Shortcomings of Dose-Response Data
- Introduction to Toxicology
- Toxicity Tests
- Dose-Response Relationship
- Types of Toxic Hazards
i. Introduction to Toxicology
Toxicology is defined as the study of chemical or physical agents which
interact with biologic systems to produce a response in the organisms.
Toxicity is the relative ability of a substance to cause injury to biologic
tissue. Given the broad range of toxicities any substance might eventually
invoke in an organism, it is easy to understand the wisdom of Paracelsus
(1493-1541) when he said,
All substances are poisons; there is
none which is not a poison. The right dose differentiates a poison and
The dose determines if the effects of any substance are toxic, non-toxic,
or beneficial. Dose is the quantity of a substance to which an organism
is exposed. In toxicological studies, the dose given to test animals
is expressed in terms of quantity administered:
• Per unit weight, usually expressed as milligrams of substance
per kilogram of body weight (mg/kg).
• Per area of skin surface, also expressed as mg/kg.
• Per unit volume of air inhaled, usually expressed as parts of
vapor or gas per million parts (ppm) of air by volume. Solids would
be expressed as milligrams of material per cubic meter of air (mg/m3).
Inhaled doses can also be expressed by time.
ii. Toxicity Tests
The design of any toxicity test incorporates selection of:
• A test organism, which can range from cellular material and
selected strains of bacteria through higher order plants and animals.
• The response or biological endpoint, which can range from subtle
changes in physiology and behavior to death.
• An exposure or test period.
• Dose or series of doses.
The objective is to select a test species that is a good model of humans,
a response that is not subjective and can be consistently determined,
and a test period that is relatively short. Often tests must be selected
that yield indirect measurements or responses that supposedly correlate
well with the response of interest — for example, determining
carcinogenic potential by measuring mutagenic potential.
iii. Dose-Response Relationship
A particular toxicity test exhibits a dose-response relationship when
there is a consistent mathematical relationship between the proportion
of individuals responding and a given dose for a given exposure period.
For example, the number of mortalities increases as the dose of chemical
given to a group of organisms increases.
Measurement of Response
Different species of test organisms differ in how they respond to a
specific chemical. In addition, there are variations in response to
a given dose within a group of test organisms of the same species. Typically,
this intraspecies variation follows a normal (Gaussian) distribution
when the number of organisms responding is plotted against the degree
of response for a given dose.
Several basic relationships can be readily identified from the plots.
A dose is often described as either a lethal dose (LD) in a test where
the response is mortality, or effective dose (ED) in a test where the
response is some other observable effect.
Constructing an ultimate dose-response curve enables the identification
of doses which affect a given percent of the exposed population, e.g.,
the LD50 is that dose which is lethal to 50 percent of the test organisms.
The National Institute for Occupational Safety and Health (NIOSH) defines
a number of dose-response terms:
• Lethal dose fifty (LD50) calculated dose of a substance which
is expected to cause the death of 50 percent of an entire defined experimental
• Lethal dose low (LDLo) The lowest dose of substance introduced
by any route other than inhalation which has been reported to have caused
death in humans or animals.
• Toxic dose low (TDLo): The lowest dose of a substance introduced
by any route other than inhalation, over any given period of time, and
reported to produce any toxic effect in humans or carcinogenic, neoplastigenic,
or teratogenic effects in animals or humans.
• Toxic concentration low (TCLo) The lowest concentration of a
substance in air to which humans or animals have been exposed for any
given period of time that has produced any toxic effect in humans or
carcinogenic, neoplastigenic, or teratogenic effects in animals or humans.
• Lethal concentration low (LCLo): The lowest concentration
of a substance in air which has been reported to have caused death in
humans or animals.
Use of Dose-Response Relationship
Comparing the LD50 of chemicals in animals gives a relative ranking
of acute toxicity of each. For example, DDT (LD50 for rats = 113 mg/kg)
would be considered more toxic than ethyl alcohol (LD50 for rats = 1400
mg/kg). Using this LD50 (mg/kg) and multiplying by 70 kg (average mass
of human) gives a rough extrapolation to humans, assuming they are as
sensitive as the species tested to the substance tested. However,
LD50 serves only as a rough estimate of one aspect of the toxic potential
of a substance.
RATING CHART (oral)
||Oral Acute LD50 for Rats
||1 mg/kg or less (e.g., dioxin, butulin toxin)
||1 to 50 mg/kg (e.g., strychnine)
||50 to 500 mg/kg (e.g., DDT)
||0.5 to 5 gm/kg (e.g., morphine)
||5 to 15 gm/kg (e.g., ethyl alcohol)
Several shortcomings must be recognized in utilizing LD50 dose-response
data for assessing the overall toxicity of a compound. One is that an
LD50 is a single value and does not indicate the shape of the curve
— that is, what the dose-response interval is, which is as important
as how high or low the LD50 is. Thus, comparing these values can give
the wrong impression.
Second, the LD50 measures only acute
full range of toxicity testing includes subacute, chronic, carcinogenic
and reproductive toxicity, amongst others. Therefore, while useful,
the LD50 must be used with considerable discretion and caution.
Routes of Exposure
There are three main pathways for substances to enter the body:
· Absorption (through contact with
skin, and eyes)
The primary function of the skin is to act as a barrier against entry
of foreign materials into the body. If this protective barrier is overcome,
toxic chemicals enter. The barrier is greatly diminished by lacerations
and abrasions. Also, many organic solvents greatly increase the permeability
of the skin to materials that would otherwise not pass through it. Another
factor is that the skin provides a large surface area for contact with
is the most rapid route, immediately introducing
toxic chemicals to respiratory tissues and the bloodstream. Once admitted
to the blood through the lungs, these chemicals are quickly transported
throughout the body.
Health hazards to personnel from ingestion
of materials are
a lesser concern than skin and respiratory hazards. The number of substances
that can be ingested are limited — that is, it is difficult to
swallow vapors and gases.
iv. Types of Toxic Hazards
- chemical agents which act on specific target
organs or organ systems.
— agents which deprive the tissues
of oxygen, a condition called anoxia. This group is divided into simple
and chemical asphyxiants. The simple
asphyxiants act by diluting
or displacing atmospheric oxygen, which lowers the concentration of
oxygen in air.
act in one of two ways. Some prevent the
uptake of oxygen in the blood. Carbon monoxide, forexample, interferes
with the transport of oxygen to the tissues by strongly binding with
hemoglobin to form carboxyhemoglobin, which leaves inadequate hemoglobin
available for oxygen transport.
A second type of chemical asphyxiant does not permit the normal oxygen
transfer either from the blood to the tissues or within the cell itself.
Hydrogen cyanide is an example of this type.
— materials that cause inflammation
of membranes. The mechanism of irritation is either by corrosive or
drying action, and may affect the eyes, skin, respiratory membranes,
or gastrointestinal tract. The irritant must come in direct contact
with the tissue to cause an inflammation reaction.
to a chemical involves immune mechanisms. When a foreign substance called
an antigen enters body tissue, it triggers production of antibodies,
which react with the antigen to make it innocuous. Upon first exposure
to a specific chemical, there are no antibodies in the body. After subsequent
exposures, the concentration of antibodies increases until a threshold
is reached. At this point, the antibody level is high enough that upon
exposure to the chemical the antigen- antibody reaction, also called
an allergic reaction, is severe enough to manifest itself as one or
more symptoms. The body has become “sensitized” to that
— a substance that
is known or suspected to cause cancer in an organism . The following
characteristics apply to carcinogens:
• Chemical carcinogens have distinct mechanisms compared to other
toxic agents. The biologic effect is persistent, cumulative, and delayed,
and repeated doses can be more effective than large doses.
• They are defined by their ability to induce neoplasms. An organism
can respond to a carcinogen by: 1) an increase in the incidence of one
or more types of tumors compared to controls; 2) development of tumors
not seen in controls; 3) the occurrence of tumors earlier than controls;
and 4) an increase in the number of tumors in individual animals compared
• Carcinogens are diverse chemical (and physical, e.g., ionizing
radiation) agents such as organic ( benzene) and inorganic (arsenic
compounds) chemicals, solid-state materials, and hormones. The widely
divergent properties of these chemicals appear to produce neoplasms
by different mechanisms.
• Some react directly with DNA and as a result are mutagenic,
others do not bind covalently to DNA, but produce neoplasms after the
reaction with DNA.
Reproductive Toxins (Mutagens, Teratogens)
chemicals which affect reproductive capabilities, including chromosomal
damage (mutations) and effects on fetuses teratogenesis). A mutagen
changes a gene in a sperm or egg cell of the parent. The parent is not
affected, but the offspring suffer the consequences. Teratogenesis is
also manifested in offspring but differs from mutagenesis in that it
results from exposure of the embryo or fetus to the agent itself.
Most of the information we have about reproductive hazards comes from
testing animals. Most known human reproductive toxins have similar effects
in laboratory animals. Therefore, we usually assume that most agents
which adversely affect reproduction in animals also have the potential
to harm human reproduction.
The field of toxicology is evolving at a rapid rate. Considerable strides
both in terms of elucidating the mechanism of toxicity, as well as the
specific effects of individual chemicals are being made. However, the
vast majority of chemicals have had little, incomplete or inadequate
testing done on them. Newer tests are often of greater sensitivity,
thereby indicating effects at levels previously thought safe.
The result of our increasing knowledge is increasing caution - we see
that the more we know the more we realize how little we know. The message
for us is this— minimize all exposure by reducing time, frequency
and concentration of exposures, and substituting less toxic materials