Socrates viewed philosophy as beginning in wonder,[1] but Socrates and his philosophic heirs recognized that philosophy does not get down to business until it starts to clarify the terms of discussion. By the middle of the last century, failure to understand the logic of language replaced wonder as the beginning of philosophy.[2] Even if philosophy could not cure all conceptual pathology, most writers came to see that clarifying terms, concepts, and usage was an essential starting point in thinking clearly about a subject.[3]
Hazard versus Risk
Precision in scientific exposition often follows from the use of measurements, using agreed upon quantitative units, and accepted, accurate, reliable procedures for measurements. When scientists substitute qualitative measures for what are inherently quantitative measures, they frequently lapse into error. For example, beware of rodent studies that proclaim harms at “low doses,” which turn out to low in comparison to other rodent studies, but orders of magnitude greater exposure than experienced by human beings.
Risk is a quantitative term meaning a rate of some specified outcome. A Dictionary of Epidemiology, for instance, defines risk as:
“The probability of an adverse or beneficial event in a defined population over a specified time interval. In epidemiology and in clinical research it is commonly measured through the cumulative incidence and the incidence proportion.”[4]
An increased risk thus requires a measurement of a rate or probability of an outcome greater than expected in the absence of the exposure of interest. We might be uncertain of the precise measure of the risk, or of an increased risk, but conceptually a risk connotes a rate or a probability that is, at least in theory, measurable.
Hazard is a categorical concept; something is, or is not, a hazard without regard to the rate or incidence of harm. The definition of “hazard,” in the Dictionary of Epidemiology provides a definition that captures the non-quantitative, categorical natural of some exposure’s being a hazard:
“The inherent capability of a natural or human-made agent or process to adversely affect human life, health, property, or activity, with the potential to cause a disease.”[5]
The International Agency for Research on Cancer (IARC) purports to set out a classification scheme, for human cancer hazards. As used by IARC, its classification scheme involves a set of epistemic modal terms: “known,” “probably,” “possibly,” and “indeterminate.” These epistemic modalities characterize the strength of the evidence that an agent is carcinogenic, and not the magnitude of quantitative risk of cancer from exposure at a given level. The IARC Preamble, which attempts to describe the Agency’s methodology, explains that the distinction between hazard and risk is “fundamental”:
“A cancer hazard is an agent that is capable of causing cancer, whereas a cancer risk is an estimate of the probability that cancer will occur given some level of exposure to a cancer hazard. The Monographs assess the strength of evidence that an agent is a cancer hazard. The distinction between hazard and risk is fundamental. The Monographs identify cancer hazards even when risks appear to be low in some exposure scenarios. This is because the exposure may be widespread at low levels, and because exposure levels in many populations are not known or documented.”[6]
This attempted explanation reveals an important problem in IARC’s project, as stated in the Preamble. First, there is an unproven assumption that there will be cancer hazards regardless of the exposure levels. The IARC contemplates that there may circumstances of low levels of risk from low levels of exposure, but it elides the important issue of thresholds of exposure, below which there is no risk. The Preamble suggests that IARC does not attempt to provide evidence for or against meaningful thresholds of hazardousness, but this failure greatly undermines the project. Exposure circumstances may be such that there is no hazard at all, and so the risk is zero.
The purported distinction between hazard and risk, supposedly fundamental, is often blurred by the Agency, in the monographs produced by working groups on specific exposure circumstances. Consider for instance how a working group characterized the “hazard” of inhalation of respirable crystalline silica:
“ln making the overalI evaluation, the Working Group noted that carcinogenicity in humans was not detected in all industrial circumstances studied. Carcinogenicity may be dependent on inherent characteristics of the crystalline silica or on external factors affecting its biological activity or distribution of its polymorphs.
Crystalline silica inhaled in the form of quartz or cristobalite from occupational sources is carcinogenic to humans (Group 1)”[7]
So some IARC classifications actually do specify that exposure to a substance is not a hazard in all circumstances, a qualification that implies that the same exposure in some exposure circumstances is not a hazard, and so the risk is zero.
We know something about the deliberations of the crystalline silica working group. The members were deadlocked for some time, and the switch of one vote ultimately gave a bare majority to reclassifying crystalline silica as a Group I exposure. Here is how the working group member, Corbett McDonald described the situation:
“The IARC Working Group, in 1997, had considerable difficulty in reaching a decision and might well not have done so had it not been made clear that it was concerned with hazard identification, not risk.”[8]
It was indeed Professor McDonald who changed his vote based upon this linguistic distinction between hazard and risk. His own description of the dataset, however, suggests that the elderly McDonald was railroaded by more younger, more strident members of the group:
“Of the many studies reviewed by the Working Group … nine were identified as providing the least confounded evidence. Four studies which we considered positive included two of refractory brick workers, one in the diatomite industry and our own in pottery workers; the five which seemed negative or equivocal included studies of South Dakota gold miners, Danish stone workers, US stone workers and US granite workers. This further example that the truth is seldom pure and never simple underlines the difficulty of establishing a rational control policy for some carcinogenic materials.”[9]
In defense of his vote, McDonald meekly offered that
“[s]ome equally expert panel of scientists presented with the same information on another occasion could of course have reached a different verdict. The evidence was conflicting and difficult to assess and such judgments are essentially subjective. Of course, when the evidence is conflicting, it cannot be said to be sufficient. Not only was the epidemiologic evidence conflicting, but so was the whole animal toxicology, which found a risk of tumors in rats, but not in mice or hamsters.”
Aside from endorsing a Group I classification for crystalline silica, the working group ignored the purportedly fundamental distinction between hazard and risk, by noting that not all exposure circumstances posed a hazard of cancer. The same working group did even greater violence to the supposed distinction between risk and hazard in its evaluation of coal dust exposure and human cancer. Coal miners have been studied extensively for cancer risk, and the working group reviewed and evaluated the nature of their exposures and their cancer outcomes. Coal dust virtually always contains crystalline silica, often making up a sizable percentage of the total inhalational exposures (40% or so) of coal dust.[10] And yet, when the group studied the cancer rates among coal miners, and in animals, it concluded that there was “inadequate evidence in humans, and “in experimental animals,” for carcinogenicity. The same working group that agreed, on a divided vote, to place crystalline silica in Group 1, voted that “[c]oal dust cannot be classified as to its carcinogenicity to humans (Group 3).”[11]
The conceptual confusion between hazard and risk is compounded by the IARC’s use of epistemic modalities – known, probably, possibly, and indeterminate – to characterize the existence of a hazard. The Preamble, in Table 4, summarizes the categories and “the stream of evidence” needed to place any particular exposure in a one epistemic modal class or another. What is inexplicable is how and why a single substance such as crystalline silica goes from a known cancer hazard in some unspecified occupational setting but then its putative carcinogenicity becomes indeterminate when it makes up 40% of the inhaled dust in a coal mine.
The conceptual difficulty created by IARC’s fundamental distinction between hazard and risk is that risk might well vary across exposure circumstances, but there is no basis for varying the epistemic modality for the hazard assessment simply because coal dust is only say 40% crystalline silica. Some of the exposure circumstances evaluated for the Group I silica hazard classification actually were lower than the silica content of coal. Granite quarrying, for example, involves exposure to rock dust that is roughly 30% crystalline silica.
The conceptual and epistemic confusion caused by IARC’s treatment of the same substance in different exposure circumstances is hardly unique to its treatment of crystalline silica and coal dust. Benzene has long been classified as a Group I human carcinogen, for its ability to cause a specific form of leukemia.[12] Gasoline contains, on average, about one percent benzene, and so gasoline exposure inevitably involves benzene exposure. And yet, benzene exposure in the form of inhaling gasoline fumes is only a “possible” human carcinogen, Group 2B.[13]
Similarly, in 2018, the IARC classified the evidence for the human carcinogenicity of coffee as “indeterminate,” Group 3.[14] And yet every drop of coffee inevitably contains acrylamide, which is, according to IARC, a Group 2A “probable human carcinogen.”[15] Rent-seeking groups, such as the Council for Education and Research on Toxics (founded by Carl Cranor and Martyn Smith) have tried shamelessly to weaponize the IARC 2A classification for acrylaminde by claiming a bounty against coffee sellers such as Star-Bucks in California Proposition 65 litigation.[16]
Similarly confusing, IARC designates acetaldehyde on its own a “possible” human carcinogen, group 2B, even though acetaldehyde is invariably associated with the metabolism of ethyl alcohol, which itself is a Group I human carcinogen.[17] There may well be other instances of such confusions, and I would welcome examples from readers.
These disparate conclusions strain credulity, and undermine confidence that the hazard-risk distinction does any work at all. Hazard and risk do have different meanings, and I would not want to be viewed as anti-semantic. IARC’s use of the hazard-risk distinction, however, lends itself to the interpretation that hazard is simply risk without the quantification. This usage actually is worse than having no distinction at all, because it ignores the existence of thresholds below which exposure carries no risk, as well as ignoring different routes of exposure and exposure circumstances that carry no risk at all. The vague and unquantified categorical determination that a substance is a hazard allows fear mongers to substitute subjective, emotive, and unscientific judgments for scientific assessment of carcinogenicity under realistic conditions of use and exposure.
[1] Plato, Theaetetus 155d (Fowler transl. 1921).
[2] Ludwig Wittgenstein, Philosophical Investigations (1953).
[3] See, e.g., Richard M. Rorty, ed., The Linguistic Turn: Essays in Philosophical Method (1992); Nicholas Rescher, Concept Audits: A Philosophical Method (2016); Timothy Williamson, Philosophical Method: A Very Short Introduction 32 (2020) (discussing the need to clarify terms).
[4] Miquel Porta, Sander Greenland, Miguel Hernán, Isabel dos Santos Silva, John M. Last, and Andrea Burón, A Dictionary of Epidemiology 250 (6th ed. 2014).
[5] Id. at 128.
[6] IARC Monographs on the Identification of Carcinogenic Hazards to Humans – Preamble (2019) (emphasis added).
[7] IARC Monograph on the Evaluation of Carcinogenic Risks to Humans: Volume 68, Silica, Some Silicates, Coal Dust, and para-Aramid Fibrils 210-211 (1997).
[8] Corbett McDonald & Nicola Cherry, “Crystalline Silica and Lung Cancer: The Problem of Conflicting Evidence,” 8 Indoor Built Environment 121, 121 (1999).
[9] Id.
[10] IARC Monograph on the Evaluation of Carcinogenic Risks to Humans: Volume 68, Silica, Some Silicates, Coal Dust, and para-Aramid Fibrils 340 (1997).
[11] Id. at 393.
[12] IARC Monograph, Volume 120: Benzene (2018).
[13] IARC Monographs on the Evaluation of Carcinogenic Risks to Humans: Volume 45, Occupational Exposures in Petroleum Refining; Crude Oil and Major Petroleum Fuels 194 (1989).
[14] IARC Monograph No. 116, Drinking Coffee, Mate, and Very Hot Beverages (2018).
[15] IARC Monograph no. 60, Some Industrial Chemicals (1994).
[16] See “Coffee with Cream, Sugar & a Dash of Acrylamide” (June 9, 2018); “The Council for Education and Research on Toxics” (July 9, 2013).
[17] IARC Monographs on the Evaluation of Carcinogenic Risks to Humans Volume 96 1278 (2010).
