What we really need are answers to more difficult questions: Can the current cancer risk be considered "acceptable", as suggested by the 2010 Royal Society report on the oil sands,^vi are all reasonable efforts being made to mitigate the risk, and will prompt regulatory action be taken when the risk is no longer considered acceptable (if it currently is)? These are not simple questions because first we need to know:

1. The chemical nature of the toxins from the tar sands industry. (There are potentially dozens, each with its own distribution within the environment.) Unfortunately, it is not possible to know pre-industry levels of these chemicals, and the adequacy and credibility of results obtained by the industry-supported regional aquatic monitoring program (RAMP) have come under serious question.^vii
2. Which chemicals have been tested and classified as human carcinogens. Ideally, any interactions between different chemicals that may affect cancer risk should also be known.
3. The doses of carcinogens delivered to the population (including information on the concentration, duration of exposure, and route of exposure). Ideally, biomonitoring of individuals (for example, in hair or urine) should also be performed where warranted by higher levels in the environment.
4. Regulations concerning exposure limits for each carcinogen, and whether these limits have been approached or exceeded downstream of the oil sands industry.
5. The number of individuals exposed to the carcinogens in order to estimate the number of excess cancer cases that can be expected, and the significance one can place on this estimate. 
6. What has been done, and what can be done, to mitigate the risks of developing cancer.

Taking the position that no increase in cancer risk is acceptable fails to acknowledge the many risks to our health that we accept each day, including risks of developing cancer from lifestyle choices. The government sets limits on the levels of known carcinogens in the environment, but these limits are often meant to be "as low as reasonably achievable" and therefore are typically greater than zero. For ionizing radiation, perhaps the best understood carcinogen (and my own area of expertise), the current dose limit is 1 mSv per year for the general public. Yet a single medical imaging procedure can deliver 10 times that dose, and the natural background dose (which is highly variable from one place to another) averages three times higher.^{viii, ix} To put these amounts into perspective, exposure to 1 mSv would be expected to produce five extra cancer deaths in 100,000 people.^x It would be impossible to demonstrate a statistically significant increase in cancer incidence by exposure of small numbers of individuals to one or even 10 mSv per year, yet we are still able to estimate the probability for a large population, provided we know the exposure. 

It often comes back to risk versus benefit. We all find it easier to accept risk when it is our choice to make, but First Nations and others who make their homes downstream of the oil sands may not have that option. Both risks and benefits need to be shared fairly, and that is not often the case.

Dozens of toxic chemicals are emitted and distributed during the mining and processing of the oil sands. Arsenic is a known human carcinogen, yet a 2006 report prepared by Cantox Environmental for Alberta Health and Wellness concluded that there was a negligible risk of cancer from exposure to inorganic arsenic in the Wood Buffalo region of Alberta that contains the oil sands.^xi Although the levels of arsenic used for those cancer risk estimates were provided by the industry, independently funded studies concluded that arsenic levels were rising in that area but did not exceed the regulatory limit.^{ii, xii} However, seven of 12 other toxic metals exceeded guidelines for the protection of aquatic life by -five to 300 fold.ⁱⁱ Heavy metals, including cadmium and mercury, are considered "possible" human carcinogens, a different designation that limits what can be said about the risk for developing cancer. 

Polycyclic aromatic hydrocarbons (PAHs) include known human carcinogens that are found downstream of the oil sands. Twenty-six out of 28 measured PAHs showed, on average, a six-fold increase in concentration downstream compared to upstream.^xiii Canada Health and Welfare and the World Health Organization recommend drinking water levels for total PAHs of 0.2 g/L, and for the most carcinogenic PAH, benzo(a)pyrene, the limit is set at 0.01 g/L. The estimated lifetime risk associated with drinking water containing 0.01 µg/L benzo[a]pyrene is considered "essentially negligible" by Health Canada, and one in 100,000 by the World Health Organization.^xiv A study conducted in 2007 by Timoney xv showed that concentrations of PAHs near the oil sands vary greatly, but at times exceed guidelines, suggesting potential danger to exposed people. Perhaps we should be asking, "How dangerous is the exposure to PAHs from the tar sands industry relative to smoking cigarettes or living in an urban environment? How rapidly are levels increasing downstream of the oil sands? What are the peak levels as well as average levels?" Answering these questions requires a reliable environmental monitoring program, which is currently lacking. 

Simply demonstrating that the amount of any one carcinogen is lower than government mandated limits fails to acknowledge the possible interactions between different chemicals. Co-exposure of fish to arsenic and benzo(a)pyrene can increase rates of genotoxicity eight to 18 times above rates observed after exposure to either carcinogen alone.^xvi Currently, there is little if any information on additive or multiplicative risks of cancer from exposure to several carcinogens, so the possibility is largely ignored in assigning "safe" limits. 

With known carcinogens being distributed over a large region of Alberta, reducing exposure and subsequent risk should be an industry priority. In the 1970s, stack precipitators were instrumental in reducing airborne particulates, but subsequent industry expansion means that overall levels are now similar to those measured before precipitators were installed.ⁱⁱ Levels will continue to rise in coming years if no efforts are made to further reduce emissions. Tailings ponds should not leak as they do now ^xiii and they should be guarded against storm damage. River water flow should be monitored so that it is adequate to dilute particulates, and climate change effects and usage effects on river flow should be taken into consideration for future expansion. Technology should be developed to recover toxic heavy metals. 

What is needed to make this happen is a world-class, government-sponsored environmental monitoring system that can keep pace with the oil sands developments, is transparent but informative to the public, and examines a full range of potential environmental effects. Water testing should be as good if not better than the air quality measurements now provided by the Wood Buffalo Environmental Association, a multi-stakeholder group that publishes readouts on its website from more than a dozen sites in the region.^xvii Information on levels of carcinogens present in plants, animals, and people in the region are also needed.

A special review panel recently convened by the Alberta government has already concluded that more stringent oversight of environmental contamination in the Athabasca oil sands is necessary.^xviii The full report is due in June 2011, but recognizing that the current monitoring program is flawed and doing something about it are two separate things. Maximum toxic contaminant levels need to be set, and not just for water, but also for soil, sediment, plant, and animal life. There should be recognition that adhering to these levels may mean curtailing expansion at some future point. The pressure to accomplish these goals must come from many directions and should not rest exclusively on the inhabitants of Northern Alberta.

Back to the question, Is there a cancer threat from the oil sands? The answer is yes, because the levels of known carcinogens in the regions downstream of the industry have increased. Have these increases actually caused cancer? Perhaps, but the available data do not support an unequivocal conclusion. Cancer is too prevalent, and the number of exposed people is too small to be sure. Does this mean that there is no reason for concern, at least at present? Absolutely not. Cancer can take many years to develop and levels of carcinogens from the industry continue to increase. Until a reliable monitoring system is in place, we will have insufficient information to base estimates of cancer risk. 

The oil sands industry has the opportunity and the responsibility to mitigate these risks, but we have a responsibility to understand these risks in relation to others we encounter in our daily lives. Hall, in an earlier edition of his book,^ix examined the chances of dying from a radiation-induced cancer in relation to the risk of dying from smoking cigarettes or driving a given number of highway miles. I've used his analogy to compare PAH-induced cancer with these risks. If drinking water containing 0.01 g/L benzo(a)pyrene causes one additional fatal cancer in 100,000 people, this would be equivalent to the risk of dying from smoking 73 cigarettes or driving 178 miles. This doesn't sound too bad until we remember that we are also exposed to many carcinogens not only in drinking water but in the air we breathe and the food we eat. One of those chemicals is arsenic. The risk of dying from cancer by drinking water containing 0.01 mg/L arsenic (the government-mandated limit) is equivalent to the risk of dying by smoking 1,500 cigarettes or driving 3,500 miles. If you're wondering why maximum allowable arsenic levels are so high, it's partly because of the difficulties in estimating both exposure and risk from cancer caused by arsenic. However, Health Canada also states that its regulation represents "the lowest level of arsenic in drinking water that can be technically achieved at reasonable cost",^xix which is even more reason for close monitoring of the carcinogens produced by the oil sands industry.