INTRODUCTION TO TOXICOLOGY
The concepts of toxicology useful for the understanding of the health pages are discussed here, and in particular for the page “Analysis”.
DIFFERENT TYPES OF TOXICITY
Influence of dose and durationThe toxicity of a substance depends on the absorbed dose and the length of time that dose is absorbed. We distinguish:
- acute toxicity, for which the duration of exposure is less than 15 days;
- subacute toxicity for exposure from 15 days to a few years;
- chronic toxicity for lifetime exposure.
Contrary to what it seems logical to think, the effects of the same substance are different according to the type of toxicity:
|Benzene||cerebral coma||bone marrow||leukemia|
|Carbon disulfide||narcotic||hepatotoxic||disorder of vision|
|Ethyl alcohol||ethylic coma||liver cirrhosis||CNS|
Toxicity with threshold – toxicity without thresholdThe detection of toxicity is done by dose / response curves. These curves show for a given toxic agent, what effects are observed according to the dose received. The effects observed are expressed as a percentage of the population studied, the y-axis is therefore graduated from 0 to 100%. The nature of the effects is, however, variable according to the study. This may be the occurrence of any effect in the case of the LOEL (Lowest Observed Effect Level) study or death for the LD50 (lethal dose for 50% of deaths) study.
Two types of curves exist. For non-carcinogenic substances the curves have a threshold, see curve (2) below. Below this threshold no effect is observed.
For carcinogenic and mutagenic substances, the curves show no threshold, at least in the range of doses available for experimentation. See curve (1) above. For very low concentrations, it is assumed that the curve (in this case substantially a straight line) continues linearly until the origin, thus without threshold. According to this hypothesis, even a single molecule can cause cancer.
For a discussion of this hypothesis, see Health Canada, Assessment and Management of Cancer Risks from Radiological and Chemical Hazards , Chapter 3 Risk Assessment.
Non-carcinogenic toxic agentsAs indicated above, these substances have the particularity of presenting a threshold below which toxicity does not occur.
In general, epidemiological studies can not be used and therefore toxicological studies are carried out on animals, often mice. They make it possible to determine the LD50 as well as the NOAELs.
Lethal dose 50% (LD50 or LD50)
Dose for which 50% of animals die.
No Observable Effect Level (NOAEL)
The NOAEL is the dose per kg of body weight that can be absorbed daily without any harmful effects being observed. It is expressed in mg / kg / day or in μg / kg / day.
The NOAEL is determined by toxicological studies.
Lowest Observed Adverse Effect Level (LOAEL)
The LOAEL is very close to the NOAEL.
Maximum Allowable Dose (DMA)
(or tolerable daily intake: TDI )
DMA is the dose that we consider “acceptable” to absorb daily on a human being over a long period of time: several years, or even lifetime.
DMA = NOAEL / uncertainty factor.
The uncertainty factor takes into account the following elements:
- inter-species variability;
- intra-species variability;
- the necessarily limited duration of studies;
- Increased risks for embryos and the elderly.
For further details see SAN, Pollution des eaux de surface, eutrophisation et cyanobactéries, 2006.
Acceptable daily intake : ADI
Same meaning as DMA / TDI.
Maximum Allowable Concentration (MAC)
The maximum concentration in drinking water is then calculated on the basis of a 70 kg adult drinking two liters of water a day.
Toxicological Reference Values (TRVs)
(or refernce dose: RfD)
TRVs are defined for threshold substances but also for substances without thresholds. In the case of threshold substances they are similar to acceptable or tolerable daily doses or concentrations (ADI, TDI).
What is a carcinogen?
This is a substance that causes an uncontrolled multiplication of cells.
What differentiates carcinogens from other toxic substances?
Changing the genome of a single cell may be enough to cause cancer. So a single molecule of the carcinogen can cause cancer. Unlike conventional toxic substances, there is no minimum dose below which a carcinogenic substance can have no effect.
The maximum dose is based on statistical risk considerations: it is the dose likely to produce an additional case of cancer in a population of N people (with a probability of 95%). The choice of N is arbitrary, in general N is taken between 10,000 and 1,000,000.
How are the maximum doses determined?
Setting a maximum dose for a carcinogen is significantly more complicated than for another substance. This is reflected by the evolution of the methods set by the US EPA over the years (1986, 1999, 2003) which makes comparisons sometimes difficult. The summary given here is very simplified (see for more details Toxicology Excellence for Risk Assessment, Approaches to Determining Unreasonable Risk to Health, 2003). The methods of the US EPA are important because they are public and the results obtained, even if they are often contested, are “references” for the whole world.
In general, epidemiological studies are not appropriate for determining maximum doses. It is therefore necessary to use toxicological studies.
The doses to which animals are subjected are much higher than those to which men are exposed. Two extrapolations will be used. The first to take into account differences in doses between experiences and the environment, the second to pass from animals to humans.
To extrapolate high doses to low doses, a mathematical model is used, usually a linear model for carcinogens.
It is therefore possible to determine (with a large margin of uncertainty) what dose is acceptable to absorb during the whole life and therefore deduce a maximum concentration in the drinking water.
The values thus determined are values that should be respected (maximum contaminant level goal MCLG) but this is sometimes unrealistic in the current state of the art or even unmeasurable. In this case the standard to be respected (maximum contaminant level MCL) is set to a higher value.
In general, MCLs are set to induce less than 1 additional cancer case per 10,000 people. This only concerns, of course, substances assimilated through drinking water.
How does one define dangerousness? Since the curve representing the probability of developing cancer as a function of daily absorbed dose is assumed to be linear, it is possible to define a coefficient representing the slope of the curve. This coefficient is designated by SF (slope factor) or CSF (cancer slope factor):
CSF = probability of developing cancer (risk) / daily dose
Unit: mg-1*kg *day
|Substance||CSF (mg-1*kg *day)|
|DDT, DDE, DDD||0,34|
|Dioxine||1,56 * 105|
Classification of carcinogens
Each country or organization has its own classification which makes comparisons difficult.
USA (US EPA)
A: carcinogenic to humans
B1: probably carcinogenic to humans B2: probably carcinogenic based on animal studies
C: can be carcinogenic
D: not classifiable
WHO CIRC / IARC
(The International Agency for Research on Cancer or International Agency for Research on Cancer, IARC is an intergovernmental agency part of the WHO).
Category 1: Carcinogenic to humans.
Category 2A: Probable carcinogen for humans.
Category 2B: Possible carcinogen for humans.
Category 3: unclassifiable as to its carcinogenicity to humans.
Category 4: probably not carcinogenic to humans.
SUPPLEMENTSHow do toxic substances work?
Toxic substances act in very different ways on the organism:
- neurotoxic: toxic to the nervous system. Many insecticides are nerve agents;
- hepatotoxic: toxic to the liver;
- dermatotoxic: toxic to the skin;
- cardiotoxic: toxic to the heart;
- cytotoxic: toxic to cells in general;
- genotoxic (or mutagenic): which creates damage to DNA and increases the frequency of genetic mutations. Most modifications to DNA are repaired by natural mechanisms but this is not systematic. There is then creation of mutant cells;
- carcinogen: which produces an uncontrolled multiplication of cells. The action of the carcinogenic substance can be at any of the various stages of a cancer;
- teratogenic: which can cause abnormalities or malformations of the embryo;
- immunotoxic: which causes a breakdown of the immune system;
- endocrine disruptors that act primarily on reproductive mechanisms;
- disruptive behavior.
What are the factors that modify the toxicity?
Regardless of the intrinsic toxicity of a molecule two important factors influence the actual dangerousness.
The first factor is related to the property of fats to concentrate certain substances, it is bioconcentration. This point is discussed in more detail in the next section.
The second factor is molecular stability. The more stable a molecule is, the longer it will persist in the environment and manifest its toxicity. To characterize the stability, the half-life time is used: the time at which 50% of the molecules are destroyed. The half-life is more than 10 years for DDT, a few years for dioxin and PCBs.
On the other hand, some molecules by degrading will form other substances whose toxicity may be higher than the starting molecule.
What are the water pollutants?
Any substance that can dissolve in water is likely to be or become a pollutant. Many substances, usually considered insoluble, are in fact poorly soluble in water and therefore potentially dangerous. This is the case of oil. The main categories of water pollutants are:
- hydrocarbons whose molecules are formed of hydrogen and carbon (octane …);
- polycyclic aromatic hydrocarbons (PAHs) which are a subclass of the previous ones but which include carcinogens (benzene);
- organohalogens, which in addition to hydrogen and carbon atoms contain halogens (chlorine, bromine …). Many of these substances are toxic and some carcinogens (eg DCE, trichlorethylene TCE, many DDT pesticides, aldrin, toxaphene …);
- organophosphorus compounds, mainly pesticides but also combat gases (tabun, sarin);
- heavy metals (mercury, lead …);
- by-products of disinfection which are the result of the treatment of water (chlorite, bromoform …);
- radioactive substances;
- chemical fertilizers (based on nitrates and phosphates).
Some pollutants are insoluble. They settle at the bottom of rivers and lakes with sediments. They can be concentrated in algae and fish that feed on it.
How many chemicals are there?
Here is a summary of chemical substances:
- 60 000 000 chemical compounds are known (polymers excluded)
- 40,000 are used
- 7,000 have been tested on animals
- 1500 are positive on animals (ie toxic and / or carcinogenic)
- 58 are in class 1 (carcinogenic proven in humans).
What are CMR?
These are particularly dangerous substances because they are either carcinogenic, mutagenic or reprotoxic.
Mutagenic substances cause genetic mutations.
Reprotoxic substances interfere with the fertility of the man or the woman, or the development of the fetus.
What are POPs?
Persistent organic pollutants (POPs) are persistent, bioaccumulative, toxic and mobile substances.
POPs include dioxins, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and several pesticides.