departments). Sometimes the questionnaire was filled by employees alone (6%), another organization (3%) or another combination of respondents (2%).
The respondents were first asked to specify the carcinogens and work tasks whose exposure was concerned. Then they were asked about if exposure was discontinued, substantially reduced or had not changed/had increased (multiple choice question, see Table 2 for alternatives). The validation of the reported changes at workplaces was not feasible due to the large number of workplaces.
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Study on cancer morbidity
The cohort followed in this study included all workers who had been registered to the ASA Register in 1979–1988 (n = 35 138). In addition to the analyses of the entire cohort, separate analyses were performed for those carcinogens, in which the number of exposed workers was at least 200. For >100 rare ASA carcinogens, the number of exposed workers was <200, and they were excluded because the expected total number of cancer cases was too small (<10) to allow statistical analysis. The identification of the persons and follow-up for death and emigration was done in the Population Register Centre based on the unique personal identification number (PID) given to everybody residing in Finland since 1 January 1967.
The follow-up for cancer was done at the Finnish Cancer Registry, using PID. The calculation of person-years started from 1 January of the year following the first notification to the ASA Register. The calculation of person-years ended at emigration, at death or on 31 December 2003, whichever occurred first. The average follow-up time was 19 years.
The numbers of cancer cases and person-years were counted as stratified by gender and 5-year age group. The expected numbers of cases of cancer were calculated by multiplying the number of person-years in each stratum by the corresponding average cancer incidence in Finland. The standardized incidence ratio (SIR) was calculated by dividing the observed number of cases by the expected number of cases. The 95% confidence interval (CI) for SIR was calculated from the Poisson distribution.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Study on preventive measures taken
Exposure to carcinogens was reported to be discontinued or substantially decreased in 73% of work departments notified to ASA in 1996 (Table 2). No improvement in carcinogenic exposures was reported by 27% of departments. The most common reasons for changes were the discontinuation of work entailing exposure (22%), use of personal protective equipment (18%) and enclosure of the source of exposure or a new exhaust system (17%).
Most of responding departments (79%) that had departed ASA in 1991–1996 reported acceptable reasons leading to the departure (Table 2). The most common reasons were the re-evaluation of exposure (24%), discontinuation of work entailing exposure (22%) and substitution for a carcinogen (18%). The re-evaluation of exposure was typical for laboratory work where potential exposure was often notified once, but was later found to be negligible or non-existing. About 80 departments (21%) had departed ASA without an acceptable reason (i.e. discontinuation of carcinogen exposure), and they were returned to register. The reason for the notification failure was often that the ASA contact person responsible for the annual notifications had resigned.
Only a portion of the changes were due to ASA. The respondents reported that 69% of the changes were independent of ASA. One significant reason not related to ASA was the legal prohibition of asbestos use which has resulted in discontinuation of use of asbestos. The ASA notification process had directly prompted measures to reduce exposure in 8% of changes both in departments that were in ASA and those that had departed ASA. In addition, ASA had contributed to 23% of the changes (24% if in ASA and 19% if departed from ASA). Responses to this question were not dependent of the type of responder (representative of employer, representative of workers or joint responding).
Information on substituting materials for carcinogens was collected by agent and work task. A carcinogen was reported to have been replaced in 349 cases, 120 of which (35%) concerned asbestos in brake maintenance and 20 (5%) asbestos in heat isolation work. Chromium (VI) compounds were substituted in painting (by several chromium-free pigments), impregnation of wood (by copper compounds) and cleaning of laboratory ware (by alkaline agents). Hydrazine, which is widely used as anticorrosive agent in water systems of power plants, had been replaced with several other chemicals containing, e.g. morpholine, oximes and amines. Nickel was substituted by copper and cobalt compounds in some applications. Toluene and other organic solvents were commonly used instead of benzene. Carbon tetrachloride, chloroform and other carcinogenic solvents were replaced by non-carcinogenic solvents. Cadmium-free soldering material had also become more popular in soldering. Arsenic compounds were replaced by non-arsenic compounds in wood impregnation, and by antimony in glass manufacturing. Alternative chemicals were also available for carcinogenic analytic reagents and indicator dyes used in laboratories. Carcinogens to which no substituting agents were reported by respondents of the present survey included butadiene, Direct Red 28, Trypan Blue, acrylonitrile, benzidine, epichlorohydrin, cytostatic drugs (iso-phosphamide, chlorambusil, melphalan), 4,4'-methylene bis-2-chloroaniline (MOCA) and thioacetamide.
Apart from measures directly reducing exposure to carcinogens, other activities related to ASA registration had been carried out. According to responses, 74% of departments had reviewed material safety data sheets (i.e. identified carcinogenic products used at the workplace), 83% had assessed levels of exposure (e.g. by personal judgment), 55% had measured levels of exposure, 94% had informed workers on carcinogenic hazards, 88% had provided training for workers on safe handling of carcinogens and 65% had arranged medical examinations. Most workplaces had accomplished these measures both during first notification year and in later years.
The annual workload related to ASA registration was inquired from departments that notified workers in 1996. Filling in notifications, maintenance of data and discussions in safety committees on issues related to ASA registration typically required <1 person-day per year per department (median 3 h, mean 6 h). The first notification year required slightly more time (median 5 h, mean 10 h), because exposed workers were listed for the first time, and notifications therefore prompted more discussions on worker groups to be included. Assuming that non-responding departments spent on average the same time for ASA, the total annual workload summed up to 1100 person-days (5 person-years) in all Finnish workplaces notified to ASA (n = 1448) in 1996. This estimate does not include time taken by preventive measures related or unrelated to ASA (median 2 h and mean 10 h per department during the first year; and median 1 h, mean 7 h per department and year in consecutive years).
The labour safety inspectors in regional offices spent altogether 8–9 person-months per year for the activities related to ASA. This time was distributed between checking of ASA notifications of workplaces (3–4 person-months), inspections of ASA matters at workplaces (3–4 person-months) and treatment of other ASA-related issues (1–2 person-months). FIOH coded and entered ASA notifications to the register of employees exposed to carcinogens. In addition, FIOH produced annual ASA statistics, pre-filled forms for ASA workplaces, check lists for regional labour safety offices and used ASA data periodically for epidemiological studies and hazard surveillance. In a typical year, when 1500 departments notified workers, these activities required 1.5–2 person-years of time. The total time taken by activities related to ASA Register added up to 7–8 person-years per year in Finland at the time of the present study.
The representatives of the workplaces wished to continue registration on the annual basis (77% of respondents). Also regional authorities (80% of respondents) considered that the annual registration was the most practical procedure. Proposals to exclude exposures covered by ASA registration were rare, and they concerned mainly notifications of small amounts of carcinogens (e.g. laboratory work) or occasional exposures (e.g. during <20 workdays per year).
Space for opinions on any other issues related to ASA Register was reserved at the end of the questionnaire mailed to workplaces which notified in 1996. About 37% (n = 349) of responding departments gave their opinions. About 45% of respondents considered ASA useful and 10% useless. The comments of the rest were not classifiable. Usually they included miscellaneous proposals or addressed details of the system. The respondents in favour of ASA argued often that the system had decreased carcinogen exposure, it served as a regular reminder on cancer risks and on need to prevent them, and that appropriate handling of ASA matters increased the feeling of safety among workers. The critical respondents argued usually that ASA had not resulted in improvements at their workplaces. The most common proposals concerned information and feedback from the authorities and FIOH to workplaces. The opportunities of the Internet were regarded as essential in the future in providing information and guidance as well as aiding the notification process (e.g. notification forms through the Internet). Since the study, electronic forms and the annual ASA statistics have been made freely available at the Internet (see FIOH Web site, www.ttl.fi).
Study on cancer morbidity
Altogether 1902 cases of cancer were observed. The SIR of total cancer was not increased (Table 3). The SIR of mesothelioma was significantly increased, while the SIRs of lung cancer and of cancer of oral cavity and pharynx were significantly decreased.
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Of the sub-cohorts exposed to specific carcinogens, the SIR of total cancer was significantly increased only among those exposed to ethylene oxide. The numbers of cases of specific cancer sites were usually low in these subcohorts and very few significantly increased SIRs were found for specific cancer sites (listed in Table 4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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The number of occupational cancer cases in Finland was estimated to be annually 200–900 (1–5% of all incident cancers) in 1992. The best estimate was 400–500 cases per year (3%), most of them among men (5% of cancers among men, <1% of cancers among women) (Aitio and Kauppinen, 1992). A more recent estimate from a Nordic study resulted in
400 cases per year for the year 2000 (Dreyer et al., 1997). Past exposure to asbestos is probably the main cause of recent occupational cancers in Finland. In 2002, 48 cases of mesothelioma and 84 cases of lung cancer were notified to Finnish Register of Occupational Diseases due to exposure to asbestos (Riihimäki et al., 2004). Only a few cancer cases were notified due to all other agents than asbestos. Occupational cancer notifications are known to be somewhat incomplete for asbestos-induced cancers, and very incomplete for all other occupational cancers.
The preventive effect of ASA registration on occupational cancers cannot be directly estimated based on the present study. However, the theoretical contribution of ASA registration on carcinogen exposure in Finland can be roughly estimated on the basis of this study. Because exposure had discontinued or substantially decreased in 73% of departments in ASA in 1996 (Table 2), the improvements can be estimated to have concerned 1060 departments (10 000–11 000 workers) out of 1448 departments (all departments in ASA in 1996) assuming that the same percentage (73%) can also be applied to the non-responding departments. This improvement took place between the date of the first notification of these departments (1986 on average) and the date of responding (in 1998, i.e. during 12 years on average). This suggests that annually 800–900 workers of these departments had gained from improvements. The corresponding figures for departments that had departed ASA in 1991–1996 (i.e. during 6 years) were 725 departments and 7000–7500 workers (1200–1250 annually). The contribution of ASA to the decrease of exposure was 32% among departments in ASA in 1996 and 27% among departments which had departed ASA (see Results). If these proportions (32 and 27%) are applied to the above numbers of workers (800–900 and 1200–1250), ASA may be estimated to have discontinued or significantly reduced exposure of 600 workers annually, which is 4% of workers annually notified to ASA. According to the database of FCR, the lifetime probability of cancer among Finns is 30%. Therefore, 150–200 of these workers (mainly men) may be expected to contract cancer in the future. If 5% of these cancers is caused by occupational factors (Aitio and Kauppinen, 1992), the expected number of occupational cancers possibly prevented among 600 workers would be 8–10 out of 400–500 occupational cancers in Finland. However, this number is based on many debatable assumptions (e.g. the risk of occupational cancer among ASA workers is assumed to be similar as among all men in Finland), and it should therefore be considered only to suggest the order of magnitude of annual cancer cases prevented by ASA.
The ASA registration may also bring along other benefits than prevention of a part of occupational cancers and their consequences including medical treatment costs of cancers and human suffering of cancer patients and their families. The reduction of exposure decreases simultaneously other health risks of carcinogenic agents, such as allergic reactions (e.g. chromates, nickel), asbestosis, neurologic effects (carcinogenic solvents), systemic toxicity (e.g. cadmium, arsenic), skin damage (e.g. hydrazine), as well as possible genotoxic and reproductive effects (many carcinogens). Registration may also result in behavioural changes due to improved awareness of occupational health risk in general. The identification of carcinogens, exposure assessment, medical examinations, dissemination of information and training on chemical safety were often associated with ASA, and they may have improved safe handling of carcinogens at most workplaces, including those that did not report a substantial reduction of carcinogen exposure. Many of the respondents also noted that information on carcinogens and ASA had alleviated psychological stress among exposed workers.
Almost unanimously, the respondents of regional labour safety offices considered ASA a useful tool in their work. Accurate information on occurrence of carcinogen exposure from ASA notifications had helped them during their workplace inspections to focus on possibilities to prevent carcinogen exposures. ASA is also potentially a good information channel between workplaces, labour safety authorities and researchers. It provides surveillance information to authorities on the extent of exposure to selected carcinogens among Finnish workplaces, and enables authorities to direct their control activities and information dissemination efficiently to the right workplaces.
An important limitation of the work presented is that the benefits of the registration scheme cannot be unambiguously identified because it was not feasible to include a control group against which to measure the effect of the intervention. Theoretically, a valid control group would have been ‘workplaces using ASA carcinogens, but not registered in ASA'. Because the registration is obligatory, no such workplaces were known to us. Workplaces using other chemicals than ASA carcinogens as the control group could also be criticized because the exposure reduction of carcinogens may be expected to be more extensive than that of non-carcinogenic chemicals (e.g. due to the prohibition of asbestos, which is independent of ASA).
The reliability of these results may also be discussed because the response rate was rather low and the respondents may have been inclined to exaggerate preventive measures whose appropriateness could not be verified by the researchers. We think that the joint responding by the representatives of the employers and the workers, which was very common, decreases the possibility of this kind of bias. In addition, the responses of the employers and those of the workers were not significantly different from the joint responses. The situation in the non-responding departments is unknown to us. According to our experience, the major reason for not responding is the lack of time or interest, but it is also conceivable that some departments may have not responded because the preventive measures were neglected. Therefore, we cannot rule out the possibility of a ‘positive’ bias, although there is no evidence of it.
Our epidemiological study cannot be interpreted to measure the effect of ASA Register on cancer risk among notified workers. This would have required a follow-up of the same cohort before and after ASA registration which was not possible. However, we found the follow-up of this cohort useful for the surveillance purposes, i.e. to find out if there were any indications of occupational cancer risks after all among these workers. The latency period of occupational cancers make epidemiological studies reasonable only when exposure has continued for a lengthy period. Therefore, the results of this first systematic study on cancer risks are reported only now, >25 years after the onset of ASA. Even here, the average duration of follow-up for cancer incidence was relatively short (19 years) for workers notified to ASA in 1979–1988. Because most of the workers were probably exposed to the same carcinogens already before the introduction of ASA, the time elapsed since first exposure may well be substantially >19 years on average, which adds to the validity of these results. However, these results should still be cautiously interpreted because of possibly insufficient follow-up. At the time of this study, the only cancer site with a significantly increased incidence was mesothelioma (pleura) (Table 3). Given the long latency time of mesothelioma, this increase is most likely due to asbestos exposure during the period before ASA, i.e. before 1979. The risk of mesothelioma among workers exposed to asbestos was slightly elevated but did not quite reach statistical significance (Table 4). The probable reason for the weak association between mesothelioma and asbestos exposure is the rather low level of exposure among car mechanics which is the predominant occupation in the ASA sub-cohort exposed to asbestos since 1979. It is reassuring, however, that so far no overall excess of cancer was observed, although it is impossible to conclude whether this is due to the preventive activities prompted by ASA or other factors.
The results concerning sub-cohorts notified for specific agents or from specific occupations are based only on a few observed cases of cancer and no information was available for control of confounding by exposure to non-occupational carcinogens. None of the statistically significant cancer site–carcinogen combinations observed (in Table 4) are established human cancer risks. It is likely that many of these associations are chance findings because multiple comparisons were made between 25 different carcinogens and 40 cancer sites. Further follow-up of the ASA cohort is needed to learn more about the etiology of these associations.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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The results suggest that a national exposure register stimulates preventive measures at workplaces. The direct impact on workers’ health is difficult to assess because a number of parallel prevention measures have been taken in the society due to the high concern and priority given to occupational cancer. Yet, probably only a small part of occupational cancers may be directly prevented by this kind of registration system. Partially based on the present study, the Finnish Ministry of Social Affairs and Health is continuing ASA registration on annual basis. To avoid the registration becoming a routine operation without desired effects, the dissemination of information related to ASA, occupational cancer risks and their effective prevention has been improved by labour safety authorities and FIOH. The evaluation of costs and benefits of this type of registration is methodologically challenging and the Finnish results are not directly applicable to other countries which may wish to design similar interventions.
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FUNDING |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Partially financed by Finnish Work Environment Fund.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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We are grateful for the support and advice provided by the members of the ASA Steering Committee of the Chemical Safety Council.
Received January 22, 2007; in final form April 4, 2007
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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