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Introduction to Toxic Substances (Adapted from the NY State Dept of Health's booklet "What You Know Can Help You: An Introduction to Toxic Substances," #2710, 5/91.) Here's a primer on toxic materials and the vocabulary used to describe them and their effects on human health.
Some chemicals are hazardous because of their physical properties. They may explode or burn easily, or react violently when exposed to certain other chemicals. A chemical may be toxic, hazardous, or both. The toxicity of a chemical is measured by the types of effects it produces, and its potency, or strength. For example, one chemical may have no noticeable effects during exposure, but may cause cancer years later. Another may produce vomiting upon exposure, but not have any known long-term effects. The more potent a chemical is, the more toxic. if smaller amounts of Chemical A can cause greater effects than larger amounts of Chemical C, Chemical A would be considered more toxic. Potency, and therefore toxicity, can be affected by the chemical's breakdown once it is in the body. The body can metabolize, or change the chemical into another chemical (or multiple chemicals) that may have more or less toxicity than the original chemical. A chemical can cause health problems only after it contacts or enters the body. There are three primary routes of exposure: inhalation, ingestion, or direct contact. Inhalation, or breathing, of gases, vapors, dusts or mists is a common route of exposure. Chemicals can enter and irritate the nose, air passages and lungs. They can be deposited in the airways or absorbed into the bloodstream by the lungs. Blood circulation can then carry chemicals throughout the body. Ingestion refers to chemicals that get in or on food, utensils, hands or cigarettes; anything that can be swallowed. Children may be at particular risk of ingesting things found in dust or soil because they are more likely to put their hands into their mouths. Once swallowed, some chemicals can be absorbed into the blood and carried throughout the body.
Exposure medium is the term for the method of "delivery" of the chemical to the body. The amount of a substance in the air we breathe, the water we drink, or the food we eat is called a concentration. The standard ways of reporting concentrations are parts per million (ppm), milligrams per liter (mpl) and milligrams per cubic meter (mpcm) The affected individual's dose can be determined by multiplying the amount of water, air, food or soil taken in, times the concentration of the chemical. Acute exposure is a short contact with a chemical, ranging from a few seconds to a few hours. Chronic exposure is continuous or repeated contact with a toxic substance over months or years. Some chemicals can build up on the body with a long-term exposure, causing effects possibly not seen with acute exposure. Residents who drink water slightly contaminated by the local landfill may develop health effects from such a chronic exposure. Not all people are equally sensitive to chemicals and therefore they are not equally affected. The factors of sensitivity include genetic differences, allergies, developed after earlier exposures, age, illness, diet, alcohol abuse, pregnancy and any drug use. Information gathered from human exposures at work or in an accident can be very useful, if incomplete. The exact amount of a spill and length of exposure may not be known, making the dose hard to determine. If someone has been exposed to more than one substance on the job, it may be difficult to find which substance caused which effects. Even when a specific substance is tied to a specific effect, the exact dose necessary to cause such an effect may not be known. Sometimes a population exposed to a chemical is compared to another group not known to be exposed. If the exposed group has a higher incidence of a certain health effect, researchers say the effect may be "related" to the exposure, but often the exact cause of a particular effect cannot be determined. Animal studies are often good indicators of chemical toxicity in humans, although many factors such as relative size and differences in metabolism must be accounted for. Many combinations of chemicals have never been tested to see if together they produce more or less effects. (This Web site does not advocate or endorse the use of animal studies.) Effects A chemical exposure can produce an effect at the site of contact (local) or elsewhere in the body (systemic), and those effects can be either immediate or delayed. Chemicals can affect any system in the body, including respiratory (nose, air, passages and lungs), digestive (mouth, throat, stomach, intestines, etc.), circulatory (heart, arteries, veins and blood), nervous (brain and nerve cells), and reproductive (sperm/egg production, etc.) systems. Some chemicals are nonspecific, such as acids, and cause damage on contact, wherever that contact is. Nonspecifics may also move and have effects throughout the body. Other chemicals are specific and attack only certain organs or systems. Since each organ and organ system has different functions and characteristics, the effects of a chemical on each system must be evaluated differently. Immediate effects happen right away. They can occur at the site of contact or elsewhere. These are sometimes reversible and they subside after the exposure stops. But some immediate effects do not go away, such as an acid burn. Delayed effects may take months or years to appear and can result from either acute or chronic exposure. This lag time between exposure and the discovery of effects is called the latency period. Delayed effects can be temporary or permanent. Cancer, for example, can occur between 5 and 40 years after exposure. After such a delay, tracking down the original cause may not be possible. |
