Environmental toxicology, also known as entox,
is a multidisciplinary field of science concerned with
the study of the harmful effects of various chemical, biological and physical agents
on living organisms. Ecotoxicology
is a subdiscipline of environmental toxicology concerned with studying the
harmful effects of toxicants at the population and ecosystem
levels.
Rachel Carson is considered the mother of
environmental toxicology, as she made it a distinct field within toxicology
in 1962 with the publication of her book Silent
Spring, which covered the effects of uncontrolled pesticide
use.
Organisms can be introduced to toxicants at various stages
of their life cycle. The degree of toxicity can vary depending on where the
organism is found within its food web. Bioaccumulation
occurs when molecular compounds are stored in an organism's fatty tissues. Over
time, this leads to the establishment of a trophic
cascade and the biomagnification of specific toxicants. Biodegradation
releases CO2 and water as by-products into the environment. This process is
typically limited in areas affected by environmental toxicants.
Harmful effects of chemical and biological agents can
include toxicants from pollutants, insecticides,
pesticides,
and fertilizers
all of which can impact an organism and its community through shifts in species
diversity and abundance. Resulting changes in population dynamics impact the
ecosystem by altering its productivity and stability.
Legislation has been implemented since the early 1970s to
ensure that harmful effects of environmental toxicants are minimized for all
species. Unfortunalety, according to McCarty (2013) we are facing the risk of
entering in a "dark age" due to longstanding limitations in the
implementation of the simple conceptual modes
Sources of Environmental Toxicity
There are many sources of environmental toxicity that
can lead to the presence of toxicants in our food, water and air. These sources
include organic and inorganic pollutants, pesticides
and biological agents, all of which can have harmful effects on living
organisms.
PCBs
Polychlorinated biphenyls (PCBs) are
organic pollutants that are still present in our environment today despite
being banned in many countries such as the United States and Canada. Due to the
persistent nature of PCBs in aquatic ecosystems, many aquatic species contain high
levels of this chemical. For example, fish farmed salmon have been shown to
have significantly higher PCB levels.
Heavy Metals
Heavy metals found in food sources, such as fish can
also have harmful effects. These metals can include mercury, lead, aluminum and cadmium. It has
been shown that fish (i.e. rainbow trout) are exposed to higher cadmium levels
and grow at a slower rate than fish exposed to lower levels or none.[5]
Moreover, cadmium can potentially alter the productivity and mating behaviours
of these fish.
Pesticides
Pesticides are a major source of environmental toxicity.
These chemically synthesized agents have been known to persist in the
environment long after their administration. The poor biodegradability of
pesticides can result in bioaccumulation of chemicals in various organisms
along with biomagnification within a food web. Pesticides can
be categorized according to the pests they target. Insecticides
are used to eliminate agricultural pests that attack various fruits and crops. Herbicides
target herbal pests such as weeds and other unwanted plants that reduce crop
production.
DDT
Dichlorodiphenyltrichloroethane (DDT)
is an organochlorine insecticide that has been
banned due to its adverse effects on both humans and wildlife. DDT’s
insecticidal properties were first discovered in 1939.[6] Following this
discovery, DDT was widely used by farmers in order to kill agricultural pests
such as the potato beetle, coddling moth and corn earworm.[6] In 1962, the
harmful effects of the widespread and uncontrolled use of DDT were detailed by
Rachel Carson in her book The Silent Spring. Such large quantities of DDT and
its metabolite Dichlorodiphenyldichloroethylene
(DDE) that were released into the environment were toxic to both animals and
humans.
DDT
is not easily biodegradable and thus the chemical accumulates in soil and
sediment runoff. Water systems become polluted and marine
life such as fish and shellfish accumulate DDT in their tissues.[8]
Furthermore, this effect is amplified when animals who consume the fish also
consume the chemical, demonstrating biomagnification within the food web.[8]
The process of biomagnification has detrimental effects on various bird species
because DDT and DDE accumulate in their tissues
inducing egg-shell thinning. Rapid declines in bird populations have been seen
in Europe and North America as a result.
Humans who consume animals or plants that are contaminated
with DDT experience
adverse health effects. Various studies have shown that DDT has damaging
effects on the liver,
nervous
system and reproductive system of humans.
By 1972, the United States
Environmental Protection Agency (EPA) banned the use of DDT in the United
States. Despite the regulation of this pesticide in North America, it is
still used in certain areas of the world. Traces of this chemical have been
found in noticeable amounts in a tributary of the Yangtze
River in China,
suggesting the pesticide is still in use in this region.
Sulfuryl fluoride
Sulfuryl fluoride is an insecticide
that is broken down into fluoride and sulfate when released into the environment. Fluoride has been
known to negatively affect aquatic wildlife. Elevated levels of fluoride have
been proven to impair the feeding efficiency and growth of the common carp
(Cyprinus carpio).[10] Exposure to fluoride alters ion balance, total
protein and lipid levels within these fish, which changes their body
composition and disrupts various biochemical processes.
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