Exposure

Exposure Assessment

Exposure Assessment attempts to determine the concentration of the contaminant of concern at the target receptor. The objective of Exposure Assessment is to answer

'What concentration of contaminants might receptors be exposed to?'

There are several main aspects to Exposure Assessment:

source characteristics (media, contaminant concentration)
current distribution (location, depth, media)
transport pathways (physical, +/- biological)
fate
estimated receptor exposure/dose.

The USEPA (1998) poses six primary questions that the exposure assessment should answer:

How does exposure occur?
What is exposed?
How much exposure occurs? When and where does it occur?
How does exposure vary?
How uncertain are exposure estimates?
What is the likelihood that exposure will occur?

While many of the documents describing RA concentrate on on-site contamination effects, many New Zealand sites will be equally concerned with off-site contaminant migration. Off-site release of contaminants can be defined as 'the migration of contaminants across the site boundary' (USEPA 1998). Depending on the characteristics of the contaminant (e.g. solubility, soil adsorption) the possible mechanisms of release include:

volatilisation
wind erosion
surface runoff
leachate
groundwater
direct uptake by organisms.

(Environment Canada 1994).

The following table, taken from Environment Canada�s ERA Framework (1994) illustrates the potential contaminant release mechanisms from various sources into the six main media.

Receiving Medium	Release Mechanism	Release Source
Air	Volatisation Fugitive dust generation	Surface wastes � lagoons, ponds, pits, spills Contaminated surface water Contaminated surface soil Contaminated wetlands Leaking drums Contaminated surface soil Waste piles
Surface water	Surface runoff Episodic overland flow Groundwater seepage	Contaminated surface soil Lagoon overflow Spills, leaking containers Contaminated groundwater
Groundwater	Leaching	Surface of buried wastes Contaminated soil
Soil	Leaching Surface runoff Episodic overland flow Fugitive dust generation/deposition Tracking/earthworks	Surface of buried wastes Contaminated surface soil Lagoon overflow Spills, leaking containers Contaminated surface soil Waste piles Contaminated surface soil
Sediment	Surface runoff, Episodic overland flow Groundwater seepage Leaching	Surface wastes � lagoons, ponds, pits, spills Contaminated surface soil Contaminated groundwater Surface or buried wastes Contaminated soil
Biota	Uptake (direct contact, ingestion, inhalation)	Contaminated soil, surface water, sediment, ground water or air Other biota

The following paragraph, taken from Section 4.3.2 of Environment Canada�s ERA Framework (1994) summarises the typical environmental exposure mechanisms:

'Aquatic biota are most likely to be exposed to contaminants through direct contact with water or through ingestion of surface water, sediment, and contaminated food (prey organisms). In aquatic systems, organisms are exposed to concentrations of contaminants. In some cases (e.g. plants, some soil organisms), the exposure for terrestrial organisms may also be to a contaminant concentration.

Terrestrial animals can also be exposed through ingestion of contaminated surface water, soil, or foods, generally as a dose. These foods include plants that can take up contaminants from surface water, groundwater, soil, or air. Surface water, sediment, soil, and prey organisms can therefore be thought of as exposure media.

Groundwater and air (e.g. dust emissions and volatilization), however, are likely only important as transport media (i.e. transporting contaminants to media from which chemicals are directly taken up by organisms)'.

See Page 15 of the Risk Assessment Procedures Manual (Alaska Department of Environmental Conservation) for an example ecological conceptual site model.

Landuses

In the same way that you need to consider the nature and sensitivity of receiving environments for ecological risk assessment, human health risk assessment requires the consideration of landuses. The land use of the site itself and surrounding land will directly affect the potential for people to be exposed to contaminants. For example, residential land involves people present permanently on a site, whereas commercial and industrial sites have people present for, say, 8 hours per day, 5 days per week. Recreational and residential land uses are more likely to involve potential exposure of children, as are childcare centers and schools. Industrial and commercial land is more likely to be almost entirely covered with concrete or asphalt, whereas residential homes tend to retain grassed areas and gardens, and piled homes have the potential to accumulate the vapours of volatile contaminants.

Exposure Pathways

In determining the potential hazards and risks posed to human health, three primary exposure routes exist:

Oral exposure (ingestion/eating)
Inhalation exposure (breathing)
Dermal exposure (skin).

Each of these exposure routes consist of many potential pathways. For oral exposure, the pathways include:

Daily intake of fluid,
Direct ingestion of soil (especially toddlers and infants, and eating home grown vegetables),
Indirect through food crops/vegetables raised in contaminated soil,
Indirect through meat, dairy products, and eggs from animals raised on contaminated sites, and
Indirect through aquatic animals and plants (fish, shellfish, watercress, etc) caught in surface waters receiving contaminated runoff and groundwater.

Generally, the first two pathways are the primary exposure pathways.

For inhalation exposure, the pathways include:

Inhalation of contaminated dust,
Inhalation of contaminant vapours,
Inhalation of air with chemicals evaporated in water, especially during a shower.

Generally, the first two pathways are the primary exposure pathways.

For dermal exposure, the pathways include:

Contact with contaminated soils, and
Contact with contaminated water (especially where there is a high sediment load).

The life stages of humans and their activities on or near a site will affect their potential for exposure to contaminants. Infants may be exposed to contaminants from mother's breast milk or formula made up with tap or bore water. The risk from contaminants may be heightened if, for example, the contaminants are lipophilic (preferentially partition to and accumulate in fatty tissues) because breast tissue and breast milk contains a high proportion of fat, and may be an infant's only source of nourishment.

Infants, toddlers and children tend to play and crawl on the ground more, giving them more opportunity for exposure to contaminated soils and surface water through inhalation, oral or dermal exposure.

People with gardens may be exposed to contaminants that are taken up by the vegetables they eat, with risk being related to the bioavailability of the compound. However, there may also be amenity effects to consider if the contaminants are phytotoxic preventing people from growing the plants they would like.

If contamination is present in groundwater, people using bore water as a water supply may be at risk, not only from consumption of water, but also from inhalation of water vapour in the shower and dermal contact with shower and bath water.

These are the kinds of details that Exposure Assessment seeks to document.

See Page 15 of the Risk Assessment Procedures Manual (Alaska Department of Environmental Conservation) for an example human health conceptual site model.

Exposure Effects

For many contaminants, levels of no observable adverse effects, maximum advisable levels, and other measures of exposure applied to humans have been based on tests conducted solely on animals. The physiological systems of these animals, often mice or rats, may or may not react in a similar way to the systems of people. Epidemiological studies of people are almost always based on a very restricted group of people, for example miners who are, for the most part, males aged 20-45. The results of these studies are then extrapolated to provide a safe value for people, and sometimes for different types of people (e.g. pregnant women, infants, etc.). As a result of this extrapolation, there is a degree of uncertainty in using a guideline value. Where direct toxicological data is available for a particular species (e.g. the effect of dissolved ammonia on trout), this uncertainty may be negligible. Exposure assessment needs to take these uncertainties into account specifically in relation to the group of receptors you are considering.

Humans are typically only considered at an individual scale (rather than community or population scale), although many epidemiological studies refer to specific populations (e.g. number of cancer cases per 1,000 workers). For human health, added to the toxicity characteristics of contaminants is the potential for carcinogenicity. Human health model end points are more specific than those for ecological risk assessment in that the target organs or tissues for individual chemicals may have been determined in epidemiological and animal testing.

Tier 1 – Exposure Assessment tasks

A Tier 1 Exposure Assessment will involve the following:

a preliminary description of the priority contaminants including contamination concentrations in key media
identify the potential pathways for contamination and the media involved
develop a conceptual model for the major exposure routes
identify uncertainties and data gaps.

Much of this stage will involve identifying contaminant pathways to a receptor and determining source concentrations. You may choose to use a flow chart to illustrate the various likely pathways.

You may also wish to perform some simple calculations to assess whether key receiving media will be affected by contaminants. This may include, for example, estimating the amount of dilution available to a contaminant along a groundwater or surface water pathway.

	Follow link for the XYZ Enterprises example at RA Tier 1.
	Introductory risk assessment information on the next RA stage: Toxicity Assessment
	Advanced risk assessment information on Exposure Assessment