The Environment Canada Policy Research Seminar Series

Dr. Donald Mackay December 12, 2002

On December 12, 2003, the Environment Canada Policy Research Seminar Series hosted Dr. Donald Mackay, who delivered an informative overview of the contribution of green chemistry to the sound management of commercial chemicals. The following is a summary of his presentation.

Donald Mackay has led a very active career in the field of green chemistry. He is internationally renowned for his pioneering work to help identify how society can enjoy the benefits of modern chemistry while guarding against past problems from chemical toxicity in the future. He used his address at Environment Canada to reflect on how perspectives about the environment's capacity to absorb chemical releases changed over the course of his professional career, and how modeling has contributed to the management of commercial chemicals.

Evolving Perspectives on Chemicals in the Environment

In the early part of the century, there was a perception that the environment had an infinite capacity to absorb wastes. Any level of human industrial emissions would be absorbed, broken down and recycled. Even by the mid-20th century, our understanding of the environment's capacity to absorb industrial emissions would be considered shockingly weak by today's level of knowledge. Having grown up in an industrial town in the 1930s and 40s, Mackay vowed to study the impacts of industrial processes on a community using a big picture analysis with the hopes that commercial activities would appropriately balance the benefits of these activities with their impacts on human health and wildlife.

In 1962, marine biologist Rachel Carson published Silent Spring, a book that revolutionized our thinking on the environment's capacity to absorb human pollution. Carson discussed DDT, a chemical insecticide that, at first glance seemed a wonderous human-made compound; it was inexpensive, yet highly effective at reducing the populations of mosquitoes that spread malaria and lice that carry typhus. However, after several decades of use, Carson observed that DDT was accumulating in the tissues of some non-target organisms, and was highly toxic to some life forms, especially birds and fish. With the increased concern about how chemicals can interact with living beings, we began to develop a far better understanding of how they enter and become dispersed throughout the environment. Greater attention was paid to multi-media processes, that is, the movement of chemicals, often over thousands of miles, as they change form, perhaps dissolving in rain droplets or lakes and streams, then finding their way into soils, plants, animals and humans.

Commercial Chemicals in Modern Societies

Over 20 million chemicals presently exist. About 50,000 to 100,000 of them are used commercially in a wide variety of products such as drugs and medicines, plastics, metals, fertilizers, pesticides, fuels, detergents, adhesives, and food additives. Progress in analytical chemistry has led us to realize that any commercial chemical used in considerable quantities can be found in any part of the world, at least in trace amounts.

Assessing Chemical Impacts: Decision Criteria

How can we assess the risk of adverse health effects of commercial chemicals? Two simple principles guide modern toxicology.

First, is the age-old maxim advanced by 16th century physician, Paracelsus: “It is the dose that makes the poison.” Any chemical can be found to be either toxic if doses are sufficiently high, or non-toxic if doses are sufficiently small.

Second, exposure to a chemical is key; a chemical cannot be toxic until it enters an organism. The health impacts of a chemical must take into account both the exposure of the chemical to organisms, and in what doses.

Toxicity

Toxicity denotes the capacity of a chemical to cause harm to a living organism, and to indicate the adverse effects caused by a chemical. The degree of harm caused to an organism by exposure to a toxic chemical generally increases with exposure level, but is also dependent upon the type of organism, the length of exposure and its developmental stage.

Persistence

Persistence refers to how long a chemical remains in the environment without being chemically or biologically broken down or altered. Highly persistent chemicals remain in the environment for a long time, though they may move through different media (e.g., from soil to water to sediment). The more persistent is a chemical, the longer the temporal window for plant or animal exposure and hence the higher the likelihood of toxic effects.

Persistence is frequently expressed in terms of half-life. For instance, if a chemical has a half-life of 2 days, it will take 2 days for a given quantity of the chemical to be reduced by one-half due to chemical and biological processes. The longer the half-life, the more persistent will be the chemical in the environment. Dr. Mackay cautions that introducing chemicals with half lives of more than a few months should be avoided.

Bioaccumulation Potential

The hydrophobicity (that is, insolubility) and stability of some chemicals causes them to accumulate in the tissues of animals, particularly fish. Bioaccumulation potential refers to a chemical's tendency to accumulate in plants and animals. Plants may accumulate chemicals from the soil through their roots. Some of these chemicals are transformed or combined with others and used by the plant. Other chemicals are simply eliminated, or accumulate in its roots, leaves, or edible parts.

Understanding how chemicals accumulate in plants is important because plants are the basis of all food chains. Animals also bioaccumulate certain chemicals in different tissues or organs.

For instance, DDT is metabolized only very slowly by most animals. Instead, it is stored in their fatty tissues. The biological half-life of DDT is about eight years, that is, it can take about eight years for an animal to metabolize half of the amount it assimilates. If ingestion continues at a steady rate, DDT builds up within the animal over time. In mammals, it may be passed from mother to offspring by birth and lactation.

Environmental Long-range Transport

Environmental Long-Range Transport (LRT) models calculate a chemical's characteristic travel distances in air and water. The transport potential of a chemical is based on a combination of its ability to persist in the environment and its degree of mobility. The greater the potential of a chemical to travel, the greater the likelihood of exposure to living organisms along the way, and hence the greater the concern.

This issue is of particular Canadian importance because of transport of chemicals to Canada's arctic ecosystem and its residents.

The Benefits of Green Modelling

Dr. Mackay has become widely known within the worldwide scientific community as the originator of Mackay models, systems of mathematical equations that help track the movement of chemicals as they disperse and change form with the goal of assessing their health impacts on living organisms. These models have been developed to predict the tendencies of a chemical to persist, bioaccumulate and travel long distances in the environment as a function of both its chemical attributes and the environmental setting.

Models, can assist in finding the best combination of chemical properties in designing new chemical compounds. Modeling is particularly helpful for processes that we cannot measure for practical reasons, like the evaporation taking place over an entire lake.

The Limitations of Green Modelling

Though Dr. Mackay has been one of the leading advocates of the use of modeling to determine the impacts of chemicals, he also cautions there are limitations. These models are highly useful in evaluating the impacts of some chemicals, but are less reliable for others. Predictions are more tenuous for models of biodegradation and metabolic processes. They also fail to assess the synergistic effects of different chemicals used in combination.

Regulatory Approaches

As our understanding of the impacts of chemicals advanced, so too did our regulatory approach. We've recognized that chemical pollution need not be perceived as an inevitable by-product of prosperity. We've moved beyond the old reactive approach (develop, use, monitor, ban) toward a more proactive approach involving registering the chemicals used in commerce, assessing in advance their environmental and health impacts, choosing suitable uses for them, and monitoring their presence and impacts with an eye to detecting any health risks they pose.

As an example of the progress we've made due to advances in analytical chemistry, the process has helped identify about 1,000 chemicals of concern. Among the most dangerous are the 'dirty dozen', twelve Persistent Organic Pollutants (POPs) linked to disease and birth defects among humans and animals. Once the detrimental impacts of these chemicals was recognized, it led to UNEP's Stockholm Convention on Persistent Organic Pollutants, which aims to control the production, use and disposal of POPs globally.

The On-going Challenge of Chemical Management

Around the world, the public continues to demand the careful assessment of all chemicals used in the world of commerce. We continue to face challenges in assessing the chemicals of commerce using a consistent, transparent, feasible and preferably rapid framework to evaluate both human and ecosystem impacts. The application of existing models and the development of new models can play a key role in this challenging task.