Module Two - Lecture Notes
Types of Environments
Before one can understand the routes and pathways of exposure, it is important to have an understanding of the term “environment.” Environment can be defined in a number of ways (7):
- Inner versus outer environment
- Personal versus ambient environment
- Gaseous, liquid, and solid environments
- Chemical, biological, physical, and socioeconomic environments
- Inner Versus Outer Environment
This refers to the human body and consists of the environment within the body and the environment outside of the body. The human body has three protective barriers against outside environmental contaminants.
- The skin, which protects the body from contaminants outside the body;
- The gastrointestinal (GI) tract, which protects the inner body from contaminants (toxins) that have been ingested,
- The membranes within the lungs, which protect the inner body from contaminants that have been inhaled.
However, each of these barriers are liable to damage under certain conditions. Contaminants can penetrate to the inner body through the skin by dissolving the layer of wax that we have covering our oil (sebaceous) glands. The GI Tract is also very vulnerable to compounds that are soluble and can be easily absorbed and taken into the body cells. However, the body has ways of protecting the GI tract. For example, unwanted material can be vomited through the mouth, or rapidly excreted through the bowels (as in the case of diarrhea).
The lungs are the most important route for toxic substances, and they are also the most fragile. Airborne materials that are inhaled can be deposited in the lungs, and, if they are soluble, they can be absorbed. A number of mechanisms protect the lungs, such as, simple coughing, or cleansing by “macrophages” that engulf and promote the removal of anything foreign.
Unless a toxicological agent or environmental contaminant penetrates one of the three barriers that protect the body, it will not get into the inner environment, and even if it does get in, there are other ways to remove it. For example, materials entering the circulatory system (arteries, veins, etc.) can be detoxified in the liver or excreted through the kidneys.
- Personal Versus Ambient Environment
Your personal environment represents the environment that you can control. The ambient (working) environment represents the environment over which you have no control. It is thought that the working environment poses the greatest threat to health, but some health experts believe that the personal environment, influenced by a number of factors, is the most important for our well being. Factors which are important for the personal environment include hygiene, diet, sexual practices, exercise, use of tobacco, drug and alcohol use, and frequency of medical checkups.
- Gaseous, Liquid, and Solid Environments
Our environment exists in one of three forms: gas, liquid, or solid, each of which can be polluted. Particulate (large particles) and gases are released into the air (gaseous), sewage and liquid wastes are discharged into water (liquid); and solid wastes, such as plastics and toxic chemicals are disposed of on land (solid). People interact with all of these environments.
- Chemical, Biological, Physical, and Socioeconomic Environments
These types of environments could affect people’s health.
- Chemical factors and contaminants include toxic wastes and pesticides in the environment; chemicals used at home (cleaning products) and by industry; and preservatives for food.
- Biological factors include different forms of disease organisms in food and water, and those that can be transmitted by insects and animals, and person-to-person contact.
- Physical factors include elements that may influence health and well-being, such as injuries and deaths from accidents, loud and excessive noise, extreme temperatures (heat and cold), and the effects of radiation.
- Socioeconomic factors – are hard to measure, but significantly affect the lives and the health of people. Low socioeconomic status increases death and illness rates.
- Inner Versus Outer Environment
Routes of Exposure
It has been estimated that about 70,000 chemicals are used worldwide, and the chemical industry introduces about 200 to 1,000 new chemicals each year (8). Because of this, we are exposed to a number of chemicals in our home, at work, and in the general environment. Trace amounts of toxic chemicals are present in the food, the air, and the drinking water. Exposure to toxic substances occurs through the three major routes listed below.
Refer to Handout 2.1. Routes of Exposure
- The skin (dermal absorption)
- The respiratory tract (inhalation)
- The digestive tract (ingestion)
Elements for a Pathway of Exposure
ATSDR defines an exposure pathway as the process by which an individual is exposed to contaminants that originate from some source of contamination (9). For exposure to occur, a completed exposure pathway must exist. A completed exposure pathway exists when all of the following five elements are present:
A potential exposure pathway exists when one or more of the elements is missing, but available information indicates that exposure is likely. An incomplete exposure pathway exists when one or more of the elements is missing and available information indicates that exposure is not expected to occur.
Types of Exposure
Toxic chemicals generally produce the greatest effect and the most rapid response when inserted directly into the bloodstream (2). Occupational exposure generally occurs from breathing contaminated air (inhalation) and/or direct or extended contact of the skin with the substance (dermal exposure). In contrast, accidental and suicidal poisoning occurs most frequently by oral ingestion. The types of exposures are
- Acute, which is exposure to a chemical for 24 hours or less.
- Chronic, which is exposure to a chemical for more than 3 months.
- Sub-acute, which is exposure to a chemical for 1 month or less.
- Sub-chronic, which is exposure to a chemical between 1 to 3 months.
Effects After Exposure
Local effects are seen at or near the body part or parts where exposure occurred. For example, inhaling particles can result in irritation of the respiratory tract, resulting in effects ranging from sneezing to chest pains and difficulty in breathing. An ant bite leads to redness and swelling at the bite location.
Some substances are absorbed into the bloodstream and are then carried to other parts of the body, where they cause their effect. These types of substances usually cause their effect in one or two target body organs. Whether or not these effects occur depends on the concentration of the chemical in the target organ. The concentration in the organ is dependent on the absorption, distribution, biotransformation, and excretion of the substance. Biotransformation occurs when a substance is changed from one form to another, which may also change the toxic properties of the substance. It usually occurs in several steps, primarily in the liver, but it may also occur in other tissue like the kidneys, lungs, and digestive tract.
Some substances are absorbed from the bloodstream and stored in tissues where they may not cause an adverse effect. For example, lead can be stored primarily in the long bones of the body, but when released, has a toxic effect on the nervous system.
Excretion of Toxins
The rate (speed) at which a toxic substance is removed from the body determines whether it will have a toxic effect. The longer a chemical is in the body, the greater the likelihood of damage. The main way a chemical is excreted from the human body is through the urine, but the kidneys, the lungs and the liver are also important in removing certain chemicals from the body. The kidney eliminates the greatest number of toxins than any other tissue/organ. The lungs eliminate substances that are in the gaseous phase, like carbon dioxide. The liver removes substances like lead or DDT (pesticide) by excreting them into bile, which is made by the liver and travels to the small intestine. From there the substance can be absorbed in the feces and then eliminated through excretion. Neither the sweat glands nor the GI tract are important routes for excreting of toxic substances (2). Skin, hair and breast milk are pathways, although minor, for excretion.
- Page last reviewed: September 1, 2015
- Page last updated: September 16, 2009
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