Exposure assessment

Exposure assessment consists in determining doses of particular chemicals ab-sorbed by the individuals over all pathways defined in the exposure scenarios. This step requires precise data on the contaminants concentration and population structure, especially when residents scenario is concerned. Calculations of the chemicals intake transported into the individual’s body by different exposure pathways can be con-ducted using equations recommended in the Risk Assessment Guidance for Superfund (EPA 1989). The universal formula for calculation of the intake of chemical can be expressed as:

I = C  CR  EFD/(BW  AT) (1)

where:

I – intake – amount of the chemical taken [mg(kg body weight)^-1d^–1], C – average concentration of the chemical over the exposure period, [mg/dm³

water], [mg/kg soil], [mg/m³ air], etc.,

CR – contact rate, amount of contaminated medium contacted per unit time, [mg soil/d], [m³ water /d], [m³ air/d], etc.,

EFD – exposure frequency and duration. It describes how often and how fre-quently exposure occurs [hours], [days], [years],

BW – average body weight [kg],

AT – averaging time, period over which exposure is averaged [d].

The exposure and risk assessment require establishing of exposure scenario models adjusted to the site and population characteristics. The scenarios enable to adapt for-mula (1) to the real cases of exposure, taking into consideration the following basic exposure pathways:

 ingestion of the water polluted with chemicals,

 inhalation of air containing toxic or carcinogenic substances,

 inhalation of air polluted with airborne dust originating from soil dusting,

 inhalation of air contaminated with volatile chemical substances released from the soil,

 consumption of vegetables and agricultural products grown on the polluted soil,

 consumption of meat and dairy products from animal breeding on the polluted area,

 incidental ingestion of the contaminated soil,

 dermal contact with the contaminated soil (EPA 1989).

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One should be aware that much more exposure scenarios are possible. They in-clude a combination of the above pathways and number of other variants, resulting from the type of the receptor contact with the polluted medium. Thus, variety of spe-cific formulas can be derived from the formula (1) enabling to calculate the chemicals intake for variety of pathways and exposure scenarios. Below some most used exam-ples are given which may vary when real conditions are applied, depending on par-ticular exposure scenario (EPA 1989).

Intake of chemicals by ingestion with drinking water

The amount of chemicals ingested with drinking water can be calculated from the formula:

I = Cw  IR  (EF  ED)/(BW  AT) (2) where:

Cw – average concentration of the chemical in water over the exposure period [mg/dm],

IR – ingestion rate – daily consumption of water [dm³/d], EF – exposure frequency [d/y],

ED – exposure duration [y], BW – average body weight [kg],

AT – averaging time, period over which exposure is averaged [d].

Intake of chemicals by inhalation with air

I = Ca  IR  ET  EF  ED/(BW  AT) (3) where:

Ca – average concentration of the chemical in air over the exposure period [mg/m³],

IR – inhalation rate [m³/h], ET – exposure time [h/d], the remaining symbols are the same.

Intake of chemicals by ingestion of contaminated fruits and vegetables

I = C_f  IR  FI  EF  ED/(BW  AT) (4) where:

Cf – average concentration of the chemical in food [mg/kg], IR – ingestion rate [kg/meal],

FI – fraction ingested from contaminated source – unitless, EF – exposure frequency [meals/y],

ED – exposure duration [y].

Similar formula is applied for calculations of the chemicals intake by ingestion of contaminated meat, eggs and dairy products.

Intake of chemicals by accidental ingestion of contaminated soil

I = Cs  IR  CF  FI  EF  ED/(BW  AT) (5) where:

C_s – concentration of chemical in soil [mg/kg], IR – ingestion rate [mg soil/day],

CF – conversion factor [10^–6 kg/mg], EF – exposure frequency [d/y].

Absorbed dose of chemicals by dermal contact with the contaminated soil I = Cs  CF  SA  AF  ABS  EF  ED/BW  AT (6) where:

SA – skin surface area contacting with the soil [cm²/event], AF – soil to skin adherence factor [mg/cm²],

ABS – absorption factor – unitless, EF – exposure frequency [events/y].

The above examples illustrate some of cases from among numerous possibilities reflecting variety of exposure scenarios.

When calculating the intakes particular concern should be devoted to considera-tions of the exposure duration and averaging time. The standard exposure duraconsidera-tions and averaging times are classified as chronic, subchronic and shorter-term exposures.

Applying the proper exposure and averaging time values has great weight for correct-ness of the whole risk assessment process. The averaging time which has to be chosen for exposure calculations depends on the type of pollutant – carcinogenic or non-carcinogenic – and on the type of toxic effect being assessed. Below the possible cases are given:

Toxic (non-carcinogenic) chemicals

In the case of non-carcinogenic substances the intakes are always determined by averaging them over the period of exposure, that is:

 developmental toxicants – when evaluation exposures to developmental toxi-cants is made, intakes are calculated by averaging over the exposure event, for instance over a day or a single exposure incident.

 acute toxic substances – for calculation of acute toxic substances intakes, the shortest exposure period that could produce an adverse effect is used for aver-aging. It is usually an exposure event or a day.

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 longer term exposure to non-carcinogenic toxicants (developmental and acute) – intakes are determined by averaging them over the period of exposure (i.e., subchronic or chronic daily intakes).

Generally the intake calculated in this way are called average daily dose (ADD).

Carcinogenic chemicals

For carcinogenic chemicals the intakes are calculated by proportional distribution of the total cumulative dose absorbed, over a lifetime. It is assumed for carcinogens that high doses of the chemicals absorbed by an individual over a short period of time is equivalent to corresponding low doses spread over a lifetime (EPA 1986b). It means that intakes are determined by averaging them over the lifetime, that is 70 years by convention (70 years  365 days/year = 25 550 days, actually). Such averaged intakes are called chronic daily intakes (CDI) or lifetime average daily doses (LADD).