The key understanding between the epidemiologic triangle and Hill’s criteria of causality is indeed the fact that there is a correlation revealing a cause and effect mechanism of exposures and health outcomes within the environment of a population. Therefore, the epidemiologic triangle consists of three major factors, which are an agent, a host, and the environment. The environment is defined as the domain in which disease-causing agents may exist, survive, or originate; it consists of “all that which is external to the individual human host”.
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The host is “a person or other living animal, including birds and arthropods, that affords subsistence or lodgment to an infectious agent under natural conditions”. An agent (of disease) refers to a factor such as a microorganism, chemical substance, or form of radiation whose presence, excessive presence, or (in deficiency diseases) relative absence is essential for the occurrence of a disease. These concepts provide the background of observing hypothetical interrelationships factors that may cause disease in epidemiologic studies, in which the agent affects the host within an environment. For example, data documented that air pollution and smoking causes lung cancer.
However, Hill’ criteria of causality was proposed as a form of action that must be thoroughly examined before concluding on these observations. There are nine areas of causality, but only seven are needed for the environmental epidemiology study. The seven criteria of causality include strength, consistency, specificity, temporality, biological gradient, plausibility, and coherence. The overall approach of this criteria of causality towards disease factors suggests that there may be more that one cause towards the correlation between exposures and health conditions within the environment, and that the cause has to be established first before the effect as well.
The father of modern epidemiology was John Snow, who was an English anesthesiologist in the mid 1800’s linked an epidemiology study called “the natural experiment” that contributed to discovering the cholera outbreak in London that affected many residents negatively by drinking contaminated water. The results lead to a comparison between two companies in which one used water from a less contaminated river and the other company utilizing a more contaminated water system resulting into a higher mortality rate (Friis).
LD50 is a lethal dose that refers to the dosage mortality rate of a chemical at 50 percent in living organisms. (Friis, page 54) The degree in which something is poisonous is defined as toxicity, which is determined by a substance’s physical and chemical properties such as low and high toxicity. Therefore, I believe that anything can be poisonous or toxic if misused or abused because of the chemical make-up of living organisms.
There are many routes of exposure; however, mainly they are through ingestion through food and drink, injections into the bloodstream, skin and eye contact, and inhalation. These routes of exposure can lead to serious physical damage as well such as lung and brain diseases. (Friis). Duration of Exposure: (Friis)
Acute- usually a single exposure for less than 24 hours
Subacute- exposure for one month or less
Subchronic- exposure for one to three months
Chronic- exposure for more than three months.
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