Risk Assessment is the process of determining the probability and magnitude of harm to human life, welfare and environment, potentially caused by the release of hazardous chemical, physical or biological pollutants.
Assessment of risks is performed in four steps: 1. Hazard Identification 2. Toxicity Assessment 3. Exposure Assessment 4. Risk Characterisation.
Step # 1. Hazard Identification:
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The first step in the risk assessment process is ‘hazard identification’. Hazard Identification is designed to screen the hazardous contaminants/ pollutants and select those of greatest public health and of environmental concern. Essentially, this process assists to identify those environmental pollutants which can cause an increase in the incidence of adverse health.
Hazard Identification conditions can be studied in three major steps:
(a) Structure-Activity Relationships
(b) In vitro and short-term tests
(c) Animal Bioassays.
The hazard potential of a chemical is chiefly associated with its intrinsic toxic properties; but some characteristics of its source and the environment receiving inputs also determine the totality of the adverse effects.
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Major properties of a substance signifying its hazardous potential are its acute toxic potential, water solubility/lipophilicity, persistence, pH, stability, sensitivity, electrophilicity, chemical reactivity and carcinogenicity etc. Chief properties of the pollutant source to be considered are the quantities released, geographical distribution, distribution time of release, and the medium into which pollutants are released.
The properties of the environment receiving inputs include its physical and chemical characteristics, water removal rates, ventilation pattern, species present and their trophic status. All these factors together determine the hazardous potential of a substance.
The properties of a pollutant primarily determine the degree of harmfulness resulting from its exposure when the properties of the pollutant source and of the environment receiving inputs do not have much influence. A useful classification of hazardous substances, adopted by the EEC nations, was first developed by Canton and Sloof (1975).
With the help of this, it is easy to identify safe and harmful substance and their relative potential of either property. Of course, along with these guidelines, such properties like carcinogenicity, teratogenicity and mutagenic potentials of the substance need be given serious consideration.
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Classification of Hazardous Substances:
Substances that are positive carcinogens in animals and/or humans are placed in “black” and negative carcinogens in “white” lists, however, wherever the evidence is unequivocal they may be placed in “grey” list, depending upon the degree of conclusive evidence obtained in various test species. Biocides are not included in the grey-list. Thus, at the end of this stage, the utility/justification of making risk estimation for exposure to a chemical can be made.
Step # 2. Toxicity Assessment:
Hazard is defined as the potential of a chemical to cause an adverse health effect as a result of sufficient exposure. Hazard assessment involves the determination of the nature and extent of adverse health or environmental effects affiliated with exposure to the pollutant in question.
This is essentially a two-step process involving:
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(i) Characterisation of the dose-response relationship, and
(ii) Extrapolation to human exposure conditions.
Toxicological evaluation is done by exposing laboratory animals to varying doses of a pollutant via inhalation; ingestion (oral) or injection and the observed toxic effects are then extrapolated to human exposure conditions, this warranting the inclusion and uncertainty factor.
The dose response assessment actually determines the relationship between the dose of a chemical and the incidence of an adverse effect. Toxic effects (excluding carcinogenicity) are evaluated by estimating and acceptable daily intake. This assumes that there is a dose below which no adverse effects occur; this dose level is called the threshold dose (for non-carcinogens).
Further extrapolations of the low dose in animals to low dose in humans are done to complete the process. When we try to convert dose data from laboratory experiments to humans, there are several issues to consider, viz., scaling for the difference in size, life-span, metabolism, difference in routes of exposure, difference in biochemical and pharmacokinetic behaviour and the differential susceptibility between species to the deleterious effects of a pollutant.
The process of extrapolation is a complicated and a controversial one, which involves the inclusion of a safety factor, to provide a range of safety to humans from the toxic effects of xenobiotics that have been predicted with other animal systems experimentally.
For extrapolating non-carcinogenic effects from animal studies to humans a safety factor of 100 is used (a factor of 10 for compensating for the species variation e.g., rats and human) and another 10 to account for the inter-individual human variation. Thus, it is believed that there exists a threshold for such compounds and that no observable effect levels (NOEL) can be established for them. An example is the case of nitrites.
Nitrites induce methaemo- globinemia by oxidizing the iron moiety in the hemoglobin. But at doses at or below NOEL, the production of methemoglobinemia is non-functional or physiologically insignificant. Only when the amount of methemoglobinemia produced by nitrites is high, as to interrupt the normal O2 supply to cell, it is of concern.
On the other hand, dose-response relations for carcinogens do not assume a threshold. This is because the cells affected by these chemicals are thought to be incapable of repairing completely or compensating for the damage done.
Even though carcinogens have so far been considered to exert such damages, recent observations suggest that the central nervous system damage caused by lead in children may assume no threshold as well. Even exposure to small levels of carcinogens is assumed to cause some damage. Thus, toxic effects resulting from the exposure to carcinogens at lower levels have always been a matter of scientific interest.
Step # 3. Exposure Assessment:
Exposure assessment is the process of measuring or estimating the intensity, frequency, and duration of human exposures to an environmental pollutant. This exposure to pollutants can be calculated as a daily dose rate averaged for a population.
The major factors in the process of assessing the exposure of human populations to pollutants present in the environment are:
(i) Intensity
(ii) Frequency, and
(iii) Duration of exposure.
When the pollutant in question is in the fluid medium, its concentration and the contact time with the respiratory tract and skin are very important in assessing the exposure. But when it is in the solid materials, the amount of solid material ingested and the concentration of the pollutant in those tissues/ organs are of prime importance.
One simple approach of determining exposure of a toxicant/pollutant is to consider its physiochemical characteristics, for example, vapour pressure of a chemical. The higher the vapour pressure of a chemical, the greater is the possibility that it would be found in the air. Similarly, the higher the solubility of a chemical in gaseous solutions, the more would be the chances of detecting it in water. The more insoluble is an inorganic compound in air and water, the greater are the probabilities of soil/ sediments being a major pathway of exposure.
Finally, the higher the lipophilicity, the brighter are the possibilities of the food chain being the major pathway.
Step # 4. Risk Characterisation:
Risk characterisation is the process of estimating the incidence and severity of a health effect resulting from the exposure to a xenobiotic. For the characterisation of risk, exposure to a xenobiotic through different pathways should be considered.
Risk is most often determined as the uncertainty concerning an undesired event, where uncertainty is expressed as the probability of occurrence. The probability of a detrimental effect for an individual or a population, though difficult to quantify, can be estimated. This estimate is called population risk (PR). It is the number of cases of effects per lifetime for existing exposure level and can be expressed as –
PR = ΣCiRiPi
Where,
Ci is the concentration of xenobiotic (in water/ air) to which the population is exposed.
Ri is the number of cases or effects per lifetime per unit concentration per individual.
Pj is the number of people exposed to concentration C of xenobiotic.
Though it is not possible to accurately predict the levels of ill-effects caused by human actions, the concept of risk has led to substantial improvements in the environmental assessment and protection process.
Risk analysis provides a more rational basis for decision that may otherwise be highly subjective by:
(i) Emphasizing probabilities and frequencies of events, and
(ii) Explicitly quantifying uncertainty.
Sampocolo and Binetti (1986) suggested the models for quantifying risk arising from personal, domestic and professional exposures, based on prioritisation concepts.
With the use of the approach the risk resulting from exposure to environmental pollutants can be calculated by the following expression:
Risk (environmental exposure)
= (PCP + TP + ETP) x R x Q x ED x P x BC x RP
Where,
PCP- Sum of points ascribed to physiochemical properties of the substance.
TP- Sum of points ascribed to toxicological properties of the substance.
ETP- Sum of points ascribed to ecotoxicological properties of the substance.
R- Coefficient of priority relevant to PCP + TP + ETP.
Q- Quantity present.
ED- Environmental diffusion.
P- Persistence.
BC- Bio-concentration.
RP- Size of population at risk.
In general, three major components determine the risk (the probability associated with the occurrence of a hazard due to a pollutant):
(a) Intrinsic toxic properties of the pollutants (hazard).
(b) Quantities of the pollutant released into the environment (exposure), and
(c) The size of population at risk.
Thus, a chemical may be extremely toxic but if it has not been released into the environment, apparently there is no risk associated with it. On the other hand, if the toxicity of a pollutant is high and also the quantities released into environment are sufficiently large to pose a potential risk to the exposed population, a decision is needed to be taken to minimize its use in the future or force for an immediate ban (depending upon the outcome of the risk-assessment process).
Risk assessment forms the basis of risk management, which is the policy-making process.