When reporters discuss the impact of environmental activi- ties on human health, they use the language of medicine, science, and statistics. Understanding this language in the context of a discussion about environmental impacts can help you make decisions related to the issues of risk. This fact sheet identifies and defines commonly used words in risk communication.
The human body, with its various systems and organs, is linked intricately to an individual's life habits, work, home, and the wider environment. Environmental threats to human health can affect specific organs or body systems. With some individuals, environmental threats can have a general affect that is difficult to ascribe to any specific context. Any possible toxic relationship between an environmental threat and human health is the focus of environmental risk assessment.
A toxin is a chemical that potentially can cause harm inside the human body. Toxins enter the body in three ways: 1) through the skin (dermatologically); 2) through the digestive system, (ingestion); or 3) through breathing, (inhalation).
When a toxin enters the body, the body registers a reaction in one of two ways. The first type of reaction is acute and refers to a recurrent or constant reaction. A chronic reaction, the second type, often occurs in individuals exposed to low levels of a toxic or allergic stimulant over a period of time.
Determining cause and effect (or causal) relationships between environmental stimuli and impacts in the body in the form of chronic effects is difficult because of the latency period in which the body doesn't register any effects. Another difficulty is isolating the effect of a particular toxin in relation to all the variables, agents, elements, compounds, and materials with which an individual comes into contact daily.
A toxic reaction is different from an allergic reaction. Allergens cause an individual to react hypersensitively or pathologically to amounts of pollens, dust, microorganisms, foods, or pollutants that do not affect most people.
The human body has the ability to cleanse itself of many toxins. The degree to which each body can take in foreign or overload substances is its threshold. Each person's threshold is different and is determined by body mass, density, weight, and unique genetic characteristics. Children, people with illnesses, and the elderly often have lower thresholds than the average adolescent or adult.
Chemicals are part of nature, and without some even highly toxic chemicals humans would not be able to survive. The concentration of the chemicals and the amount of exposure create a level of chemical in the body that may or may not be toxic.
When the levels of some chemicals exceed the body's ability to rid itself of them, they accumulate. Other chemicals have an additive effect, meaning the toxicity of one adds to the toxicity of another. When one chemical increases the toxicity of another or itself becomes more toxic, it has a synergistic relationship (either with itself or the other chemical). When one chemical in the body stimulates the breakdown of another chemical, it has an antagonistic relationship with that chemical.
The effects of impacts on the human body from environmental stimuli range from allergic reactions to cellular change to death. Common low-level reactions to chemicals include sneezing, headaches, congestion, or rashes. A more severe reaction results in the growth of cells in the human body.
Carcinogens are substances or agents that may cause cancer, which is a condition of rapidly spreading, abnormal cell growth. Mutagens are substances or agents that may cause changes in the genetic material of living cells. Mutagens may be passed on through sperm or eggs, and may cause abnormal cell growth either immediately or after a period of latency. Approximately one-half of all carcinogens are mutagens; most, but not all, mutagens are also carcinogens. A third type of substance or agent is the teratogen which can cross the placenta of a pregnant woman and can cause spontaneous abortion, birth defects, or developmental abnormalities in a fetus.
Because of lower body weights, lower metabolism rates, and fewer detoxifying enzymes, children, the elderly, and infirm individuals are more susceptible than healthy adults to toxins. Also, children eat, drink, and breathe more than adults per pound of body weight.
Official reports often identify an environmental risk in terms of x in y being affected. For example, the report might say "3 people in 100 are likely to be affected," or "26 in 1,000 people are at risk." This is a statistical probability. This does not mean 3 out of 100 people will or will not be affected. Rather, the likelihood of 3 additional cases of reaction, in a group of 100 people similar to the test group but not exposed to the toxin or disease, will be affected.
Statistical probability is based on a bell curve or normal distribution. It assumes there is a baseline of occurrence of a reaction, cancer, or disease, and that a particular source will affect a normal population at a certain, predictable rate. Such a result is statistically likely, but not certain. The individuals exposed will change the likelihood. Some groups of people already have higher rates of disease or reaction, and the statistical significance of impact of exposure is lower. People who smoke, people who use excessive chemicals in their homes, people with respiratory problems, or people who live in inner cities, for example, affect the statistical probability of a group of people having a reaction to an environmental threat.
Most reports of environmental threats include measurements of toxins in the environment. These measurements are reported in parts per thousand, million, or billion (ppt, ppm, ppb). A measurement of one part per thousand, for example, means that for every thousand molecules of air, liquid, or material, one molecule is likely to be the toxin. Most toxins have established limits of so many parts per, below which most people will not be affected or the difference is within normal fluctuations in the environment. Above this level, the toxins may be a threat to certain populations or to the general public.
Remember, statistical risk is a prediction comparing people exposed to a toxin with people not exposed to that toxin. Risk is not a guarantee of occurrence. Individual life choices change the likelihood of risk for the individual. Making changes to minimize risk from controllable sources in the home, office, school, yard, and other environments is a useful and wise activity.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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