Hazardous substances seldom stay at the point of release. They move and are transported among the environmental mediums (air, water, soil and sediments, and biota). Pathways refer to the mediums by which hazardous substances get to a living organism and the environment.
Examples include the air we breathe, the water we drink, the food we eat, and even the soil we work with, play with and use to grow much of our foods. Understanding these mediums will enable us to mitigate the health and environmental impacts of the identified hazardous substances on human beings. The pathways are as a result of the interaction of these components: exposure pathways, medium of exposure and route of exposure.
Humans can be exposed to chemicals and physical agents through various exposure pathways. An exposure pathway describes the course that a substance takes from the source of the hazardous substance or chemical to the exposed individual or environment.
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Using a human being as an example, an exposure pathway (see Figure 1) generally consists of the following:
(1) The source of the chemical;
(2) A medium (i.e. soil, water, or air);
(3) An exposure point, the potential point of contact between the human and the contaminated medium; and
(4) A route of exposure (inhalation, ingestion, dermal absorption), the way in which a substance enters the human body.
Figure 1: Illustration of exposure pathways. Source: United States Department of Energy, (1989)
Humans are exposed to hazardous substances in a number of ways. Sometimes, exposure occurs directly at the source, as in the case of workers, while in some, through contact with a medium.
The primary media, soil, water, air, and biota, become contaminated by various mechanisms. Table 1 presents some typical release sources, release mechanisms, and receiving media.
Air is the gaseous component of the earth, which serves as a veritable medium for airborne chemicals and hazardous substances to exist in the environment. Exposure to airborne chemicals varies widely among inhalation microenvironments.
They can occur virtually everywhere, such as at residence, workplace, outdoor ambient air, transport vehicles, recreational spaces and public spaces.
Water refers to the liquid part of the earth’s surface. Water can occur either at the earth’s surface or underground. Most hazardous substances released into the environment end up in water bodies, such as rivers, ponds, lakes and underground aquifers.
Surface and ground water used for domestic and other production purposes are the most likely exposure medium of hazardous substances for those living near the sites.
Soil and sediments serve as one of the mediums most exposed to deposits of hazardous substances. Although, soil is not ordinarily a direct exposure source for humans, contaminated soil could affect plants and animals and can also transfer the hazardous substances to other mediums, such as water and air.
Biota: plants and animals in the environment also serve as exposure medium as hazardous substances absorbed from the environment can be transmitted to human beings when there is a contact.
Route of Exposure
After soil, water, air and biota become contaminated, human exposure to the contaminants can occur through contact with the medium. These potential points of contact are called exposure points. Any sources or contaminated media at a source site can be considered a point of exposure.
If contaminants have travelled offsite, significant exposure points would typically be locations close to or down gradient or downwind of the site; however, chemicals can sometimes be transported from a site and deposited in a distant water body.
At the point of exposure, chemicals can enter the human body typically through the following exposure routes: inhalation, ingestion, and direct contact.
Breathing of gases, vapours, dust or mists is a common route of exposure. Inhalation of polluted air or chemicals in air essentially occurs continuously because ambient air contains a variety of gaseous and particulate pollutants.
These substances enter and irritate the nose, air passages, lungs and other regions.
Human exposure to toxic substances can occur via ingestion, the process of taking a substance into the body through the mouth. Ingestion of mediums, such as water, food, and soil, can result in the inadvertent consumption of toxic substances.
Ingestion of contaminated drinking water, as well as ingestion of contaminated groundwater and surface water (such as during swimming) can result in exposure to toxic chemicals. Ingestion of biota including fruits, vegetables, grains, meats and dairy products, fish and shellfish can all result in chemical exposure.
Exposure to chemicals through the ingestion of contaminated soil, although typically inadvertent, occurs frequently with children at greater risk than adults as a result of childhood behaviours (US EPA, 1997).
The skin is generally an effective barrier against the entry of environmental chemicals. Although, skin is relatively impermeable and acts as a barrier, certain chemicals can diffuse through the epidermis.
Touching of hazardous substances with the skin or eyes is also a route of exposure and this can occur during various activities.
Contact with contaminated water and soil can occur during bathing or swimming, gardening, construction, or recreation or even during the use of domestic or commercial products.
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The route of exposure can determine whether or not the hazardous substance will have an effect. For example, inhaling a polluted air can result in health challenges, while touching a lead may not cause any health challenges or physical injury.
In summary, hazardous substances are found in nature and are produced by various human activities.
Hazardous substances seldom stay at the point of release, they move and are transported among the environmental mediums (air, water, soil and sediments, and biota).
Hazardous substances on the different media enter the human body through three exposure routes namely: inhalation, ingestion and direct contact.