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Intake fractions

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Scope

Purpose: 

The purpose of intake fraction is to provide a representation of the emissions-to-intake relationship.  This is a significant part of the risk assessment of chemicals.  Quantification of this relationship provides an indication of the potential impact of emissions on exposed populations and allows for the determination of the effect of source control on health outcomes.  Thus, an important tool for risk assessment is the derivation of a value that relates emissions to exposure in an efficient manner for both screening level assessments and policy comparisons. An appealing aspect of iF is that it allows for an estimate of population exposure to a substance for which no exposure data are available, even new substances, as long as certain basic characteristics are known.

Boundaries: 

Intake fraction is subject to the same uncertainties associated with any modelling assessment (e.g. parameter uncertainty, model specification uncertainty).  There are, therefore, several assumptions and limitations to be aware of when using intake fraction.

  1. There is an assumption that the relationship between emissions and concentration (and intake) is linear. Intake fraction has been less frequently applied for reactive or secondary pollutants.
  2. With an aggregate measure such as iF, one must be careful to include changes over time in the model.
  3. Difficulties arise in how to deal with multiple exposures - i.e. repeated intake of the same pollutant entity.  On the one hand, one needs to be careful not to double-count exposure:
  • long-lived substances, especially, may recycle though the environment and be available for multiple intake;
  • some substances (e.g. dioxins) may be  passed between mother and child.

On the other hand, such recycling is still a part of the impact of the emissions that perhaps should not be ignored.

» Read more Examining the distribution of individual iFs or the spatial distribution might provide more information on the variability in a population or geographical area of the intake relative to the source emissions and could be useful in examining equity issues. Other issues that should be considered when estimating iFs are the background level and the dose-response of the substance.  It is important when using measured values to subtract out background values, which are not due to the source of interest. Also, the concept of iF is most useful with linear dose-response curves. For exposures with threshold or non-linear functions iF may be poorly applicable for estimating population risks.

Method description

Input: 

Intake fraction requires two types of inputs, both of which can be derived from measurements, modelling, or a combination of the two.  Since iF is a fraction, one input is the numerator, which is the population or individual intake value, generally in units of mass.  The other input is the denominator, which is the emissions (in mass) from the source(s).  The units may also be in rates (e.g. mass/time).  The intake and emissions must be of the same units.

Output: 

The output comprises a dimensionless number that summarises, for every unit emission of a pollutant from a source or source type, the fraction that is taken in by the exposed population: e.g. for every tonne of benzene emitted from motor vehicles in a given city, 1 gram is inhaled by the exposed population in that city. 

Rationale: 

The intake fraction is based on the simple concept that only a proportion of the total mass of a pollutant emitted into the environment is actually taken up by humans as a result of exposure.  This proportion provides an indication of the 'efficiency' of exposure - or, in reverse, the effectiveness of dilution, breakdown and removal processes in the environment to avoid exposures.  It can thereby be used to provide useful information on the relative advantages or disadvantages of different policies for siting or dispersing emissions, or on policies that affect the spatial relationships between the population and emission sources. 

A further, appealing aspect of iF is that it allows for an estimate of population exposure to a substance for which no exposure data are available, even new substances, as long as certain basic characteristics are known. Intake fraction tends to be relatively consistent and comparable across exposure pathways and source categories.

For example, studies have found the following general ranges for pathways:

  • inhalation dominant: range 10-9 – 10-5
  • for primary PM2.5 typically 10-6 – 10-5
  • multipathway: range 10-7 – 10-5
  • ingestion dominant: range 10-6 – 10-4

Proximity of population to source and population density are also influencing factors, as the nearer the population to the source and the higher the density, the higher the intake fraction. For example, generally power plant iFs are lower than vehicular iFs for primary particulate matter.

Method: 

Intake fraction (iF) estimates how much of a unit emission of a pollutant is taken up by an exposed population. In other words, iF is the integrated incremental intake of a pollutant released from a source or source category (such as mobile sources, power plants, or refineries) summed over all exposed individuals during a given exposure time, per unit of emitted pollutant (Bennett et al. 2002a).

\[iF=\frac{ \sum_{people, time} mass \: intake \: of \: pollutant \: by \: an \: individual}{mass \: release \: into \: the \: environment}\]

(1)

Practically speaking, this is usually quantified as

\[iF=\frac{Concentration_{Source} \ast Population \ast Intake \: Rate}{Emissions}\]

(2)

An intake fraction of 10-6 indicates that for every 1 kg of pollutant released into the environment 1 μg of the pollution will be taken up by the exposed population (Bennett et al. 2002a).

Three intake routes are included in the concept: intake through ingestion or inhalation, and dermal uptake. The different routes are related to the total intake fraction according to the following relationship for all exposure pathways (Bennett et al. 2000b).

iF(total) = iF(inhalation) + iF(ingestion) + iF(dermal) [3]

Bennett and co-workers suggest the following notation: iF(route, media, subpopulation), where:

  • route refers to ingestion, inhalation, dermal uptake or total;
  • media refers to release to air, water and soil; and
  • subpopulation refers to exposed group (e. g. workers, residents or all exposed).

While population iF is useful in determining large-scale impacts of a pollutant, the evaluation of the distribution of individual intake fractions throughout a population space can also provide useful information. The total intake fraction can then be calculated as the sum of all of the individual intake fractions (iFi) for an exposed population (Bennett et al. 2002a). 

As exposures to pollution are rarely evenly distributed, the effectiveness of control policies should account for the factors that lead to particularly high exposures. The distribution of individual intake fractions across time and space and various activities and microenvironments can provide such information. Generally iF should be applied for situations with a fairly long time frame. Calculations can be made for short periods also, but these are less useful, at least for screening level purposes. Most commonly, iF is calculated as an annual average or for a lifetime.

The numerator of iF requires an estimation of population or individual intake, derived from multiplying the media concentration or exposure of a pollutant (e.g. benzene in ambient or microenvironment air) by the appropriate intake rate (e.g. breathing rate).  The denominator requires some estimation of total emissions over a specified time period or emission rate.  This may be derived from inventories or emissions models. Both measured or modelled values may be used in the numerator and denominator; however, one must be careful to note that measured values for the exposure concentration may include contributions from sources other than the source under investigation (e.g. the benzene concentration in urban air includes several source types, such as vehicles, industry, long range transport).  Modelled values (e.g. from dispersion models) are more able to specify only the source contribution to exposure.

Further details on applying the intake fraction methodology are given in the document on Exposure-intake models (see link below).