In the 1970s, a group of U.S. workers scraped a chemical off the sides of large vats to make polyvinyl chloride, or PVC. Almost all of them developed a rare form of liver cancer.

The case involved a small number of people, but their cancers were easily traced to the workplace, said Prof. Colin Soskolne, an occupational cancer epidemiologist at the University of Alberta, where he teaches about the classic story.

"The reason we study occupational groups is because that's where people are most likely, certainly historically, to have been egregiously exposed [to carcinogens] without protections and safeguards and on an ongoing, regular basis," said Soskolne.

Figuring out the extent of carcinogens in the workplace today requires more scientific detective work.

The detectives link contact with a substance associated with cancer to higher-than-average risk of developing the disease.

People who sand wood for a living need to take precautions because wood dust is associated with some cancers. (Andrew Winning/Reuters)

The France-based International Agency for Research on Cancer (IARC) is the global authority on determining if a substance causes cancer.

Researchers consider IARC's rigorous approach to assessing causes of cancer to be the gold standard, because it carefully weighs toxicology studies on lab animals exposed to high doses and human population studies from scientific and industrial perspectives.

Some carcinogens like asbestos become politicized during IARC's reviews, experts say.

The main reason it's so difficult to make cause-and-effect links is that it takes many years from first exposure before cancer may appear, said Dr. Anthony Miller, professor emeritus at the Dalla Lana School of Public Health at the University of Toronto.

Miller, a physician and epidemiologist, said Canada has been active in identifying carcinogens. That's partly because compensation boards encouraged people suffering from work-induced diseases including cancer to receive compensation. That in turn encouraged them to adopt prevention steps to protect people and save money.

Asbestos, a known carcinogen, was mined in Canada until October, 2011, but it may just be a temporary halt for the 130-year-old Canadian industry. (Jacques Boissinot/Canadian Press)

Although it's fashionable to say industry was uncooperative, very often they were, Miller said. Industrial exposures, such as in nickel smelting, have been reduced.

People working now are exposed to lower levels, making it harder and more expensive to tell if they are still at risk.

But Miller believes more can still be done to reduce exposure.

"Do you adopt what we call the precautionary principle, or do you wait until you're absolutely certain? Well, industry wants you to wait until you're absolutely certain. But that's not especially good for public health," Miller said.

Canadians should not be exposed to carcinogens at work, says the Canadian Cancer Society. But when exposure can't be eliminated, the levels should be reduced as low as possible, the group adds.

U.S. researchers don’t have specific information about cancer caused in the workplace. But based on 1997 mortality data, they estimate that occupational deaths from a combination of cancer, respiratory diseases, heart disease and other illnesses account for 64,751 deaths a year — more than from motor vehicle deaths. .

Lin Fritschi, a cancer epidemiologist at the Western Australian Institute for Medical Research in Perth, estimated workplace carcinogens cause 5,000 cancers ever year in that country.

Industry wants to wait until a risk is absolutely certain, but that's not what is best for public health, says Dr. Anthony Miller, a Toronto-based physician and epidemiologist. (University of Toronto)

Canada doesn't have current numbers. Researchers at CAREX Canada are trying to put together the pieces. By linking exposure to carcinogens in various industries, the B.C.-based research initiative is flagging which employees might be at risk.

Sarah Miller, a researcher with the Canadian Environmental Law Association (CELA), lauds CAREX Canada's work because unlike other such groups worldwide, it is combining occupational and environmental exposures.

"People assume that governments are protecting them, but they're not necessarily, because health protection hasn't been married with environmental protection," she said.

That's important because contact with toxics including carcinogens from our diet, workplace and leisure activities get all mixed up. A smoker who is exposed to asbestos on the job, for example, could be 90 times more at risk of cancer than a non-smoker, Soskolne said.

Better safe than sorry

Joel Tickner, a professor at the University of Massachusetts Lowell, applauds Europe's better-safe-than-sorry approach to workplace carcinogens, but wants to see more innovation on alternatives. (Adrian Bisson/UML)

"If we look at informing consumers and workers about what exactly they're working with and their risk, I think that will definitely move us closer to reducing cancer prevalence," said Joanne Di Nardo, senior manager of public issues with the Ontario division of the Canadian Cancer Society.

Her group lobbied for Ontario's Toxic Reduction Act. It's the first legislation in the country that requires manufacturing and mining facilities in the province to track and quantify what substances they use and create, including carcinogens. Industries are also encouraged to reduce use of those substances and publicly release their plans starting by the end of 2012.

The law was modeled on the Massachusetts Toxics Use Reduction Act. Over 20 years, emissions from plants in the state decreased by 56 per cent and use of toxic chemicals decreased by 21 per cent, according to officials. The reductions saved the manufacturing industry $43 to $50 million US between 2000 and 2009, which doesn't include health and safety benefits.

The Massachusetts law requires companies to pay a fee for using toxics, Fritschi said.

But Ontario's law lacks enforcement powers, pollution prevention planners that are independent of industry and an institute to train them in finding and adopting the best substitutes, Di Nardo noted.

Those were key elements to success in Massachusetts, said Joel Tickner, a professor in the School of Health & Environment at the University of Massachusetts Lowell.

Innovation requires both willingness and capacity. When a company designs a more efficient spray nozzle, they'll quickly save money. It's in the more difficult work of redesigning production lines for health and environmental benefits where the training institute has paid off, he said.

Tickner applauds Europe's better-safe-than-sorry approach. The EU puts the onus on companies to show a chemical can be used safely if it causes cancer, damages DNA, is dangerous to reproduction or builds up to harmful levels in organisms or the environment.

Shifting burden of proof to industry

In North America, substances are presumed safe until deemed dangerous by regulators like the U.S. Food and Drug Administration, Health Canada or the U.S. Environmental Protection Agency.

But the same is not true in medicine. Pharmaceutical companies must prove safety before new medications get licensed. Similarly, doctors err on the side of caution when applying lab results to patients, said Ann Novogradec, a PhD candidate in the Faculty of Environmental Studies at York University in Toronto.

To gain more clues about how exposure to carcinogens in the workplace cause cancer, the Canadian Partnership Against Cancer, which funds CAREX Canada, has also launched five large studies in Atlantic Canada, Quebec, Ontario, Alberta and B.C.

Organizers will recruit tens of thousands of people and follow them for a number of years to see who develops various diseases and complications.

Instead of relying on people to recall exposures from decades ago, these studies are a powerful way to look at how our environment and genetic make-up affect disease, said Dr. Heather Bryant, vice president of cancer control for the Canadian Partnership Against Cancer.

Provincial tumour registries that track cancer cases and deaths don't include detailed occupational histories. But Quebec's project will look at employer's names, dates of employment and the type of work that participants did over periods of time, Bryant said.

The studies are already providing answers on health behaviours such as use of multivitamins and herbal supplements, but it will take longer for cancer risks to become clear.

In the meantime, workers and consumers can reduce their risk, said Maggie MacDonald, toxic program manager at Environmental Defence.

"Industry has the advantage of being able to move more quickly" than governments to protect consumers, she said.

MacDonald wants people to be more informed about harmful substances in consumer products and workplaces, avoid them as much as possible, and demand more regulation and bans against some hazards.

This is part of a three-part series, Exposed: On the Job, about carcinogens in Canadian workplaces. The series continues on Friday.