5.2 Occupational Exposure Limits
Toxicity data is used to generate occupational exposure limits (OELs). OELs for chemical hazards represent the maximum acceptable concentration of a hazardous substance in workplace air. In theory, workers exposed to a chemical substance at the OEL for their entire working life will experience no adverse health effects. Each jurisdiction in Canada sets its own OELs. As we saw in Chapter 4, there are also OELs for physical hazards such as noise, radiation, and (more rarely) vibration. There are approximately 800 OELs in Canada.
Provincial and territorial regulations can set three types of OELs, depending on the nature of the substance’s toxicity:
- A time-weighted average exposure value (TWAEV) is the maximum average concentration of a chemical in the air for a normal 8-hour working day or 40-hour working week.
- The short-term exposure value (STEV) is the maximum average concentration to which workers can be exposed for a short period (e.g., 15 minutes). The STEV is often higher than the TWAEV.
- The ceiling exposure value (CEV) is the concentration that should never be exceeded in a workplace.
OELs for a vapour or gas are often set as parts per million (ppm). Aerosols (e.g., dust, fumes, mist) are normally set as milligrams per cubic meter of air (mg/m3). Fibrous substances (e.g., asbestos) are typically set as fibres per cubic centimeter of air (f/cc or f/cm3). Compliance with OELs is often assessed via air sampling. Periodic air samples do not necessarily capture normal working conditions because the act of testing may temporarily change workplace behaviour. This dynamic is called the observer effect.
When establishing OELs, governments often follow threshold limit values (TLVs) published by the ACGIH. The TLVs are the ACGIH’s recommendations for allowable chemical exposure. While it is an arms-length body, concerns about its recommendations have been raised. Nearly one sixth of all the ACGIH’s TLVs have been set based upon unpublished corporate data, which raises concerns about the validity and reliability of the results. Further, the committees that set these standards have included a significant number of industry representatives and consultants—many of whose relationships to industry were hidden while they were members—thereby raising concerns about conflict of interest in the establishment of TLVs.[1]
Indeed, many scientists dispute the notion that there is any safe level of exposure for carcinogens and reproductive hazards. In this view, so-called safe levels of exposure reflect simply the point below which scientists are (at present) unable to detect ill effects. Box 5.2 takes on the thorny issue of why the ongoing reduction in OELs—while doubtlessly beneficial to workers—is evidence that OELs have not been very effective at protecting them.
Why are declining OELs so concerning?
A concerning trend in OELs is that so-called safe levels of exposure go down over time, often dramatically. The exposure level for benzene, for example, dropped from 100 ppm to 10 ppm between 1945 and 1988, and exposure limits on vinyl chloride dropped from 500 ppm to 5 ppm. This phenomenon is not just a part of the distant past. Alberta reduced its OEL for chrysotile asbestos from 2 f/cc in 1982 to 0.5 f/cc in 1988 to 0.1 f/cc in 2004.
On the surface, this trend toward ever-lower OELs seems to indicate the system works: as new scientific evidence about chemical hazards becomes available, regulators revise their OELs. Yet let us think about this a bit more deeply. The law of probability suggests that, all else being equal, sometimes initial OELs will set be too high and sometimes they will be set too low. So why do OELs always go downward? Shouldn’t they go up at least some of the time?[2]
The constant downward trend in OELs actually demonstrates a systemic underestimation of risk to workers by regulators. That is to say, regulators almost always err on the side of over-exposing workers to chemical hazards. Why is this? There are likely three reasons.
The first is that the science underlying OELs has not been very good. For example, in, 90% of cases where TLVs have been set, there is insufficient data on the long-term effects of exposure from either animal or human studies.[3]This introduces uncertainty into the regulatory process. This uncertainty is exacerbated when employers hide evidence that substances negatively affect workers, sometimes by producing studies of questionable validity.[4] The second reason (explored later in this chapter) is that the threshold of scientific certitude is often set very high and this makes it hard to “prove” substances are hazardous.
The third reason is that regulators operate in a political environment, where workers, employers, and the state all seek to advance their interests. It follows that regulators setting standards must ask what actions will be politically palatable. In this way, setting exposure limits is not a purely scientific process, but also a political one. Among the findings of researchers is that most exposure limits have been set at levels industries were already achieving. [5]That is to say, “safe” OELs appear to be defined in practice as “convenient for employers” rather than “posing no hazard to workers.” Even with processes that involve multiple stakeholders at the table (i.e., labour and employers), the outcomes tend to favour employers due to imbalances in political power and access.[6]
This discussion expands our understanding of how the social construction of hazards affects workplace safety. By labelling levels of exposure as “safe” (even when they are not), the state is able to define some hazards out of existence. This benefits employers because many of these substances are integral to industrial processes or are the least expensive substance available to do the job. The effect of such hazardous substances on workers is ignored. After all, how can a “safe” substance cause harm to a worker?
Compounding concerns about the validity of OELs is their usefulness in today’s labour market. OELs assume a standard employment relationship with a single employer and an 8-hour workday. Many workers have more than one job and may experience chemical exposures at each worksite. These combined exposures may exceed OELs or may entail complicated chemical interactions. Yet OHS regulations do not require employers to consider chemical exposures workers experience from other jobs or in the community. Employers may well not even know that workers have a second job, let alone what chemical exposures they have. In this way, the trend toward increasingly precarious employment can create workplace hazards that are essentially invisible. There is also a gendered dimension to OELs. Most OELs have been set based upon studies of healthy young men, and the resulting standards are applied to both genders.[7] OELs do not take into account individuals’ varying sensitivities to chemicals. The same exposure level may result in no ill effects for one worker, while the next person next might experience health effects.
This critique of OELs raises important questions about the validity of information contained in material safety data sheets (MSDS). An MSDS is supposed to contain information about potential hazards, safe use, storage, and handling practices, and emergency procedures. Manufacturers and suppliers must provide and employers must make available an up-to-date MSDS for any chemicals that are considered controlled products by WHMIS. Often the information in MSDSs is based upon OELs. Inaccurate OELs can undermine the utility of MSDSs, which are the key method by which information about chemical hazards is communicated. Further, analysis of the content of MSDSs has also found them to be incomplete, inaccurate, sometimes out of date, and often incomprehensible to workers.[8] These findings raise profound questions about the effectiveness of chemical hazard assessment, recognition, and control efforts. More detailed and accurate information is available in databases provided by organizations such as the Canadian Centre for Occupational Health and Safety (e.g., ChemInfo database), but these resources can be expensive to access and difficult for workers to find.
- Castleman, B., & Ziem, G. (1988). Corporate influence on threshold limit values. American Journal of Industrial Medicine, 13(188), 531–559. ↵
- Dorman, P. (2006). Is expert paternalism the answer to worker irrationality? In V. Mogensen (Ed.), Worker safety under siege: Labor, capital and the politics of workplace safety in a deregulated world (pp. 34–57). Armonk, NY: M.E. Sharpe. ↵
- Castleman, B., & Ziem, G. (1988). Corporate influence on threshold limit values. American Journal of Industrial Medicine, 13(188), 531–559. ↵
- Michaels, D. (2008). Doubt is their product: How industry’s assault on science threatens your health. Toronto: Oxford University Press. ↵
- Roach, S., & Rappaport, S. (1990). But they are not thresholds: A critical analysis of the documentation of threshold limit values. American Journal of Industrial Medicine, 17, 728–753. ↵
- Foster, J. (2011). Talking ourselves to death? Prospects for social dialogue in North America—Lessons from Alberta. Labor Studies Journal, 36(2), 288–306. ↵
- Messing, K. (1998). One-eyed Science: Occupational health and women workers. Philadelphia: Temple University Press. ↵
- Nicol, A-M, Hurrell, C., Wahyuni, D., McDowall, W., & Chu, W. (2008). Accuracy, comprehensibility, and use of material safety data sheets. American Journal of Industrial Medicine, 51(11), 861–876. ↵
The maximum acceptable concentration of a hazardous substance in workplace air.
The maximum average concentration of a chemical in the air for a normal 8-hour working day or 40-hour working week.
The maximum average concentration to which workers can be exposed for a short period.
The concentration of a substance that should never be exceeded in a workplace.
A form of testing error stemming from temporary workplace behaviour change due to the act of testing.
