In Memoriam: Don Mackay

On October 20, 2023, Donald (Don) Mackay passed away in Peterborough, Ontario, Canada. Don was one of the founding pioneers of the field of environmental organic chemistry, revered and respected around the world for his many invaluable contributions to understanding and quantifying the fate and transport of contaminants in the environment and biota. We recall some of his scientific accomplishments with the intention not only to commemorate his life, but also to acquaint a new generation of environmental scientists with some of his ground‐breaking concepts and ideas, which continue to be of immense value in environmental chemistry and toxicology. Born in 1936, Don grew up in Glasgow, Scotland, and studied Chemical Engineering at the Royal College of Science and Technology in his hometown, graduating with a BSc in 1958 and a PhD in 1961. After postdoctoral work with Prof. Olev Trass in the Department of Chemical Engineering and Applied Chemistry at the University of Toronto and a 3‐year period of employment with ICI in Teesside in the United Kingdom, Don returned to the University of Toronto to work as a professor for close to 30 years. In 1995 he took up a position at Trent University in Peterborough as Founding Director of the Canadian Environmental Modelling Centre (CEMC). Since 2002, he was Professor Emeritus at both the University of Toronto and Trent University. Don was among the first to apply principles, concepts, and tools from chemical engineering and physical chemistry to the understanding, interpretation, and prediction of the fate, transport, and effects of chemicals in the environment. In doing so, he laid the groundwork for quantifying environmental phase distribution and transfer processes, developed, refined, and popularized an approach to compartmental mass balance modelling of the physical environment and organisms, and made invaluable contributions to the science of contaminant exposure and risk, especially related to the assessment of persistence, long‐range transport, bioaccumulation, and nonspecific modes of toxic action. Before illuminating his specific contributions in this realm, we first want to highlight defining elements of Don's “scientific character” that are strongly related to each other: Although the breadth of Don's endeavors was vast, one common denominator was the desire to come up with a parsimonious solution, if not the simplest effective solution, to a question or problem. He was truly an adherent of William of Ockham's razor. Far from being easy, this process of stripping down a problem to its essential core and devising a simple approach to address it can be exceptionally challenging. The unit world modelling approach that Don designed, and that describes the environment as being composed of a small number of well‐mixed compartments, is one manifestation of his striving for simplicity, because these models can be rationalized as the simplest representations of the environment suitable for the task of quantitatively describing the environmental fate of contaminants. Other examples are simple empirical equations to describe the process of bioconcentration of persistent hydrophobic neutral organic chemicals (Mackay, 1982): or the partitioning of hydrophobic neutral organic chemicals to organic matter from the aqueous phase (Seth et al., 1999): or the rate of evaporation of an organic liquid (Mackay & van Wesenbeeck, 2014): Don underpinned these empirical relationships with the strong theoretical basis provided by thermodynamics. While it is easy to find fault with such simple models, tools, and equations, it is far more difficult to dismiss their usefulness, as illustrated by their widespread adoption and frequent application in regulatory practice. They are thus primary pieces of evidence in support of the adage that all models are wrong, but some are useful (Box, 1976). Don's contributions were above all useful. This penchant for simplicity was also apparent in Don's scientific writing, teaching, and presentations, which were remarkable for their accessibility and understandability, yet also their precision. This is even more noteworthy because his communications regularly dealt with subject matter often perceived as intimidating and obscure (e.g., physical chemistry concepts such as fugacity and activity, multimedia chemical fate). While the conceptual and mathematical models Don developed were extremely helpful for communicating complex scientific issues to diverse stakeholders for chemical regulatory applications, they also provide critically important links for applying the scientific method to systematically integrate current knowledge and address uncertainty through hypothesis generation and testing. Translational research is defined as transmitting insights, tools, and data obtained through basic research into applications and practices that directly benefit humanity. Very much the engineer, he was not content to make important, academically recognized contributions, but devoted ingenuity, time, and tireless effort to see such knowledge used by professional environmental practitioners and applied in the environmental policy arena. A large part of the mandate of the CEMC, and therefore also of the latter part of his academic career, was the transmittance of knowledge, concepts, and modeling tools to a community of users engaged in the assessment of the behavior, fate, and effects of contaminants in the environment. This took the form of extensive collaborations, numerous short courses, a very accessibly written textbook on modeling with the fugacity approach (Mackay, 1991), and the distribution and customization of free‐to‐use modeling software. The annual CEMC Partners meeting became not only an important venue for knowledge sharing, but also invaluable as an informal networking event for stakeholders to work together to address scientific and regulatory challenges. Don firmly believed that the sound management of chemical risks required the coming together of scientists working within academia, industry, and government. Accordingly, he communicated and collaborated widely and without prejudice. His research activities were supported by a wide range of government departments, businesses, and industry associations. Drawing distinctions between those who are permitted to contribute to the scientific exchange of ideas and those whose contributions are judged suspect and should be dismissed merely because of their affiliation (Schäffer et al., 2023) would have been anathema to him. The Society of Environmental Toxicology and Chemistry (SETAC) was Don's scientific home throughout his career, because of the organization's explicit ambition to have academia, business, and government represented in all its activities. Don and other co‐founders of SETAC were driven by the belief that the astute application of scientific knowledge by academics, industry, and governments will allow humanity to enjoy the many potential benefits of chemicals while minimizing the potential detriment to humans and their environment. The ISI Web of knowledge lists close to 500 articles by Don Mackay, which is far from a complete list of his contributions, which have been cited almost 30,000 times. These numbers are indeed exceptional and speak to the enormous impact his work has had, and continues to have, on the field of environmental chemistry. His most noteworthy contributions to environmental chemistry can be summarized as follows. Don's background in chemical engineering provided him with the ability to recognize the importance of physical–chemical properties for understanding chemical behavior in natural and fabricated environments, including laboratory test systems (Mackay et al., 2017). Don and colleagues generated, collected, curated, and published databases of physical–chemical properties (Mackay et al., 2006); demonstrated relationships between solubilities and partition ratios; and developed methods for addressing the uncertainty inherent in measured and predicted chemical properties (Beyer et al., 2002; Cole & Mackay, 2000; Mackay, Bobra, et al., 1980; Shiu et al., 1988). Don repeatedly showed the principal relationships between partitioning properties and chemical activity with baseline toxicity (“narcosis”) in algae, invertebrates, fish, and mammals (Abernethy et al., 1986, 1988; Bobra et al., 1983, 1985; Celsie et al., 2016; Mackay et al., 2009, 2014; McCarty et al., 1992; Schmidt et al., 2018). He wrote and edited texts and numerous articles seeking to transfer this foundational knowledge to the research and regulatory communities (see Boethling & Mackay, 2000; Mackay, 1991; Thibodeaux & Mackay, 2010). Don first developed (Mackay & Leinonen, 1975; Mackay & Wolkoff, 1973) and later refined (Mackay et al., 1986) the framework for understanding and quantifying the exchange of organic chemicals between air and water. He developed one of the most widely used techniques for measuring the air–water partitioning equilibrium of organic chemicals (Mackay et al., 1979), and developed theoretical and laboratory‐based techniques for quantifying the kinetics of air–water exchange (Mackay & Yeun, 1983). His extensive compilations of air–water equilibrium constants are widely relied‐on resources to this day (Mackay & Shiu, 1981; Suntio et al., 1988). In a series of now famous articles (Mackay, 1979; Mackay & Paterson, 1981, 1982), Don introduced the concept of fugacity into the field of environmental organic chemistry. This innovative approach quickly found widespread acceptance, consolidated by a well‐known textbook (Mackay, 1991) and the freely available modeling software he developed (Trent University, 2024). The concept and the models, known as Mackay‐type models the world over, are used by countless students, researchers, chemical regulators, and environmental practitioners every day. Highly quoted multimedia modeling papers are Mackay and Paterson (1991) and Mackay et al. (1996), and other relevant texts include Cowan et al. (1995) and Thibodeaux and Mackay (2010). Among Don's most influential work are publications on the topic of global contaminant transport and accumulation, identifying the role that temperature‐dependent environmental phase partitioning plays in the large‐scale distribution of contaminants (Mackay & Wania, 1995; Wania & Mackay, 1993, 1996). He further made seminal contributions to the development of models to serve in a quantitative understanding of global dispersion and accumulation processes (Wania & Mackay, 1995, 1999) and in the assessment of the potential of organic chemicals for long‐range transport (Beyer et al., 2000). Starting out with a groundbreaking paper on the bioconcentration of organic chemicals (Mackay, 1982), Don's research has continuously highlighted the crucial role of thermodynamics and kinetics in the mechanism of organic chemical uptake in aquatic organisms, both from the water (Gobas et al., 1989; Gobas & Mackay, 1987) and with food (Gobas et al., 1988). Later, he also successfully tackled the issue of contaminant uptake in vegetation (Harner & Mackay, 1995; Paterson et al., 1990, 1991). Again, his contributions led to valuable quantitative approaches to describing key processes of uptake, distribution, and elimination of contaminants in various organisms (Cahill et al., 2003; Clark et al., 1990; Paterson et al., 1990). Don's unique and comprehensive knowledge base on chemical properties, environmental fate, bioaccumulation, and toxicity resulted in holistic mass balance modeling approaches for chemical exposure to humans and ecological receptors and potential risks. He developed models for oil spills (Mackay, Buist, et al., 1980; Reed et al., 1999) and multimedia mass balance models linking fate and bioaccumulation (toxicokinetics) in natural (Arnot & Mackay, 2008; Arnot et al., 2006) and indoor (Webster et al., 2016) environments. He outlined and demonstrated exposure and risk estimation frameworks, bringing the pieces of the puzzle together using chemical concentrations (Arnot et al., 2006; McCarty & Mackay, 1993), chemical activity (Gobas et al., 2015; Mackay et al., 2011), and, of course, fugacity (Mackay et al., 2020). Don's scientific legacy for the next generations of environmental chemists, toxicologists, and risk assessors is vast and a testament to the success of the scientific method whereby models based on first principles and informed and continuously tested with empirical data provide useful solutions to environmental problems. We invite future environmental researchers to become familiar with the works of Don Mackay, so they can truly stand on the shoulders of a giant in environmental research to advance their work toward a healthy environment.

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