Generally speaking, a risk is anything with the potential to cause us harm or loss or to put us in danger. How we assess risk in any given situation, however, will be influenced by our circumstances and the kinds of possible hazards we are considering. We evaluate a possible risk both qualitatively—how dangerous it seems to us personally, for instance—and quantitatively—its statistical likelihood of harming us or its economic cost. Environmental scientists, economists, and epidemiologists have developed a number of tools for assessing risk.

In the 1970s, vinyl chloride gas—the building block for PVC pipes, automobile interiors, and dishes—became a ubiquitous industrial chemical. The United States was its number one producer. But researchers began to notice that the chemical could cause liver and bone damage in animals at very low doses. The pivotal moment came on January 23, 1974, when the B. F. Goodrich Company announced that it was investigating the cause of cancer deaths among three of its workers. In response, the EPA published its first-ever assessment of environmental risk for a chemical.

The process of risk assessment became formalized with the National Academy of Science’s (NAS) groundbreaking report Risk Assessment in the Federal Government: Managing the Process. Since its publication in 1983, the EPA has integrated the principles of risk assessment into its practices and outlines four basic steps in the assessment of a potentially toxic substance: Identification of the hazard, dose–response assessment, exposure assessment, and risk characterization. Not every risk assessment encompasses all four steps. In most cases, however, risk assessment is an expensive process, costing from $1 to $2 million and taking 3 to 5 years for a single chemical.

1. Identification of the hazard. What health problems are caused by a given pollutant? During the first step of risk assessment, contaminants that are suspected to pose health hazards are identified. They are then tested to determine whether exposure may cause specific health problems (e.g., cancer, chronic disease) and whether the adverse health effect is likely to occur in humans. To obtain information for this step, existing scientific data for a specific chemical are evaluated. In addition, researchers may study populations that have been exposed to the chemical, or they may experimentally test the effects of the chemical on an animal (e.g., rats, mice, or monkeys).

2. Dose–response assessment. What are the health problems associated with different exposures? The likelihood and severity of adverse health effects (response) are related to the amount of exposure to an agent (dose). This relationship is described in a dose–response assessment. Typically, as the dose increases, the measured response also increases. At low doses there may be no response until the exposure reaches a threshold dose, which is the lowest dose (concentration) of a toxic substance that induces a toxicity response in an organism.

The most common type of dose–response testing is done on animals such as mice or rats; it involves exposing the subjects to varying amounts of the toxic substance. Initial toxicity testing is intended to find acute or short-term toxicity effects. This normally requires an LD₅₀ test, which determines the amount and exposure of the toxic substance that is a lethal dose to 50% of the animals in the test at the end of 14 days (Figure 11.16). Then researchers examine the animals further to discover which organs are affected, determine the reversibility of the toxic response, and develop dosages for continuing experiments. Researchers can then follow with more toxicity tests, such as chronic (long-term) exposure to the toxic substance at low levels of exposure.

3. Exposure assessment. How much of the pollutant are people exposed to over a given period of time? How many people are exposed? Exposure assessment defines the population that might be exposed to the agent of concern and identifies the routes through which exposure can occur. Exposure assessment also estimates the amount, duration, and frequency of the doses that people might receive as a result of their exposure.

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4. Risk characterization. What is the extra risk of health problems in the exposed population? This is the step in which risk assessment results are articulated. It includes the analysis of information from the first three steps to develop a qualitative or quantitative estimate of the likelihood that any of the hazards associated with the agent of concern will occur in exposed people.

The EPA conducts risk assessments for a variety of agents, such as diesel exhaust, mercury, secondhand smoke, and ozone. The toxic metal lead is one such agent. Known to damage the nervous system, kidneys, and other internal organs, ingestion of lead in children can cause developmental delays or mental retardation. Since the 1980s, the EPA has phased out lead in gasoline and has banned or limited lead used in consumer products like residential paint. As a result, the levels of lead in the air have decreased by 94% between 1980 and 1999. The amount of lead in people’s blood has also decreased significantly in recent years. Of particular importance is the decrease in blood lead concentrations among U.S. children (Figure 11.17).

Furthermore, the EPA’s National Center for Environmental Assessment periodically evaluates the latest research concerning the public health and welfare effects of lead and publishes the most up-to-date findings. The data are used for the establishment of the most current national air-quality standards for lead.

Health and environmental regulations today are designed to keep the amount of a given contaminant released into the environment at a “safe” level, or to clean it up after it’s already entered the environment. New products and chemicals are often subjected to limited testing and assumed to be “innocent until proven guilty”—that is, until scientific evidence demonstrates them to be harmful. With this approach, it is possible for toxic chemicals to be released into the environment or into our bodies until sufficient evidence suggests that harm is being done. Instead of the assumption of safety, the precautionary principle (see Chapter 1, page 12) offers protection before harm is done—an approach that can be characterized by the phrase “Better safe than sorry” (Figure 11.18). Invoking the precautionary principle, Canada banned the use of BPA in baby bottles. The European Union has also integrated the precautionary principle into its process for making decisions on environmental and health-related issues.

In the 1992 Rio Declaration on Environment and Development of the Earth Summit, the precautionary principle was proposed in the context of protecting the environment. Principle 15 of the Rio Declaration states that “in order to protect the environment, where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation.” The basic tenets of the precautionary principle involve taking preventative action before scientific certainty of cause and effect, seeking out and evaluating alternative products or services, and disclosing the potential impact on human health and environment associated with the selection of products or services.

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Some cities have successfully used the precautionary principle to guide policy. In 2005 San Francisco passed a purchasing ordinance that requires the city to use safer alternatives when purchasing commodities for the city, such as cleaning products or electronics. The idea is to minimize harm by using the best available science to identify safer, cost-effective alternatives. The logistics of that implementation requires many different groups to participate. City employees with scientific backgrounds read and interpret academic research and then identify alternative products with less toxicity. Purchasers accustomed to prioritizing cost over other factors now think more holistically about purchasing decisions, and janitors and other maintenance workers find the best way to use new cleaning materials. Following implementation in San Francisco, the ideas of the precautionary principle have been applied in other U.S. cities, including Portland, Oregon, and Berkeley, California.

The precautionary principle is one useful tool to apply to environmental analysis and decision making, but it is not without its critics. Some say that regulation of products and chemicals could deprive society of significant benefits. In the United States, for example, the FDA requires all new drugs to be tested before they are put on the market. In other words, the FDA requires testing as a precaution to prevent harm to human health. Careful safety testing protects people from dangerous side effects, but it slows the introduction of new drugs. In the interest of precaution, the sickest of the sick may be prevented from receiving beneficial new medications.

One promising approach to studying how the environment impacts your personal health has been dubbed “the exposome.” The exposome consists of all the environmental exposures in an individual’s life and how those exposures affect that individual’s health. Because everyone is exposed to a wide range of pollutants, stresses, and other environmental variables, it has often been difficult to link a specific disease, say, a type of liver cancer, to a specific cause. Recently, researchers have begun measuring the exposome using a variety of high-tech devices alongside chemical and genetic tools. For instance, a scientist at Columbia University has created special backpacks that children can wear to continuously collect air samples while they are at home and at school. Other researchers are analyzing exposures by identifying chemicals in a person’s blood or looking for the fingerprint of chemical exposure on a person’s genome. Such work is still in the early stages, but it provides more data to give us greater confidence in our risk assessments.