Environmental Engineering Reference
In-Depth Information
generally, the initial step for the assessment and evaluation of the toxic characteristics of a substance is to determine the
acute oral toxicity. This provides information on health hazards likely to arise from short-term exposure by the oral route [66].
The term acute toxicity is used to describe the adverse effects of a substance that may result from a single dose of a substance
or multiple doses given within a 24 h period. The studies are carried out via oral, dermal, and inhalation routes of exposure for
the purpose of estimating doses that cause lethality. Acute effects may be local and/or systemic [66, 68].
Acute toxicity tests are performed to obtain information on the biological activity of a chemical and its mechanism of action
at different levels including cell components. This permits us to obtain information for the identification and risk management
in the related context of production, handling, and use of the chemical [69]. The Ld
50
(median lethal dose) value is currently
the basis for toxicologic classification of chemicals and is defined as the statistically derived dose that, when administered in
an acute toxicity test, is expected to cause death in 50% of the treated animals in a given period. The Ld
50
value is expressed in
terms of weight of test substance per unit weight of test animal (milligrams per kilogram). In the last years, acute systemic tox-
icity studies have been among the most criticized of all toxicology tests on both scientific and ethical grounds. Newer, preferred
methods are now trying to employ dose selection lethality limits instead of Ld
50
, applying the 3Rs principle (refinement,
reduction, and replacement of animal use) [66, 68].
In case there is a need to use animals, the requirement should be reviewed and approved by the institutional animal ethics
committee. The National Toxicology Program (NTP) regularly evaluates substances for a variety of health-related effects.
Rodents are the most common animal models used by the NTP [70]. Studies for general toxicology using rodents include sin-
gle-dose acute studies, repeat-dose studies at 2, 4, 13, 26, or 52 week's duration, carcinogenic studies with or without genetically
modified animal models, as well as sensitization and irritation studies.
generally, the testing laboratories adhere to the principles enunciated in the
Guide for the Care and Use of Laboratory
Animals
[71]. The regular NTP
in vivo
procedures are as follows:
Perinatal exposure.
This range-finding study determines whether there is maternal toxicity and/or toxicity to the pups in
order to provide a basis for determining the doses for the subsequent toxicity study (13-week or 2-year study). The ani-
mals are exposed to the substance during in utero development, through their mother's milk, and via dosed feed, dosed
water, or gavage administration.
14-Day Toxicity Protocol.
The goal of this is to provide a basis for identifying potential target organs and toxicities and
to assist in setting doses for the 13-week exposure study. After a 10-14-day quarantine period, animals are assigned
randomly to treatment groups. The study includes five treatment groups, each administered a different concentration of
test article per sex per species plus a control group. Each group per sex per species contains five animals. The animals
receive the test article through a designated route of exposure and the control animals receive the vehicle alone.
13-Week Toxicity Study.
In addition to obtaining toxicological data, the purpose of this study is to determine the treatments
for each strain and species to be used in the 2-year toxicology/carcinogenesis study. Basically, after a 10-14-day quaran-
tine period, animals are assigned randomly to treatment groups. The study includes five treatment groups, each adminis-
tered a different concentration of the problem material plus a control group. Each group contains 10 animals per sex per
species. The animals receive the test chemical by a designated route of exposure. Controls receive untreated water or feed
or vehicle alone in gavage and dermal studies. For dosed-feed and dosed-water studies, animals are exposed for 90 days,
after which they are sacrificed with no recovery period. For inhalation, gavage, and dermal studies, animals are exposed
five times per week, on weekdays only, until the day prior to necropsy.
2-Year Study Protocol.
The purpose of this study is to determine the toxicological and/or carcinogenic effects of long-term
exposure on rats and mice. Typically, after a 10-14-day quarantine period, animals are assigned randomly to treatment
groups. Rats and mice receive the test agent for 104 weeks via a defined route of exposure at three treatment concen-
trations plus controls. For inhalation, gavage, and dermal studies, animals are treated five times per week, on week-
days only [70].
In
in vivo
assays, the evaluation includes identifying the treatment-related lesions in target organs. In mammals, the organ
weights of at least liver, thymus, right kidney, right testis, heart, and lung are recorded from all animals surviving until the end
of the study. A complete necropsy is performed on all treated and control animals that either die or are sacrificed. All tissues
required for complete histopathology are prepared and stained with hematoxylin and eosin for histopathology evaluation.
In the NTP's
in vivo
procedures, all the studied animals are weighed individually on day one of the test, after 7 days, and at
weekly periods thereafter. Animals are observed twice daily, at least 6 h apart, including on holidays and weekends, for mor-
bidity and death. Animals found moribund or showing clinical signs of pain or distress are humanely euthanized. Formal
clinical observations are performed and recorded weekly. For dosed-feed or dosed-water studies, food consumption/water
