Annals of Occupational Hygiene Advance Access published online on October 10, 2007
Annals of Occupational Hygiene, doi:10.1093/annhyg/mem050
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Trends in Inhalation Exposure—A Review of the Data in the Published Scientific Literature
1 Research Division, Institute of Occupational Medicine, Research Avenue North, Riccarton, Edinburgh, UK
2 Centre of Occupational and Environmental Health, University of Manchester, Manchester, UK
3 Environmental Epidemiology Division, Institute for Risk Assessment Sciences, University of Utrecht, Utrecht, The Netherlands
4 Independent Consultant, Ainsdale, Southport, UK
* Author to whom correspondence should be addressed. Tel: +44(0)870 850 5131; fax: +44(0)870 850 5132; e-mail: karen.creely{at}iom-world.org
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGACKNOWLEDGEMENTSREFERENCES |
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As part of a larger study aimed at identifying the long-term changes in inhalation exposure for selected hazardous substances in a number of industrial sectors within the UK, we have reviewed the published literature on temporal changes in inhalation exposure. Scientific papers and reports of interest were identified using standard literature review techniques. Most studies did not express the results as relative annual trends in exposure, and so where possible the data were reanalysed using regression methods to produce estimates of the average annual percentage change in concentration. In the majority of instances, there were significant reductions in exposure, with percentage yearly declines up to 32%. In many studies, information about changes in the working environment, process conditions or other factors that may have influenced the change in exposure over time was lacking. Factors commonly cited as being responsible for exposure reductions included the introduction of new standards and response to regulatory requirements as well as changes in production methods. A large number of exposure measurement datasets exist for many industrial sectors for most of the second half of the 20th century and this resource has allowed us to identify trends in occupational exposure. It is most important that longitudinal exposure data continue to be collected along with relevant contextual information to enable future changes to be adequately assessed.
exposure data • review • temporal trends
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Exposure data are collected and used for several purposes, e.g., to provide evidence for setting statutory occupational exposure limits (OEL), as well as to determine compliance with OELs, to obtain exposure estimates for epidemiological studies and to assess the effectiveness of control measures. A great number of exposure measurements have been collected by scientists and occupational hygienists over the last few decades. As part of a larger study aimed at identifying the long-term changes in inhalation exposure to selected hazardous substances within the UK (Creely et al., 2006), we have undertaken a review of the published literature discussing temporal changes in inhalation exposure. The introduction and subsequent changes in health and safety legislation [e.g. the Control of Substances Hazardous to Health Regulations (COSHH)], changes and reductions in OELs and the production of guidance on control and good practice are just a few of the factors that may have had some impact on temporal trends in exposure.
The main aim of this paper was to identify and review the published scientific literature that could inform temporal changes in personal inhalation exposure to hazardous substances. The review focused on estimates of the magnitude of temporal trends in inhalation exposures to aerosols, gases, vapours and fibres. The secondary aim of this paper was to identify and summarize those factors cited in the published literature that may have been responsible for any observed changes.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Identification of applicable published articles
Standard literature review techniques were used to identify relevant articles published. Several online bibliographic databases were interrogated including
- Barbour Index Health & Safety Professional (http://www.barbour-index.co.uk)
- EBSCOhost Electronic Journals Service (EBSCO) (http://ejournals.ebsco.com)
- Canadian Centre for Occupational Safety and Health (http://www.ccohs.ca)
- ISI Web of Science Service for UK Education (http://wos.mimas.ac.uk)
- Science Direct (http://www.sciencedirect.com)
- PubMed (http://www.ncbi.nlm.nih.gov/)
Keyword search terms included ‘trends', ‘inhalation', ‘time', ‘temporal’ and ‘occupational’ and ‘exposure’ which were used singly and in combination. The abstracts from the resulting searches were reviewed, with the full paper being obtained if the abstract was judged to include any data or information on time trends related to inhalation exposures. The search included all types of hazardous substances although abstracts were limited to those written in English. When no abstract was available, or if it was deficient, copies of the full paper were obtained for further consideration. Secondary references were chosen from the primary paper references and additional papers were also identified through discussions with colleagues. The full papers were reviewed by an occupational hygienist, with only those papers which provided details relating to personal exposure measurements being considered further.
Statistical analysis of inhalation trends identified
For several of the papers identified, annual percentage change in exposure was calculated by the authors. In all cases, the monitoring data were transformed to the log scale prior to analyses using statistical regression models. Around half of the papers used individual data points in the statistical models, adjusting for between and within worker differences as appropriate, and around half modelled geometric mean levels of exposure.
Where annual percentage trends in exposure were not quoted by the authors, data from the publications were analysed using regression methods to obtain trends information. Key data from each of the articles of interest were, where possible, entered into an Excel file. These included substance of interest, year measurement collected, industrial sector, exposure value, unit concentration (e.g. mg m–3), summary statistic (e.g. geometric mean, arithmetic mean) as well as the country and industry where the study was carried out. Papers were excluded from analysis if the measurement of exposure was not quantified (e.g. reported as percentage above OEL) or there was insufficient information on time period.
Geometric or arithmetic mean concentrations were analysed on a log scale in relation to year of sample (expressed as a continuous variable). The resulting regression coefficient was used to calculate average annual change in concentration. We have assumed that the relative changes in exposure were likely to be similar whether the data were represented as arithmetic or geometric means. In some instances, the time period of the data was presented as a range of years and where this occurred, we assigned a single year in the middle of the range. Where there was no unique central value (e.g. when the range covered 6 years), the lower of the two central values was used.
Generally all data points from the citation were analysed together to calculate an overall annual percentage change in concentration. Where there was clear information on different industries or jobs, additional analyses were carried out by job and/or industry. The summary of time trends from the published literature is given as the average annual percentage change in concentration.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Articles that included some discussion or information relating to personal inhalation exposures at different time periods were identified in the published literature. These papers included information on a wide range of hazardous substances in a variety of industries, with several also providing some information on the factors thought to be responsible for the changes observed which are discussed in the following sections.
Trends in exposure to aerosols
Several studies have looked at the US Occupational Safety and Health Administration (OSHA) Integrated Management Information System (IMIS) database to identify time trends for various agents including dusts and fumes (Froines et al., 1990; Stewart and Rice, 1990; Gomez, 1997; Teschke et al., 1999; Coble et al., 2001; Okun et al., 2004; Yassin et al., 2005). Gomez (1997) evaluated 2111 8-hour time-weighted average (TWA) personal exposures to lead in the battery industry and reported an average decline of 5–9% per year in mean exposures for the period 1979–1989 for four of the five exposure groups further analysed, although no temporal trends were observed for welder exposures to iron oxide. Okun et al. (2004) also evaluated trends in occupational lead exposures after the establishment of the general industry lead standard in 1978 and the construction standard in 1993 and found that the median exposure levels declined 5- to 10-fold over the time period for general industry [based on a dataset of 32 385 samples spanning 19 years (1979–1997)] but not for the construction industry (based on a dataset of 1998 samples). This was thought to be due to the limited number of years of data that were available following the implementation of the revised standard. Froines et al. (1990) however reported limited improvement in the prevalence and severity of airborne lead exposures collected in the battery manufacturing (dataset of 1149 measurements), secondary smelting (dataset of 718 measurements), pigment manufacturing (dataset of 143 measurements) and brass/bronze manufacturing industries (dataset of 822 measurements) for the period 1980–1985.
Using 7209 respirable crystalline silica dust compliance inspection data collected from 1988 to 2003, Yassin et al. (2005) provided an update to the study undertaken by Stewart and Rice (1990). Overall, a non-statistically significant decline in exposure of 10% per year was observed between 1988 and 2003. The geometric mean in the general contractor industry was found to be nearly 6.2 times lower in 1988–2003 compared to exposure reported by Stewart and Rice (1990) in 1979–1987, with similar reductions noted in the bridge tunnel construction sector (5.5 times lower) and stonework masonry sector (9.8 times lower). The authors suggest that the significant decline observed in the construction industry may be explained by the implementation of advanced health and safety programmes and effective engineering controls. In the grey iron industry, silica exposure levels were significantly lower in 1988–2003 than the levels reported by Stewart and Rice (1990) (e.g. exposure levels for ‘furnace workers’ declined by 53.5%, ‘grinders’ by 28.6% and ‘cleaning department’ by 50.8%), which was thought to be due to OSHA's inspections and enforcement actions.
Coble et al. (2001) analysed time trends in personal exposures to various air contaminants measured during compliance inspections of pulp and paper manufacturing facilities conducted between 1979 and 1997 (database contains results of 3568 personal TWA measurements for 171 air contaminants). They observed decreasing trends for three of the four metals (although increasing trends for chromium) and all the dusts/mists assessed (ranging from 2 to 14% per annum), with particulates not otherwise regulated decreasing annually by 6%. Lastly, Teschke et al. (1999) analysed 1632 measurements of airborne wood dust and found that the unadjusted geometric mean in 1979 of 4.59 mg m–3 reduced to 0.14 mg m–3 in 1997.
Decreasing trends in datasets other than the US IMIS database have also been reported. Jones and Smith (1986) surveyed seven woodwork companies in the furniture industry in the UK in 1976/1977 and again in 1983. Results from the surveys were categorized into various groups and inspection of these revealed that for 12 of the 15 groups, dust concentrations were less in 1983 than in 1976/1977 although the extent of the improvement varied from one group to another. Factors suggested to have contributed to these reductions included changes in production methods, modifications or upgrading in the capacity of dust control and extraction equipment and advice and inspection from the enforcement agency. A decreasing time trend in airborne dust exposure in the US textile industry was reported by Abdel-Kader and Rando (1987), with the percentage of respirable dust samples with results <0.2 mg m–3 increasing from 53.1 to 77.5 and 81.6% for years 1, 2 and 3 sampled, respectively (296, 306 and 264 samples being collected in each of the respective years). These changes were thought to be the result of the introduction of modern automated yarn processing equipment and elimination of some dusty processes. Kononen et al. (1989) reports declines in median 8-h TWA exposures to lead of 10, 12 and 20% in assembly, battery plants and foundries from 1980 to 1985.
Van Tongeren et al. (2000) investigated the trends in exposure to inhalable dust in 18 carbon black manufacturing facilities in seven countries in Western Europe. During this period, a large number of personal inhalable (n = 8015) dust exposure measurements were taken during three phases of data collection between 1987 and 1995. The results showed that the personal inhalable dust exposure had reduced significantly since 1987, particularly for the job categories ‘warehousemen’, ‘site crew’ and ‘fitter/welder’. Similarly, Muranko et al. (2001), when comparing total dust exposures collected in the US carbon black industry between 1993 and 1995 with those collected during 1970–1980 by Smith and Musch (1982) reported 58, 59 and 70% reductions in geometric mean exposure for the ‘maintenance’, ‘production’ and ‘materials handling’ categories, respectively. Although the laboratory department showed an increase in geometric mean exposure by 325%, the mean exposure was still well below the permissible exposure limit and it was viewed that this increase may be due to greater job diversity. The decreases in exposure were thought to be due to several factors such as upgraded process equipment design, increased usage of local ventilation systems for material handling and packaging, introduction of low dust producing cleaning methods such as water washing and vacuuming and the greater use of bulk shipping containers for transportation.
Deubner et al. (2001) analysed 34 307 airborne beryllium samples collected between 1970 and 1999 at three sites in the US beryllium mining and extraction industry (of which only 3% were personal breathing zone samples) with the analysis demonstrating that personal exposures decreased over the time period. Stefaniak et al. (2003) summarized 4528 personal breathing zone and area air samples collected by the Los Alamos National Laboratory in New Mexico for beryllium by decade for the time period 1949–1989 and found that while exposures decreased dramatically from the 1940s to the 1950s (although only a few measurements were available from the 1940s), exposures were generally stable thereafter. Kolanz et al. (2001) in their critique of historical and current exposure assessment methods for beryllium state that industrial airborne beryllium concentrations have generally decreased over time because of regulatory requirements, improved occupational hygiene programmes and ventilation/engineering controls. Seiler et al. (1996) found few trends in exposure estimates from >2200 TWA exposures collected from the US beryllium processing industry from 1950s to 1978; however, this was possibly due to the small number of data points available for each of the 284 plant-specific job titles.
Reductions in exposures to total dust and respirable silica have also been reported for various activities. Dosemeci et al. (1995) assessed exposures in 20 mines and nine pottery factories in China from 1950 to 1987, resulting in 4846 total dust and 9678 respirable silica dust samples being available for analysis. The total dust concentration, across all operations and facilities, decreased from an arithmetic mean of 17.7 mg m–3 in the 1950s to 6.26, 4.91 and then 3.85 mg m–3 in the 1960s, 1970s and 1980s, respectively, while the mean respirable silica dust concentration decreased from 3.9 mg m–3 to 0.90, 0.56 and 0.43 mg m–3over the respective decades. Focusing only on data collected from the underground mine operators, in the 1950s the maximum mean total dust level was 21 mg m–3, which, with the introduction of effective control measures, decreased to 2.5 mg m–3 in the 1980s. Exposures to respirable quartz (n = 4269) were reported by Sanderson et al. (2000) to decrease in a stepwise fashion across 18 US silica sand plants as control measures were implemented, with the geometric mean personal measurements being 51.2, 25.6, 11.6 and 7.5 µg.m–3 for the time periods 1974–1979, 1980–1984, 1985–1988 and 1989–1996, respectively. However, it was also observed that milling and drying jobs, which had some of the highest exposures in 1946, did not show a decrease in the 30-year period, with Sanderson et al. (2000), suggesting that this may be a reflection of the historical sampling undertaken or that dust control was not implemented for these jobs to the same extent that they were for others. Sanderson et al. (2000) also noted that respirable quartz exposures appeared to increase in 1988, possibly being a reflection of changes in the analytical method for quartz analysis to improve accuracy and to conform to international methodology.
Analysis of 1258 measurements collected during 10 welding fume surveys performed during 1983–2003 in The Netherlands found a 4% decrease in exposure per year for this time period (Kromhout et al., 2004). It was noted that welding fume exposures tended to decrease at a lower rate than other chemical exposures, possibly due to the process-related nature of the task remaining unchanged over this time. As part of an epidemiological study in the European ceramic fibre industry, several hundred measurements of total inhalable dust were collected in six plants throughout Europe in 1987 and again in 1995/1996 (Cherrie et al., 1989; Groat et al., 1999), with concentrations in 1995/1996 generally being less than half of those found in 1987. Following conversion of available data collected in the diatomaceous earth industry during 1948–1988, the geometric mean respirable dust concentrations decreased from 0.372 mg m–3 during the 1950s to 0.17 mg m–3 during the later periods for all jobs (Seixas et al., 1997).
Kiilunen (1997) analysed a dataset of personal nickel exposure measurements accumulated from 1980 to 1989 at the Finnish Institute of Occupational Health, with neither increasing nor decreasing temporal trends being observed. Symanski et al. (2000) conducted an evaluation of temporal changes in exposure to nickel aerosols from data collected in 10 different nickel-producing and -using industries in North America, Western Australia and Europe, during 1973–1995 using a database of 20 108 measurements. Significant negative linear annual trends were detected for total nickel exposures in mining (7%), smelting (9%) and refining (7%), although a statistically significant increase of 4% per annum was shown in milling. Declining exposures were found to be greater in workplaces with no ventilation systems, for those workers who performed similar rather than diverse tasks, and in workplaces in North American compared to European and West Australian workplaces.
A study of time trends in exposure to inhalable particulate in the rubber industry in The Netherlands detected an average annual decrease of 5.7% per year between 1988 and 1997, based on 903 measurements (Vermeulen et al., 2000). Workers with seniority within the same department (more experienced) were reported to show an even steeper decline in exposure levels. Elimination of sources significantly reduced the inhalable particulate by two-thirds, with measures designed to control contaminant levels reducing exposures by 34%. Changes in anode technology and quality were also thought to result in declining exposures (2629 measurements collected from 1978 to 1996) to polycyclic aromatic hydrocarbons in Vertical Stud Soderberg pot rooms at two Norwegian aluminium smelters (Romundstad et al., 1999).
Wells and Greenall (2005) extracted >50 000 data points from archived UK foundry industry monitoring reports. These mainly related to dusts and their components, however, a wide range of data for gaseous and vapour phase pollutants were also extracted. Although the project only allowed a fairly limited analysis of the data, the authors considered that the results demonstrated that the Health and Safety at Work Act 1974 and COSHH, together with trade associations’ proactive approach, had a profound effect on reducing exposures.
Symanski et al. (1998a,b), using a database containing exposure data from a broad range of industries and substances collected world wide, explored various factors such as industry and type of sampling on declining temporal trends in exposure. Their findings suggest that exposures decreased more rapidly in manufacturing than in mining industries and more rapidly for aerosols than vapours. Exposures collected more recently (first year of sampling in 1972 or later) fell more rapidly than exposures first evaluated during earlier periods. Symanski et al. (1998b) also found that most exposures declined between 4 and 14% per year (although downward trends ranged from 1 to 62% per annum), with a median decline of 8% per annum.
Trends in exposure to gases and vapours
The US OSHA IMIS database has also been studied to identify time trends in workplace exposure to various gases and vapours. An investigation using OSHA data from 1979 to 1989 found decreases of 7% per year for perchloroethylene exposures among dry cleaners (Gomez, 1997). Coble et al. (2001) analysis of time trends in employee exposures to air contaminants measured during compliance inspections of pulp and paper manufacturing facilities conducted between 1979 and 1997 also showed decreasing trends for 17 of the 18 solvents (ranging from 6 to 19% per annum) although exposures for Stoddard solvent were reported to increase by 8% per annum. Decreasing trends were also reported for four of the five gases investigated (ranging from 6 to 17% per annum) although acrolein exposures were reported to increase by 6% per annum. LaMontagne et al. (2002) assessed long-term trends in ethylene oxide exposures (1984–1998, n = 87 628 work shift and 45 816 short-term samples) to evaluate the potential impact of OHSA's 1984 and 1988 standards. Although exposures declined steadily following implementation of both standards, the authors report that from 1996 to 1999, the probability of exceeding the short-term limit increased and work shift exposures might also have increased. LaMontagne et al. (2002) stated that investigation of the possible reasons for upward trends was beyond the scope of their study but possibilities include decreased enforcement or the diversion of employer's attention from ethylene oxide hazards to other hospital hazards such as blood-borne pathogens or glutaraldehyde.
Decreasing trends in gases and vapours from other datasets have also been reported. Priha et al. (1986) collected 394 personal solvent measurements during a 10-year period (1975–1984) from Finnish furniture factories (lacquering departments) and found a decreasing time trend for formaldehyde, although not for other solvents. Although the authors emphasize caution when interpreting the results they suggest that the reduction in formaldehyde was probably a consequence of increasing concern about the hazards, which led to the development of paints, lacquers and particle board that emitted less formaldehyde. Nielmela et al. (1997) presented historical trends in formaldehyde exposure in the wood panel industry in Finland based on measurements taken at eight facilities between 1980 and 1994. They report successive measurement medians of 0.91, 0.26 and 0.46 ppm for the periods 1980–1985, 1986–1990 and 1991–1994, respectively. Cherrie et al. (1999) observed that median acrylonitrile exposure measurements from a single UK company (which included >1000 measurements) decreased from 0.44 mg m–3 in 1980 to 0.22 mg m–3 in 1990, although no further information had been provided by the company to help explain the changes observed.
Improved occupational hygiene programmes at chemical facilities, following the introduction of more stringent OELs were thought to be responsible for the reductions in benzene exposures from an acetic acid manufacturing facility in the US from 1976 to 1987 (n = 3655) (Williams and Paustenbach, 2005), with manufacturing operation workers likely TWA exposures decreasing from 2 to 1ppm over this time period. Due to high solvent levels in the workplace, an occupational health programme was introduced in 1997 to reduce workers exposure to solvents within a synthetic leather manufacturing plant in Taiwan (Kuo et al., 2001). This included improved localized ventilation, safety awareness and machinery operation efficiencies. Equipment was installed to recycle dimethylformamide waste and work shifts were also rotated to reduce time in high solvent level areas. Airborne dimethylformamide, epichlorohydrin and toluene were significantly reduced in 1999 compared to 1997 levels in all three workplaces assessed. For example, epichlorohydrin personal exposures (n = 114) were reported to decrease by 97.9%, from 8.4 to 0.18 mg m–3. Although usage of solvents had decreased by 40% over the time period and production had decreased by over 50%, it was considered that the occupational health programme exerted the greater impact in reducing solvent concentrations.
Regression analysis showed that the trichloroethylene exposures of Danish workers (n = 1075) decreased on average by 4% per year from 1947 to 1964 and by 15% per year between 1964 and 1989 (Raaschou-Nielsen et al., 2002). Price et al. (1997) reviewed historical carbon disulphide exposure data in the viscose rayon industry from 15 published studies between 1940 and 1995 which were conducted in 11 countries and found that the average exposures decreased from 158 ppm in the 1940s to 
14 ppm (although it was noted that personal samples were available only after 1979). In a Danish study undertaken to analyse styrene concentrations to assist in the selection of cohorts for epidemiological purposes, Jensen et al. (1990) analysed 2528 air samples collected during the years 1955–1988. The data were analysed in terms of three time periods relating to a national campaign aimed at reducing styrene exposures. These were: pre-campaign (1955–1970), campaign period (1971–1980) and post-campaign (1981–1988). The authors found that the arithmetic mean concentration decreased by a factor of four from 714 mg m–3 in the early period (1955–1970) to 274 mg m–3 during the campaign period (1971–1980) and then to 172 mg m–3 in the late period (1981–1988). Acetone and toluene exposures were also found to decrease consistently over these time periods. Dichloromethane exposures were lower pre-campaign although remained fairly constant from 1971 onwards and xylene exposures were observed to fluctuate over the three time periods. The authors noted that the focus in the pre-campaign was to obtain worst-case measurements, whereas the measurements in the post-campaign were performed mainly to assess factories’ own control, although they believed that selection alone could not explain the 4-fold decrease in styrene exposure.
In a study aimed at analysing time trends in commercial painters solvent exposures collected in The Netherlands between 1980 and 1999 (n = 304 measurements of solvent exposure), the decline in solvent content of paints and the more widespread use of water-based paints since the 1990s was thought to help explain the 12% reduction per year in toluene exposures (Burstyn and Kromhout, 2002). Chouaniere et al. (2002) observed that arithmetic mean toluene exposures in a French heliogravure plant decreased during 1981 and 1996 from 174 to 5.6 ppm for printing block operators, 100 to 25 ppm for printing press operators and 79 to 28.6 ppm for operators employed in the shaping and dispatch workshops (based on a total of 231 measurements collected). Printing press shielding and extraction were thought to help to reduce toluene exposures in a Danish rotogravure plant from 100 ppm (TWA) before 1983 to <20 ppm thereafter (Eller et al., 1999). Caldwell et al. (2000) evaluated 16 880 solvent exposure measurements from 99 articles and reported that in general, hydrocarbon solvent exposures decreased 4-fold from 1960 to 1998. Caldwell et al. (2000) also discuss some articles, which demonstrate reductions in exposure over time. For example, Ekberg et al. (1986) commented that exposures were higher in the 1950s and 1960s when more solvent-based glues were used.
Burstyn et al. (2000) found a decline in bitumen vapour exposure by a factor of 2–3 each decade (14% per year time trend) in their dataset of exposure measurements collected from 1581 asphalt pavers from eight countries. The most likely explanation offered for the decline was better temperature control in the production and application of asphalt. In the ‘old days’ when asphalt was transported from the mixing plant to the paving site overheating was used to keep the product workable. With better insulation of the trucks and better temperature control, this was no longer needed (H. Kromhout, personal communication). Steinsvag et al. (2006) reported downward annual time trends of 6 and 8% for oil mist and oil vapour during 1979–2004 in offshore drilling of oil-based mud in Norway. The authors reported that these trends were mainly associated with decreases in between-rig variance, possibly due to rigs with lower exposures being included at the later sampling periods rather than exposure being reduced over time within each rig.
Trends in exposure to fibres
Few papers were identified which explicitly provided details of temporal changes in fibre inhalation exposures of fibres. Paustenbach et al. (2003) reported a 10-fold decrease in TWA asbestos concentrations during brake work from the 1970s to 1980s which was thought to be attributable to increased usage of brake-dust control measures in some US garages, changes in design of disc brakes and the introduction of non-asbestos brake pad materials in the 1980s. Hagemeyer et al. (2006) noted a continual decrease in asbestos dust concentrations in German workplaces from the 1950s to 1990 which was most likely attributable to the rapid decline in use of asbestos since 1980 corresponding to regulations and bans on the production, usage and placement of asbestos onto the market. Coble et al. (2001) reports 5% annual declines in asbestos exposures (all forms) measured during compliance inspections of pulp and paper manufacturing facilities conducted between 1979 and 1997.
As part of an epidemiological study in the European ceramic fibre industry, several hundred measurements of respirable and non-respirable fibres were collected in six plants throughout Europe on two occasions in 1987 and 1995/1996 (Cherrie et al., 1989; Groat et al., 1999), with this data being reanalysed by Miller et al. (2007). Since 1987, there appeared to be considerable success in reducing fibre concentrations, with concentrations in 1995/1996 being, in many instances, less than half those reported in 1987 (Miller et al., 2007). The authors also noted that changes in exposure differed by occupational group, possibly due to the amount of handling and manipulation of dry materials for some. Maxim et al. (2000) summarizes data from a comprehensive workplace exposure monitoring programme for refractory ceramic fibre (RCF) conducted since 1990, including data collected under a 5-year consent agreement between the US Environmental Protection Agency and the RCF Coalition. This updates data presented in an earlier paper (Maxim et al., 1997). Analysis of the data collected at plants operated by manufacturers and at customer facilities revealed that average RCF concentrations decreased during the period from 1990 to 1998.
Reanalysis of data from the published literature
The annual percentage change in concentration for aerosols, gases and vapours and, lastly, fibres for those datasets reanalysed and where annual trends in exposure were presented by the authors of the citation are summarized in Tables 1, 2 and 3, respectively. In instances where the datasets were reanalysed, details of the summary central tendency statistic used (arithmetic, geometric mean, median) are provided where possible.
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The changes in exposure ranged from a reduction of 32% per year for asbestos garages (based on reanalysed data) (Paustenbach et al., 2003) to an increase of 8% for Stoddard solvent in the pulp and paper industry (which was not statistically significant) (Coble et al., 2001). Five of the 87 datasets [chromium, acrolein, Stoddard solvent (Coble et al., 2001), total nickel (Symanski et al., 2000), dichloromethane (based on reanalysed data) (Jensen et al., 1990)] showed increasing time trends, although only one of these was statistically significant.
The calculated average annual percentage change in exposure for vapours and gases ranged from a 24% annual decline in hexane exposures to an 8% annual increase in exposure to Stoddard solvents, both within the paper and pulp industries (Coble et al., 2001). The average annual percentage change in exposure for dusts and fumes ranged from a 19% decline for polyaromatic hydrocarbons in various industries (Romundstad et al., 1999), to an increase of 4% in nickel dust exposures in the milling industry (Symanski et al., 2000) and an increase of 4% in chromium dust exposure in the paper and pulp industry (Coble et al., 2001). For fibres, annual declines in exposure ranged up to 32% for particular brake jobs carried out by motor mechanics (Paustenbach et al., 2003) and no increasing trends in annual percentage changes in exposure were identified.
Suggested factors responsible for temporal trends in exposure
The secondary aim was to identify those factors cited in the published literature that may have been responsible for any observed trends in inhalation exposure over time. Table 4 provides a summary of the factors cited in the identified papers as possibly being responsible for both decreasing and increasing temporal trends.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Several studies were identified that addressed quantitative changes in average inhalation exposure to various hazardous substances, with the majority reporting declining exposures. The published articles described data spanning a time period ranging from the 1940s to 2003, although the majority of the articles reported data collected between the 1970s and 1990s and between the 1970s and 1980s. We have not attempted to assess the quality, nor pass judgement on the quality of the data collected, analysed and reported in each of the identified papers. It should also be noted that the studies discussed were presenting data collected for many different purposes, and in some instances, not for the purpose of determining temporal trends in exposure. The quality of the data in terms of how meaningful they are in determining temporal trends will no doubt vary although we assume that the quality of the actual exposure data contained within each of the identified papers will at least have been assessed to some degree given that it has been published in the peer-reviewed literature. While some papers such as Vermeulen et al. (2000) and Van Tongeren et al. (2000) applied similar measurement strategies, the same sampling and analytical procedures, and studied the same companies and even partially the same workers over the time period in question, this was not generally the case. Given that relatively few papers actually reported annual trends in exposure, reporting instead various statistics such as arithmetic means or geometric means for a given year or time period, it is difficult to reach a general consensus for the reported data. However, estimates of the average annual percentage change in concentration have either been provided by authors or through our regression analysis, with a declining annual trend being observed in nearly all instances, ranging up to 32%. For aerosols, the temporal trend ranged from an annual decline of 19% for polyaromatic hydrocarbons to an increase of 4% per annum for chromium and total nickel. For gases and vapours, the temporal trends ranged from an annual decline of 24% per annum for hexane to an increase of 8% per annum for Stoddard solvent, whereas only declining trends were observed for fibres, ranging up to 32% per annum.
Relatively few studies have examined data obtained from UK workplaces, with most focusing on larger European and USA datasets. It was noted that a limited number of published articles offered possible explanations for the changes in exposure observed (although there are some notable exceptions such as Vermeulen et al., 2000) thus indicating gaps in the research. Such information is valuable to help identify practices, legislation and so forth which may be responsible for initiating changes in exposure. Those factors cited as resulting in a decreasing temporal trend in exposure are outlined in Table 4 and include regulatory changes, implementation of health, safety and hygiene programmes, changes in production methodologies and improvements in ventilation used. Increases in annual trends in exposure were noted in several instances, although possible explanations for these observed trends for the hazardous substances and scenarios reported were rarely provided. Those cited included changes in sampling strategies and analytical methods and diversion of attention to other hazards.
Although possible factors responsible for the reported temporal trends in exposure have been provided, it is reasonable to suggest that many others will also exert an effect. For example, as part of our larger study looking at trends in inhalation exposure, follow-up case studies were undertaken in companies where exposure measurements were collected >10 years ago, with key employees being interviewed to identify those factors responsible for any changes in exposure observed. While these interviews also suggested that legislation, both health and safety related and environmental, has had a positive impact in reducing exposure, changes in industry and market forces, resulting in processes being outsourced (particularly to countries out with the UK), were noted to have had a significant impact (Creely et al., 2006). It is however important to consider that globally these translocations may potentially lead to increased numbers of workers being exposed and at higher levels.
Possible problems in trying to establish temporal trends included insufficient number of years of follow-up data to identify any changes (Okun et al., 2004) and changes sampling strategy and methods. For example, Raaschou-Nielsen et al. (2002) found that exposures were higher in previous years, when worst-case measurements were collected whereas later data were collected to assess the general level of control. Steinsvag et al. (2006) also suggested that observed reductions in exposure may have been due to changes in sampling strategy. Care must be taken during data analysis of temporal trends to consider such variables so that trends in exposure can be fully explained. It was also noted that it is often difficult to establish the role of other, social and economic factors which may affect time trends.
Maxim et al. (2000) observed that time trends tend to flatten out in absolute terms over time as the limits of technological and economic feasibility are reached and this poses the following question: ‘What will the trends in inhalation exposure be in the future?’. Alesbury (2002) suggests that there will be a continued decrease in TWA exposures for those working in large companies due to increasing regulatory pressure, improved technology and automation although this may also be influenced by both external and internal economic pressures on industry and governments. It is also unclear to what extent trends in exposure may be influenced by the outsourcing of ‘dirty’ jobs to contractors and the movement of production to less-developed countries where control measures may not always be as strictly applied (Kromhout and Vermeulen, 2000). However, Alesbury (2002) does express some concern that the increase in automation and enclosure may actually increase the number and intensity of peak/short-term exposures during maintenance work when control measures may be compromised. It was noted that in the identified literature, there was relatively very little discussion concerning temporal trends in short-term or peak exposures, which is perhaps a reflection of the type of data most commonly reported and also by the tendency to consider only long-term exposure measurements for such assessments. When considering how we will measure temporal trends in exposure in the future, we need to take due consideration of all changes in work and exposure patterns, including maintenance/remedial activities. Alesbury (2002) suggests that in the future, small- to medium-sized enterprises (SMEs) will account for an increasing proportion of the workforce and that there is little reason to anticipate major changes in exposures in these organizations over the next 10 years without a fundamental change in the approach to regulation, health risk management and enforcement activity. There is usually a lack of reliable exposure data from SMEs which makes it difficult to determine trends in these companies.
The overall aim of reducing occupational exposures is to prevent ill-health. If the cited reductions in inhalations exposure are real, rather than being, e.g. an artefact of changes in study methods (a question raised and discussed previously by Kromhout and Vermeulen 2000), then it is reasonable to expect that reductions in the incidence and prevalence of occupational diseases should also be demonstrated (with due consideration being given to applicable latency periods). Limited literature does suggest that reductions in occupational disease have occurred over recent years which may possibly be linked to reductions in exposure levels. For example, Diller (2002) critical review of nine longitudinal case–control studies reported downward trends in the incidence of occupational asthma, attributable to toluene diisocyanate, which paralleled the downward trends in occupational exposure in some studies. McDonald et al. (2005) also reports some decline in occupational asthma reported from 1992 to 2001 but few changes in the incidence of acute work-related respiratory diseases. Kogevinas et al. (1998) also made a link between decreased cancer mortality and improved conditions in the rubber manufacturing industry. In order to obtain a more thorough and comprehensive picture, further analysis of both medical and occupational exposure surveillance data from national reporting schemes is required.
This study has highlighted some of the factors thought to be responsible for the temporal trends in exposure observed, with this information providing a focus for legislators both in terms of building upon existing programmes and the development and implementation of new strategies aimed at reducing occupational exposures and achieving long-term occupational health targets. It is only because of the efforts of many scientists over the decades that there is a legacy of data available to shed light on the time trends of exposure; however, the collection and storage of new measurement data appears to be decreasing, at least in the UK. For example, as Tickner (2001) discusses in his review of the UK Health and Safety Executive National Exposure Database, the addition of new data within this is diminishing, in part due to changes in operational procedures and also due to the increasing costs associated with data collection. It is also possible that where exposure data collection is compliance driven that exposure may stop being measured when there is evidence (sometimes qualitative) that control is adequate. It is most important that exposure data continue to be collected and systematically stored if any future changes in exposure are to be demonstrated which will help allow government agencies, industry and others to assess the effectiveness or otherwise of future initiatives aimed at reducing inhalation exposures throughout industry. The continuing collection of quantitative exposure data is also essential for establishing quantitative exposure–response relations via epidemiological research. Some initiatives have commenced and help provide a focus for others. For example, the CEFIC Exposure Management and Assessment System Database (CEMAS), a joint venture between the Institute of Occupational Medicine in Edinburgh and the Institute of Risk Assessment Sciences at the University of Utrecht, aims to help industry, researchers and collect chemical exposure data in a standardized format (Ritchie et al., 2007). The Industrial Minerals Association dust monitoring programme, which was launched in 2000, aims to collect statistically valid data on dust exposure levels from EU industrial minerals and already contains >6000 measurements (IMA Europe, 2006). Further consideration must be given to the use of standardized sampling protocols, the measurements strategies and the collection of auxiliary information by the occupational hygiene community to maximize the value of longitudinal datasets.
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UK Health and Safety Executive (D4807).
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The work was carried out as part of the study ‘Trends in inhalation exposure: mid-1980s till present’. We would like to thank Andy Phillips, Celia Elliott-Minty and Peter Griffin for their help and encouragement throughout the project and Sean Semple (University of Aberdeen) and Anne Sleeuwenhoek (Institute of Occupational Medicine) for their helpful comments during the preparation of this manuscript.
Received January 15, 2007; in final form September 6, 2007
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