22 Feb 2024

In Memoriam: Don Mackay

Frank Wania, University of Toronto Scarborough; Jon A. Arnot, ARC Arnot Research & Consulting; Frank A.P.C. Gobas, Simon Fraser University

The full memorial piece, with all subsequent references, will publish in a future issue of Environmental Toxicology and Chemistry. This has been modified for style and space.

Don Mackay in a lab setting

On 20 October 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. Here, we recall some of his scientific accomplishments with the intention to not only commemorate his life but also 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 B.Sc. in 1958 and a Ph.D. in 1961. After postdoctoral work with Olev Trass in the Department of Chemical Engineering and Applied Chemistry at the University of Toronto and a three-year period of employment with ICI in Teesside in the UK, 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 modeling 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 non-specific 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:

Don Mackay was a master of simplifying complex concepts. While 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 modeling approach that Don designed, and which describes the environment as being composed of a small number of well-mixed compartments, is one manifestation of his striving for simplicity, as 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:

BCF = 0.048 KOW

the partitioning of hydrophobic neutral organic chemicals to organic matter from the aqueous phase:

KOC = 0.35 KOW

or the rate of evaporation of an organic liquid:

ER (μg m−1 h−1) = 1464P (Pa) × M (g mol−1)

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. 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 as 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.

Don Mackay was a devotee of translational research, 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, and the distribution and customization of free-to-use modeling software. The annual CEMC Partner’s meeting became not only an important venue for knowledge sharing, but also invaluable informal networking events for stakeholders to work together to address scientific and regulatory challenges.                   

Don Mackay was a bridge builder between academia, government and industry. 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 would have been anathema to him. SETAC was Don's scientific home throughout his career because 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.

ISI Web of knowledge lists close to 500 articles by Don Mackay, which is far from a complete list of his contributions. These papers 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. Among his most noteworthy contributions are:

Understanding and Communicating the Importance of Physical-Chemical Properties for Environmental Fate and Toxicity. 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. Don and colleagues generated, collected, curated and published databases of physical–chemical properties, demonstrated relationships between solubilities and partition ratios, and developed methods for addressing uncertainty inherent in measured and predicted chemical properties. Don repeatedly showed the principal relationships between partitioning properties and chemical activity with baseline toxicity (“narcosis”) in algae, invertebrates, fish and mammals. He wrote and edited texts and numerous papers seeking to transfer this foundational knowledge to the research and regulatory communities.

Understanding and Quantifying the Air-Water Exchange of Organic Contaminants. Don first developed and later refined 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 and developed theoretical and laboratory-based techniques for quantifying the kinetics of air-water exchange. His extensive compilations of air–water equilibrium constants are widely relied upon resources to this day.

Developing and Applying Fugacity-Based Environmental Fate and Transport Models. In a series of now famous papers, Don introduced the concept of fugacity in the field of environmental organic chemistry. This innovative approach quickly found widespread acceptance, consolidated by a well-known textbook and the freely available modeling software he developed. 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.

Exploring the Mechanisms of Global Contaminant Fate and Transport and Long Range Transport Assessment. 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. He further made seminal contributions to the development of models to serve in a quantitative understanding of global dispersion and accumulation processes and in the assessment of the potential of organic chemicals for long-range transport.

Understanding and Quantifying the Bioaccumulation of Organic Chemicals in Organisms. Starting out with a groundbreaking paper on the bioconcentration of organic chemicals, 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 and with food. Later, he also successfully tackled the issue of contaminant uptake in vegetation. Again, his contributions led to valuable quantitative approaches to describing key processes of uptake, distribution and elimination of contaminants in various organisms.

Models for Chemical Exposure and Risk Evaluation. 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 oils spills and multi-media mass balance models linking fate and bioaccumulation (toxicokinetics) in natural and indoor environments. He outlined and demonstrated exposure and risk estimation frameworks, bringing the pieces of the puzzle together using chemical concentrations, chemical activity and, of course, fugacity.

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 where 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 towards a healthy environment.     

Author's contact: [email protected]