1. GENERAL TOXICOLOGY

i. Toxicology

Toxicology is the study of harmful effects to living organisms from substances which are foreign to them. The toxins may be naturally occurring in the environment or synthetic chemicals. The following definitions will describe some basic concepts in toxicology.  

    Toxicity can be generally broken down into two categories:

acute toxicity  refers to the rapid development of symptoms/effects after the intake of relatively high doses of the toxicant. Acute toxicity refers to immediate harmful effects generated by sufficiently large doses.

chronic toxicity refers to the harmful effects of long-term exposure to relatively low doses of toxicant. This would include traces of pesticides in foods, air pollution, etc.

A single compound may generate both acute and chronic toxic effects depending on the dose and duration of exposure.

  

There are two general types of toxic effect:

Lethal Effects: resulting in the death of individuals

Sublethal Effects: other effects not directly resulting in death

 

    There are four basic types of damage caused by toxic materials:

1. Physiological damage: reversible/irreversible damage to the health of the organism

2. Carcinogenesis: induction of cancer

3. Mutagenesis: induction of genetic damage / mutation(s)

4. Teratogenesis: induction of birth defects

         ii. Epidemiology        

In contrast to the rapid effects observed in toxicology, epidemiology examines the relationship between exposure to a specific toxic agent and the development of health problems in a group of humans. In essence epidemiology examines the health patterns in humans with respect to an agent/toxicant. An example might include the health effects of chronic, low-level exposure to pesticides found on un-washed fruit. Simply put, epidemiology looks for trends and effects in human health following exposure to a specific compound or other toxic agents.

iii. Effect of Dose

Relative toxicity of a substance is often expressed in the dose-response relationship. This simply relates the dose (quantity administered) of a substance to the response generated in a test organism. In order to relate this information to humans, dose is often expressed as dose per organism weight, or mg / kg (where mg represents toxicant dose and kg expresses animal weight.). It is also important to consider the method of application, whether oral, dermal, intravenous, etc., as these may greatly affect relative toxicity of a given compound.

Very commonly the quantity of a toxicant required to cause death is used as an indication of relative toxicity. This is most often expressed as: LD₅₀ . This expresses the mg/kg dose required to kill 50% of a test population. For example the LD₅₀ for sugar is >10,000 mg/kg, while for the botulism toxin is ~ 0.0001 mg/kg. Obviously the botulinum toxin is highly toxic even in relatively low doses. An alternative measurement of dose response is the ED₅₀ where there an effect on 50% of the test population. This represents a measurement of sub-lethal effects.

Some Example LD₅₀Values

Dose-response curves are generated for various agents by plotting "percent of population affected vs. Dose (mg/kg). When examining such graphs, often there is a minimum concentration (or "threshold") of dose below which there is no observable effect in the test population. This is referred to as the NOEL (no observable effects limit).

With respect to human safety, NOEL dose information is often obtained from animal experimentation and then divided by a "safety factor" (i.e. 100) in order to establish safe levels for humans. This generates two kinds of values: ADI (acceptable daily intake) and MDD (maximum daily dose). While these values suggest a large margin of safety, in reality it is quite possible that the more sensitive individuals in a population may still suffer ill effects within the dose limits set by these guidelines.

Pharmaceutical safety / effectiveness can be indicated by the pharmaceutical TI (therapeutic index). The therapeutic index is the ratio of LD50 / ED50. A high TI value indicates an effective treatment at low doses and a lethal effect at higher doses. A low TI value indicates an ineffective treatment at low doses with lethality at the same low levels. The objective of using the therapeutic index is to have a pharmaceutical that is highly effective at low doses with minimal toxic effects.

         iv. Exposure Types:

Typically exposures can be classified according to the duration of the exposure. There are four main categories of exposure:

Acute Exposure: describes exposures over a short period of the life of an individual   (which may be relatively intense). An acute exposure is often  referred to as being less than 24 hours duration.

Subacute Exposure: describes exposures over a relatively short period of time,  often less than one month.

Chronic Exposure: describes repeated exposures for a "significant" portion of an individual’s life-span. (More than three months for mammals).

Subchronic Exposure: describes exposures shorter than chronic exposures but longer than subacute (1-3 months is typical for mammals).

v. Exposure Routes:

In order for a toxic agent to have an effect an individual (or population) must be exposed. There are several exposure routes: oral (mouth), dermal (through the skin), inhalation (breathing) and /or injection (subcutaneous, intravenous, intramuscular, intraperitoneal). The specific route of exposure may have a significant impact on relative toxicity: specific substances may even have substantially reduced or enhanced toxicity dependant upon the type of exposure.

It is rare for chemical interactions with living systems to occur in complete isolation. Often there are specific interactions between multiple agents / chemicals which can be quite different than the action of an individual toxicant. Generally, these interactions can be described as being:

1. additive: compounds X + Y combine toxicity proportionally (i.e. 2+3 = 5)
2. antagonistic: compounds X + Y combine to be less toxic than either individually (i.e. 2+3 = 1)
3. synergistic: compounds X + Y combine to be more toxic than either individually (i.e. 2+3 = 9)

With these distinctly different interactive effects being possible, it is often very difficult to anticipate combined effects purely on the basis of chemical structure or other isolated data. It is obviously much easier to acquire toxicity-effect data for individual compounds as opposed to complicated mixtures with many unknown interactions.

Data on individual toxicity's and safety can be obtained from a number of sources. MSDS (Material Safety Data Sheets) are listings that describe (in detail) the known potential hazards for many individual compounds. This type of information usually accompanies commercial chemicals and is also offered on numerous websites. MSDS information includes handling, equipment, safety measures and potential risks / hazards associated with a given substance and can be an excellent starting point for assembling information on a specific compound.          

            vi. Bioaccumulation

    In general, the accumulation of a toxic agent into a living system is referred to as the process of bioaccumluation, while the concentration of such an agent in living tissues is referred to as bioconcentration. An indicator of relative solubility in organic/water phases is the partition coefficient (K_ow = [octanol] / [water] ) which demonstrates the affinity of a specific compound for polar/non-polar environments. Generally a partition coefficient of 4-7 indicates a compound will bioconcentrate to a high degree, while values >8 suggest the compound will bind so strongly to sediments that it is relatively unable to migrate into living tissues. Partition coefficients can therefore be used as indicators of potential assimilation of compounds into living tissues.

        Another consideration for potential toxic compounds is biomagnification, which occurs as small organisms ingest and bioconcentrate specific toxins, which in turn are eaten and further bioconcentrated by larger organisms (i.e. insects eat pesticides, birds eat many insects, etc.).

The relative persistence of a specific toxic agent is often expressed as "half-life" (T_0.5) or the length of time required for one half of a toxic material to be degraded / eliminated. There is a simple relationship: the longer the half-life of a given toxic agent, the greater the potential period of bioaccumulation / concentration.

vii. Risk Assessment

Risk assessment attempts to estimate the expected toxicity's or potential effects from a substance on an exposed  human population. This is conducted by examining both toxicological and epidemiological data. Risk assessment is often used to help establish permissible levels of exposure to a specific material. As stated above, the NOEL limits are adjusted with a 100-fold safety factor to establish limits that are deemed "safe" for the majority of the population. While this may work for direct toxic effects, it may not be so simple for carcinogens.

In the United States the EPA limits exposures to potential carcinogens by simply assuming there is no threshold for the dose-response relationship. The EPA then determines the quantity of MDD an individual could be exposed to in a lifetime and sets a limit that this dose should not increase the probability of cancer in more than one in a million individuals.

Risk assessment is not an exact science: there are often powerful arguments put forward by various groups for both health and/or economic reasons to alter exposure limits. Careful scrutiny is required when examining reports/data concerning toxicological information to establish relevance and non-bias in the results. No matter what safety levels are finally established for a specific compound, it is important to consider that a few individuals within a population will almost certainly suffer effects (lethal or sublethal) from exposure to doses that have no apparent effect on the majority of the population.