Canadian Occupational Safety

25 2019 AUGUST/SEPTEMBER To calculate the OEL to be used for an extended work shift, a safety man- ager follows an equation using the standard work schedule. In the case of a Category II chemical, the man- ager will divide the standard shift per day by the actual number of hours worked. Thus, if a worker works 12 hours a day, the reduction factor will be equal to 8/12. The OEL (according to the province or territory) is then multiplied by a factor of 0.67 to get the adjusted exposure limit. In the case of a Category III chemi- cal, the manager divides the standard number of hours worked per week by the average number of hours actually worked per week based on the repet- itive work cycle. Thus, for a worker working 84 hours during a two-week work cycle, the reduction factor will equal 40/42. The OEL is then mul- tiplied by a factor of 0.95 to get the adjusted exposure limit. Although it's important for safety managers to understand how the equation is reached, they can simply consult tables of factors on the IRSST website. Each table corresponds to one of the three categories of chemi- cals that require adjustment. For safety managers, the proce- dure is fairly straightforward. After identifying the chemical substance, they should check to see into which adjustment categor y the chemical falls. They then check the relevant table to find the adjustment factor. If Category III, they will also need to establish the repetitive work cycle to calculate hours worked per week. The Quebec model is also in Austra- lia, and it is referenced by the ACGIH and other health and safety organiza- tions, Aubin says. In 2018, the Ontario MOL recommended the model's expo- sure adjustment calculations be used to adjust OELs. A list of chemicals and their corresponding category is avail- able on the IRSST website. The Brief and Scala method, considered the simplest and most conservative of the three models, uses equations that take into account both the longer exposure time and corre- sponding shorter recovery time caused by an extended shift. The method is used for all chemicals regardless of their half-lives (the amount of time it takes for half an initial amount of a substance to disintegrate), with the exception of substances whose OEL is based on concentration and may not be affected by hours of exposure. The user calculates both a daily and weekly adjusted exposure limit, and the more conservative of the two cases should be used. Finally, the pharmacokinetic model is founded on the idea that the toxic effect of chemicals is related more to the concentration of chemi- cals in the body than in the air. It uses a mathematical model that requires the input of information such as the biological half-life of a chemical to predict peak body burden. Like the Brief and Scala model, the pharmaco- kinetic model takes into account both the exposure and recovery time. This model is more difficult to use because biological half-lives are not available for many chemicals. It is the least conservative of the models. Of these commonly used ways to adjust OELs, no one model is gener- ally accepted more or less than the others, says Jan Chappel, senior tech- nical specialist at CCOHS. "There's no one way to make those adjustments. You need someone with the knowledge, like an industrial hygienist, to make that assessment. You need someone who knows what chemicals you're working with, how people are exposed, whether it's inhaled or dermal. And then they take the model that best matches what's happening in the workplace," she says. In regard to provincial occupa- tional health and safety law, the Brief and Scala model is required in some cases unless, as the Alberta code says, the employer has received approval from a director of occupational hygiene to use another method. In other jurisdictions, Lavoie says, employers are not required to use a specific model. "Nova Scotia is not much different than many other jurisdictions where the legislation is not set in stone," he says. "Nova Scotia would most likely be comfortable if you apply any estab- lished model and are able to support it with a reasonable explanation." When employers do not have an industrial hygienist on staff and do not want to consult one, they should just use the Brief and Scala model to adjust exposure limits, Lavoie says. "That's because it's the most restric- tive one. The philosophy here is the more you know about your product [for example, knowing the toxico- logical half-life], the more options you have. The less you know about your product, the more restrictive the model you have to apply because of the uncertainty," he says. Moreover, he adds, employers who are not completely sure their chosen adjustment method will limit expo- sure to a safe level should introduce some medical surveillance of workers at the same time they implement an extended shift. Often, workers are exposed to more than one chemical at a time. In this situation, the safety professional must first analyze the individual chemicals to determine what kind of effect the combination will produce. Some chemicals, due to similarity of toxicity, will simply produce an increase in concentration. Others will interact in different ways to produce completely different effects. The combined effects of chemicals are classified as independent, addi- tive, antagonistic, potentiating and synergistic. Independent materials are evaluated individually and, along with their concentrations, are used in a formula to determine the adjusted OEL. There is no model for adjust- ment of the exposure limit where chemicals are thought to have syn- ergistic or potentiating effects. It is important to note, too, that the bio- logical effect of exposure to several chemicals is often unknown. The safety manager should try to find a substitute for one of the chemicals to avoid the potential effect. In adjusting an OEL, the resulting figure should always be lower, Chappel adds. An exposure limit should never be adjusted upwards for shorter shifts. "We assume a non-traditional work schedule is a longer workday. But there are people who work shorter workdays. However, they caution never to use the equations to raise the exposure limits. You can't skew the numbers to allow someone to be exposed to more than the OEL," she says. "That would not be considered good practice at all." Safety managers should also keep in mind that some workers at their site may be working at another job that the employer doesn't know about, Chappel says. For this reason alone, it is a good idea to reduce expo- sures as much as possible. "In all cases, the better approach is to keep your exposures as low as possible regardless of the OELs. You should try and minimize or elimi- nate your exposures as much as you can. OELs are good guidelines, but they don't guarantee a person won't become ill in the long term," she says. "The general term is called the precautionary principle: Take action when you don't have all the informa- tion. But in this case, take action to keep exposures as low as possible." COS Linda Johnson is a Toronto-based freelance journalist who has been writing for COS for eight years.

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