Hazard surveillance is the process of assessing the distribution of, and the secular trends in, use and exposure levels of hazards responsible for disease and injury (Wegman 1992). In a public health context, hazard surveillance identifies work processes or individual workers exposed to high levels of specific hazards in particular industries and job categories. Since hazard surveillance is not directed at disease events, its use in guiding public health intervention generally requires that a clear exposure-outcome relationship has previously been established. Surveillance can then be justified on the assumption that reduction in the exposure will result in reduced disease. Proper use of hazard surveillance data enables timely intervention, permitting the prevention of occupational illness. Its most significant benefit is therefore the elimination of the need to wait for obvious illness or even death to occur before taking measures to protect workers.
There are at least five other advantages of hazard surveillance which complement those provided by disease surveillance. First, identifying hazard events is usually much easier than identifying occupational disease events, particularly for diseases such as cancer that have long latency periods. Second, a focus on hazards (rather than illnesses) has the advantage of directing attention to the exposures which ultimately are to be controlled. For example, surveillance of lung cancer might focus on rates in asbestos workers. However, a sizeable proportion of lung cancer in this population could be due to cigarette smoking, either independently of or interacting with the asbestos exposure, so that large numbers of workers might need to be studied to detect a small number of asbestos-related cancers. On the other hand, surveillance of asbestos exposure could provide information on the levels and patterns of exposure (jobs, processes or industries) where the poorest exposure control exists. Then, even without an actual count of lung cancer cases, efforts to reduce or eliminate exposure would be appropriately implemented.
Third, since not every exposure results in disease, hazard events occur with much higher frequency than disease events, resulting in the opportunity to observe an emerging pattern or change over time more easily than with disease surveillance. Related to this advantage is the opportunity to make greater use of sentinel events. A sentinel hazard can be simply the presence of an exposure (e.g., beryllium), as indicated via direct measurement in the workplace; the presence of an excessive exposure, as indicated via biomarker monitoring (e.g., elevated blood lead levels); or a report of an accident (e.g., a chemical spill).
A fourth advantage of the surveillance of hazards is that data collected for this purpose do not infringe on an individual’s privacy. Confidentiality of medical records is not at risk and the possibility of stigmatizing an individual with a disease label is avoided. This is particularly important in industrial settings where a person’s job may be in jeopardy or a potential compensation claim may affect a physician’s choice of diagnostic options.
Finally, hazard surveillance can take advantage of systems designed for other purposes. Examples of ongoing collection of hazard information which already exists include registries of toxic substance use or hazardous material discharges, registries for specific hazardous substances and information collected by regulatory agencies for use in compliance. In many respects, the practising industrial hygienist is already quite familiar with the surveillance uses of exposure data.
Hazard surveillance data can complement disease surveillance both for research to establish or confirm a hazard-disease association, as well as for public health applications, and the data collected in either instance can be used to determine the need for remediation. Different functions are served by national surveillance data (as might be developed using the US OSHA Integrated Management Information System data on industrial hygiene compliance sample results—see below) in contrast to those served by hazard surveillance data at a plant level, where much more detailed focus and analysis are possible.
National data may be extremely important in targeting inspections for compliance activity or for determining what is the probable distribution of risks that will result in specific demands on medical services for a region. Plant-level hazard surveillance, however, provides the necessary detail for close examination of trends over time. Sometimes a trend occurs independently of changes in controls but rather in response to product changes which would not be evident in regionally grouped data. Both national and plant-level approaches can be useful in determining whether there is a need for planned scientific studies or for worker and management educational programmes.
By combining hazard surveillance data from routine inspections in a wide range of seemingly unrelated industries, it is sometimes possible to identify groups of workers for whom heavy exposure might otherwise be overlooked. For example, analysis of airborne lead concentrations as determined in OSHA compliance inspections for 1979 to 1985 identified 52 industries in which the permissible exposure limit (PEL) was exceeded in more than one-third of inspections (Froines et al. 1990). These industries included primary and secondary smelting, battery manufacture, pigment manufacture and brass/bronze foundries. As these are all industries with historically high lead exposure, excessive exposures indicated poor control of known hazards. However some of these workplaces are quite small, such as secondary lead smelter operations, and individual plant managers or operators may be unlikely to undertake systematic exposure sampling and could thus be unaware of serious lead exposure problems in their own workplaces. In contrast to high levels of ambient lead exposures that might have been expected in these basic lead industries, it was also noted that over one-third of the plants in the survey in which the PELs were exceeded resulted from painting operations in a wide variety of general industry settings. Structural steel painters are known to be at risk for lead exposure, but little attention has been directed to industries that employ painters in small operations painting machinery or machinery parts. These workers are at risk of hazardous exposures, yet they often are not considered to be lead workers because they are in an industry which is not a lead-based industry. In a sense, this survey revealed evidence of a risk that was known but had been forgotten until it was identified by analysis of these surveillance data.
Objectives of Hazard Surveillance
Programmes of hazard surveillance can have a variety of objectives and structures. First, they permit focus on intervention actions and help to evaluate existing programmes and to plan new ones. Careful use of hazard surveillance information can lead to early detection of system failure and call attention to the need for improved controls or repairs before excess exposures or diseases are actually experienced. Data from such efforts can also provide evidence of need for new or revised regulation for a specific hazard. Second, surveillance data can be incorporated into projections of future disease to permit planning of both compliance and medical resource use. Third, using standardized exposure methodologies, workers at various organizational and governmental levels can produce data which permit focus on a nation, a city, an industry, a plant or even a job. With this flexibility, surveillance can be targeted, adjusted as needed, and refined as new information becomes available or as old problems are solved or new ones appear. Finally, hazard surveillance data should prove valuable in planning epidemiological studies by identifying areas where such studies would be most fruitful.
Examples of Hazard Surveillance
Carcinogen Registry—Finland. In 1979 Finland began to require national reporting of the use of 50 different carcinogens in industry. The trends over the first seven years of surveillance were reported in 1988 (Alho, Kauppinen and Sundquist 1988). Over two-thirds of workers exposed to carcinogens were working with only three types of carcinogens: chromates, nickel and inorganic compounds, or asbestos. Hazard surveillance revealed that a surprisingly small number of compounds accounted for most carcinogen exposures, thus greatly improving the focus for efforts at toxic use reduction as well as efforts at exposure controls.
Another important use of the registry was the evaluation of reasons that listings “exited” the system—that is, why use of a carcinogen was reported once but not on subsequent surveys. Twenty per cent of exits were due to continuing but unreported exposure. This led to education for, as well as feedback to, the reporting industries about the value of accurate reporting. Thirty-eight per cent exited because exposure had stopped, and among these over half exited due to substitution by a non-carcinogen. It is possible that the results of the surveillance system reports stimulated the substitution. Most of the remainder of the exits resulted from elimination of exposures by engineering controls, process changes or considerable decrease in use or exposure time. Only 5% of exits resulted from use of personal protective equipment. This example shows how an exposure registry can provide a rich resource for understanding the use of carcinogens and for tracking the change in use over time.
National Occupational Exposure Survey (NOES). The US NIOSH carried out two National Occupational Exposure Surveys (NOES) ten years apart to estimate the number of workers and workplaces potentially exposed to each of a wide variety of hazards. National and state maps were prepared that show the items surveyed, such as the pattern of workplace and worker exposures to formaldehyde (Frazier, Lalich and Pedersen 1983). Superimposing these maps on maps of mortality for specific causes (e.g., nasal sinus cancer) provides the opportunity for simple ecological examinations designed to generate hypotheses which can then be investigated by appropriate epidemiological study.
Changes between the two surveys have also been examined—for example, the proportions of facilities in which there were potential exposures to continuous noise without functioning controls (Seta and Sundin 1984). When examined by industry, little change was seen for general building contractors (92.5% to 88.4%), whereas a striking decrease was seen for chemicals and allied products (88.8% to 38.0%) and for miscellaneous repair services (81.1% to 21.2%). Possible explanations included passage of the Occupational Safety and Health Act, collective bargaining agreements, concerns with legal liability and increased employee awareness.
Inspection (Exposure) Measures (OSHA). The US OSHA has been inspecting workplaces to evaluate the adequacy of exposure controls for over twenty years. For most of that time, the data have been placed in a database, the Integrated Management Information System (OSHA/IMIS). Overall secular trends in selected cases have been examined for 1979 to 1987. For asbestos, there is good evidence for largely successful controls. In contrast, while the number of samples collected for exposures to silica and lead declined over those years, both substances continued to show a substantial number of overexposures. The data also showed that despite reduced numbers of inspections, the proportion of inspections in which exposure limits were exceeded remained essentially constant. Such data could be highly instructive to OSHA when planning compliance strategies for silica and lead.
Another use of the workplace inspection database has been a quantitative examination of silica exposure levels for nine industries and jobs within those industries (Froines, Wegman and Dellenbaugh 1986). Exposure limits were exceeded to various degrees, from 14% (aluminium foundries) to 73% (potteries). Within the potteries, specific jobs were examined and the proportion where exposure limits were exceeded ranged from 0% (labourers) to 69% (sliphouse workers). The degree to which samples exceeded the exposure limit varied by job. For sliphouse workers excess exposures were, on average, twice the exposure limit, while slip/glaze sprayers had average excess exposures of over eight times the limit. This level of detail should prove valuable to management and workers employed in potteries as well as to government agencies responsible for regulating occupational exposures.
Summary
This article has identified the purpose of hazard surveillance, described its benefits and some of its limitations and offered several examples in which it has provided useful public health information. However, hazard surveillance should not replace disease surveillance for noninfectious diseases. In 1977 a NIOSH task force emphasized the relative interdependence of the two major types of surveillance, stating:
The surveillance of hazards and diseases cannot proceed in isolation from each other. The successful characterization of the hazards associated with different industries or occupations, in conjunction with toxicological and medical information relating to the hazards, can suggest industries or occupational groups appropriate for epidemiological surveillance (Craft et al. 1977).