Some substances are more toxic than others. The toxicity of a substance is described by the types of effects it causes and its potency.
Types of Effects: Different chemicals cause different effects. For example, Chemical A may cause vomiting, but not cancer. Chemical B may have no noticeable effects during exposure, but may cause cancer years later.
Potency: Potency (strength) is a measure of a chemical's toxicity. A more potent chemical is more toxic. For example, sodium cyanide is more potent than sodium chloride (table salt) since swallowing a smaller amount of cyanide can poison you.
The potency and, therefore, the toxicity of a chemical can be affected by its breakdown within the human body. When a substance is absorbed into the body, its chemical structure may be changed or metabolized to a substance that is more toxic or less toxic. For example, carbon tetrachloride, once a commonly used solvent, is changed by the body into a more toxic chemical that causes liver damage. For some other chemicals, metabolism changes the chemical into a form that is more easily eliminated by the body.
Exposure: A chemical can cause health effects only when it contacts or enters the body.
Routes of Exposure: Exposure to a substance can occur by inhalation, ingestion or direct contact.
Inhalation (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 become deposited in the airways or be absorbed by the lungs into the bloodstream. The blood can then carry these substances to the rest of the body.
Ingestion (swallowing) of food, drink or other substances is another route of exposure. Chemicals that get in or on food, cigarettes, utensils or hands can be swallowed. Children are at greater risk of ingesting substances found in dust or soil because they often put their fingers or other objects in their mouths. Lead in paint chips is a good example. Substances can be absorbed into the blood and then transported to the rest of the body.
Direct contact (touching) with the skin or eyes is also a route of exposure. Some substances are absorbed through the skin and enter the bloodstream. Broken, cut or cracked skin will allow substances to enter the body more easily.
The route of exposure can determine whether or not the toxic substance has an effect. Breathing or swallowing lead can result in health effects, but touching lead is not harmful because lead isn't absorbed through the skin.
Dose: The amount of a substance that enters or contacts a person is called a dose. An important consideration in evaluating a dose is body weight. If a child is exposed to the same amount of chemical as an adult, the child (who weighs less) can be affected more than the adult. For example, children are given smaller amounts of aspirin than adults because an adult dose is too large for a child's body weight.
The greater the amount of a substance a person is exposed to, the more likely that health effects will occur. Large amounts of a relatively harmless substance can be toxic. For example, two aspirin tablets can help to relieve a headache, but taking an entire bottle of aspirin can cause stomach pain, nausea, vomiting, headache, convulsions or death.
Exposure medium: Exposure to chemicals occurs when we breathe, eat or touch soil, water, food or air that contains chemicals. The amount of a chemical in the medium is called its concentration. Common ways of reporting concentrations are parts per million, milligrams per liter or milligrams per cubic meter. These and other units of measure are defined in the Glossary of Environmental Health Terms available from the New York State Department of Health.
A person's dose can be determined by multiplying the concentration of the chemical times the amount of the water, air, food or soil that a person takes in. For example, the average adult drinks about 2 liters (roughly quarts) of water and breathes about 20 cubic meters (roughly cubic yards) of air a day. If drinking water contains 1 milligram of lead per liter, then the person would take in a total of 2 milligrams of lead in a day.
Length of exposure: Short-term exposure is called acute exposure. Long-term exposure is called chronic exposure. Either may cause health effects that are immediate or health effects that may not occur for some time.
Acute exposure is a short contact with a chemical. It may last a few seconds or a few hours. For example, it might take a few minutes to clean windows with ammonia, use nail polish remover or spray a can of paint. The fumes someone might inhale during these activities are examples of acute exposures.
Chronic exposure is continuous or repeated contact with a toxic substance over a long period of time (months or years). If a chemical is used every day on the job, the exposure would be chronic. Over time, some chemicals, such as PCBs and lead, can build up in the body and cause long-term health effects. Chronic exposures can also occur at home. Some chemicals in household furniture, carpeting or cleaners can be sources of chronic exposure.
Chemicals leaking from landfills (dumps) can enter the groundwater and contaminate nearby wells or seep into basements. Unless preventive measures are taken, people may be exposed for a long time to chemicals from their drinking water or indoor air.