Home Resources Links FAQs Commentaries Search Help
Home Page for Health, Environment and WorkClick on Image

Environmental Health Impact Assessment


There are various stages in an Environmental Health Impact Assessment: 
    • Hazard Identification 
    • Exposure Assessment 
    • Exposure-Response relationship
    • Estimation of risks to health, and communication of these risks to all concerned
    • Determination of health costs (potential or actual), 
    • Option appraisal ...
Some of these aspects are considered here.

Hazard Identification:

The hazards are determined from a wide range of information sources relevant to what is planned - a matrix approach  is to be recommended. 

For example one dimension of the matrix could include the various stages and/or processes of what is proposed while the other could include the various forms of energy, hazardous chemical substances, and microbial contamination which may arise. 

You might wish to access information on occupational hazard identification and risk assessment

Exposure assessment:

Note that once a hazard matrix is constructed it does not necessarily follow that a detailed assessment of all the conceivable hazards must be undertaken. Usually common sense and a preliminary review of available data by a competent professional will indicate which exposures to hazards are likely to be trivial and need be taken no further, and which ones will need a formal assessment of likely exposure. If possible it is better to measure "dose" (actually taken up by the human body) but often we have to make do with "exposure." The parameters for measuring exposure are many and varied, for example:
  • Concentration (in air, water or solid media) of a chemical or micro-organism, or some form of measure of intensity of energy (e.g. noise or radiation) are common and important measurements ...but what is the most relevant in any particular scenario:...mean?... or ...peak? or perhaps some other .... (e.g. change in flux of electromagnetic radiation)
  • Time exposed is clearly very important too, and the total time exposed has been used as a surrogate for 'true' exposure (what are the shorcomings of this?) To what temporal extent will the population be exposed in terms of total duration, average duration, or age at commencement of exposure?
    • To cite some examples: e.g. housing, hotel or holiday accommodation in use 24 hours; day workplaces not usually occupied by the same people round the clock.
  • But what is 'true' exposure? - it might be cumulative exposure - expressed in its simplest terms as a product of concentration of exposure, and duration...or..
  • Peak exposure might need to be considered
  • 'Measurements of 'uptake' might need to be made as they can be better reflections of 'dose' than 'exposure' e.g. measuring lead in the blood
All these measures of exposure have different relevance in specific contexts, and suffer from varying degrees of confidence/(un)certainty.

How can you calculate the "dose" of the agent? 

What routes would you consider and what media?
e.g. could a route be ingestion by eating or drinking; skin contamination through washing, bathing, or exposure to contaminated soil; or inhalation of polluted air? Young children ingest much more soil, earth, dust and paint than adults.

What factors influence the rate at which the body can take up the "dose"? What does the body do to it? Does this "biotransformation" make the agent more or less harmful?

The Exposure-response relationship:

In occupational and environmental epidemiology the exposure-response relationship is important because it can fulfil a number of purposes. Thus it:
  • can provide strong evidence of causation
  • helps in planning interventions (e.g. setting occupational exposure limits) and setting priorities
  • assists in prediction - in health and economic terms (and is therefore important in environmental impact assessment)
In considering exposure-response relationships, besides understanding its purposes, one has to understand what one means by 'exposure', and by 'response' - the properties of the relationships between these parameters, and factors which may influence them.

Exposure/dose v response/effect

The relationship between exposure (concentration or "activity" multiplied by time  in an integrated way) and harm to health expressed as response (i.e. likelihood of harm) and effect (magnitude of harm in an individual) is of fundamental importance. 

 A wide range of measures of 'response' or of effect -some rather imperfect- are used e.g. various indices of incidence, prevalence, mortality etc. For outcomes which are not mortality nor cancer incidence, different criteria of severity and/or certainty may apply - based on symptoms, physical signs, special investigations etc. Sometimes biological effects (e.g. enzyme concentration changes) which are not necessarily health effects are studied.

Some important points to be considered in the exposure-response relationship are:

  • Is it a linear relationship with no threshold (as is assumed to be the case for cancer and reproductive risks from exposure to ionising radiation, or for chemical carcinogens such as benzene)?
  • Does the relationship have a threshold (e.g. noise induced hearing loss)?
  • Is it linear? (or rather to what extent is it not linear?)  - what is its 'slope' or 'gradient'?
Exposure-response relationships, like the parameters from which they are derive can suffer from varying degrees of confidence/(un)certainty
In other words, after assessing exposures, the exposure-response relationship is the means of translating estimates of exposure into risks to health. It will need an input of the exposure estimates. Information (so far as is already known to science) is needed about the relationship between a given exposure and the likelihood of a health effect.

Other data which may be needed includes information about the characteristics of the population likely to be exposed, and whether some are potentially more susceptible than the general population. Are we considering risk to "average" or "typical" persons or are we considering individuals for whom the risk may be higher (the very old and very young, for example). 

How are we going to allow for the diversity of human responses - by looking at the most critical, or by looking at the average? 

Can "safety" or "uncertainty" factors help? 

What information do we need about the exposed population? Age...state of health...etc.

To cite some examples: e.g. hospitals, homes for the elderly, schools, may contain more vulnerable people. Babies are particularly susceptible to nitrates in drinking water. People with chronic obstructive pulmonary disease (COPD) might be more susceptible to smogs. Atopic asthmatics may be more sensitive to respiratory irritants.

Health risk can be expressed as the frequency with which a given level of harm may be expected from hazards associated with a particular development. Amongst other things, we need to be clear about the nature and severity of the harm, the number of people at risk, and the period of time during which the specified number of new harmed cases will arise.  The health effects need to be characterised in various ways, for example:

Latency of the health effect:

is it an acute irritant/poison (e.g. chlorine, sulphur dioxide) or a cancer risk which may be manifest only after a period of a few years (e.g. benzene) or several years (e.g. asbestos)?

Nature of the health effect: 

  • what is the most critical health effect? - The most critical health effect is not necessarily the most severe e.g. an organic chemical might if inhaled result in [a] loss of consciousness and death or in [b] reduced fertility and adverse pregnancy outcome. Yet the critical effects which need to be studied most carefully and need to be prevented are [b]. Why? - not because we're not interested in preventing deaths, but because the exposures that carry a measurable risk of death are much higher than those carrying a measurable risk of reduced fertility and adverse pregnancy outcome. By striving to prevent these we will most certainly control the risk of death from overexposure as well.
  • Can you describe this in lay terms?

Severity of the health effect:

is it a minor effect (e.g. acne caused by dioxins) or a very serious one (e.g. leukaemia caused by ionising radiation or benzene exposure)? Is it possible to distinguish "nuisance" symptoms from serious health effects? If the health effect appears at first to be a minor one, could it be associated with something more seious?

Reversibility of the health effect:

e.g. cancers do not usually reverse; dermatitis and asthma may do.

Is there a likelihood of harm to future generations (sometimes called "hereditary mutagenicity")?

...e.g. genetic effects caused by radiation.

To what extent could other environmental exposures (whether or not related to the environmental assessment in hand) interact to produce different/more likely/more severe health effects? 

e.g. the possible interaction between various airborne irritants that may affect the respiratory tract (SO2, NOx, etc.); or a general issue - tobacco smoking.

To what extent could the proposal reduce the risk to human health, which in turn has to be balanced against possible adverse consequences? e.g.:

  • Fluoride added to drinking water can reduce the risk of dental caries (but could it have adverse consequences e.g. on bone)
  • Chlorine added to drinking water reduces the risk of serious microbial contamination (but associations have been shown, although causation not proven, between organic chlorides and increased frequency of some gastrointestinal cancers)
  • Building a sewage processing plant could reduce the risk of gastrointestinal and other infective illness attributed to the sewage pollution that had hitherto affected a beach, even though it may have other undesirable consequences.
As with exposure assessment, but to an even greater degree, our exposure-response assessment may contain considerable uncertainty. 

Worked examples, and case-studies:

This website does not yet contain a worked example of an environmental impact assessement as it relates to human health, but  you may wish to consult a student's assignment on the subject.

You may also wish to consult  a useful document: the Canadian Handbook on Health Impact Assessment.