Exposure-to-intake modelling in IEHIAS

The text on this page is taken from an equivalent page of the IEHIAS-project.

Intake can be considered either the next step in the source-impact chain after exposure, or as a substitute for exposure. The latter is particularly appropriate for the ingestion and dermal absorption pathways. For dermal absorption and inhalation intake, contact time with the contaminated media should be included. A general representation of intake is as follows:

Intake = source emission x dilution x temporal decay x removal x medium transfer x inhalation/ingestion

Ideally, intake rates and data should be specific to the population being assessed, but in reality data are likely not to be available. In such cases, closest available data should be used: for example, from the same country or region and (sub)population. Useful sources of data in this context are the exposure factor databases that have been developed to describe the social/personal factors that condition exposures and intakes of different population groups. Amongst these, the most comprehensieva dnwidely used are probably the EXPOFACTS database, which includes data from various European countries, and the U.S. Environmental Protection Agency’s (US EPA’s) Exposure Factors Handbook. The ExpoPlatform contains more information on sources of exposure factor data.

For non-cancer risk assessment, intake for risk assessment can be compared to a tolerable intake, which is defined as the "estimated maximum amount of an agent, expressed on a body mass basis, to which each individual in a (sub)population may be exposed to over a specified period without appreciable risk" (IPCS 2004). For chronic effects, the Acceptable Daily Intake (ADI), Tolerable Daily Intake (TDI), and Reference Dose (RD) similarly refer to a daily intake amount for which an individual might be exposed over a lifetime without any noticeable increase in health risk. Intake for a particular exposure scenario may be calculated and compared to the ADI to determine whether exposure is above this threshold level. For cancer risk assessment, risk can be assessed by multiplying the cancer potency factor (mg/kg per day) by the intake rate: here, the averaging time would be a lifetime. For exposures over shorter time periods of interest, the averaging time is adjusted to the appropriate time frame. Thus, the overall intake of an agent can be defined as:

AveragageDailyIntake = C x IR x ED / BW x AT

where C = concentration in media of interest; IR = the intake rate; ED = exposure duration; BW = body weight; and AT = the averaging time of interest.

Generally, intake should be in units that match the dose-response function (e.g. mg/kg per day) to provide a means of estimating health risk.

Inhalation pathway

We can define inhalation intake as the mass of an agent that passes into the respiratory tract, beginning at the nose and mouth (Ott 2007). Intake is therefore the exposure concentration (mass/volume) multiplied by the volumetric inhalation rate (volume/time).

When a person breathes, air passes through several passages through which absorption through the epithelial layers of the respiratory tract is possible. The amount absorbed through this surface is the absorbed dose. Health effects can result from the absorbed dose or from irritation through contact of the pollutant with the boundary layer of the respiratory tract. Gaseous pollutants can reach to the smallest branches of the airways, and can diffuse into the bloodstream. Particles are subject to settling and inertial forces and thus particles deposit in the airways differentially according to size. Sequentially, the first area of the respiratory system to be crossed is the oronasal (nose and mouth), then the pharyngeal region. Coarse particles (~ >10 μm) will generally deposit in the naso-pharyngeal region while finer particles deposit in the pulmonary regions and ultrafine particles (<0.5 μm) can also diffuse through the epithelial lining of the respiratory tract. Breathing rate and tidal volume also have an effect on particle deposition.

Breathing rates are in units of volume per time (e.g. m³/day). Exposure factor sources, such as the U.S. EPA’s Exposure Factors Handbook, report options for average breathing rate per day, stratified by gender and age, as well as activity-specific breathing rates, generally as an average per hour. These were derived from a review of the literature. These rates, however, have been acknowledged as overestimates of population breathing rates, and an approach developed by Layton (1993) - which provides methods for assessing breathing rates using metabolic data - are often used to derive rates. While breathing rates are usually used as point estimates, there is a move towards incorporating population variability by generating distributions of breathing rates. The U.S. EPA (1997, 2000) has provided some guidance towards developing such distributions.

Ingestion pathway

The ingestion pathway includes several media, such as plant and animal food substances, water, breast milk, and non-food items (e,g, dust or soil and objects). The last of these is particularly relevant for children, who have a tendency to crawl on the ground and put various objects into their mouths (pica activity). Intake rates for these media are addressed in various databases or handbooks (e.g. EXPOFACTS and the U.S. EPA Exposure Factors Handbook) as well as studies throughout the literature. The relevant ingestion pathways depend on the agent and population of concern. Regulatory, market, cultural, and socio-economic contexts may all have a direct impact on the ingestion pathway, affecting the types of food that are available or consumed and cooking practices, as well as rates of ingestion.

Food or dietary ingestion presents several issues for consideration in intake assessment. One is how to categorise the composition of the diet (e.g. fruits, vegetables, meat, dairy, etc.) and the frequency of eating these foods. Another issue is how to quantify the amount that is consumed: e.g. 'as consumed' versus 'dry weight', which refer respectively to the weight of the food consumed either as it is eaten, or after the moisture weight has been removed. The cooking process may either remove or increase the concentration of an agent in a food item, so the consumption and method of cooking food may be important for some agents.

Food intake data usually come from diaries kept by study subjects, 24-hour recall, or food frequency questionnaires. These tend to provide only short-term consumption data.

Dermal pathway

Exposure through dermal pathway is the most challenging of the three intake routes. There is, for a start, a question as to whether dermal contact should be considered a legitimate component of intake, since the IPCS definition excludes passage through absorption barriers (which include the skin). Complexities also arise because of:

• the wide range of activities that lead to dermal contact, including household works, personal hygiene, hobbies, and every day features of living (e.g. wearing clothes);
• the great variety of materials that may come into contact with the skin (e.g. cosmetics, clothes, cleaning products), and
• the different forms in which they may occur (e.g. as liquids, aerosols, gases, solids).

Properties of both the agent and of the exposed skin have a major effect on dermal intake. In order to penetrate the skin, the agent needs to be able to penetrate the skin - i.e. it needs appropriate partition capabilities. The condition of the exposed skin is important in this context, injuries to the skin potentially increasing the level of intake.

Several factors therefore need to be specified in estimating dermal intake. These include:

1. the concentration of the agent in contact with the skin;
2. the extent of the exposed skin surface;
3. the duration of exposure;
4. the ease of absorption of the agent through the skin - i.e. the permeability or absorption coefficient,;
5. the frequency of the exposure event.

With further information it is possible to calculate the biologically effective dose (i.e. the amount of the agent delivered to a target organ). This, likewsie, depends on the chemical properties of thr agent, and varies according to the metabolism of the exposed individual.

On this basis, the average daily dose (ADD) through dermal intake can be calculated as follows (EPA 1997):

ADD = DA_event x EV x ED x EF x SA / BW x AT

where:

DA_event = absorbed dose per event (mg/cm²);

EV = event frequency (events/day);

ED = exposure duration (years);

EF = exposure frequency (days/year);

SA = skin surface area available for contact (cm²);

BW = body weight (kg); and

AT = averaging time (days) for noncarcinogenic effects, AT = ED; and for carcinogenic effects, AT = 70 years or 25,550 days.

Links to a number of intake models are provided below.

References

• IPCS/OECD 2004 IPCS Risk Assessment Terminology Part 1: IPCS/OECD Decsriptions os selected key generic terms used in chemical hazard/risk assessment. IPCS Harmonization Project. Geneva: World Health Organization.
• Ott, W.R., Steinemann, A.C. and Wallace, L.A. (eds.) 2007 Exposure analysis. Boca Raton, Florida: Taylor and Francis.
• Layton, D.W. 1993 Metabolically consistent breathing rates for use in dose assessments. Health Physics 64(1), 23-36.
• U.S. Environmental Protection Agency 1997 Exposure factors handbook. Washington, DC: National Center for Environmental Assessment, Office of Research and Development.
• http://www.epa.gov/ncea/pdfs/paramprob4ef/chap1.pdf U.S. Environmental Protection Agency 2000 Options for development of parametric probability distributions for exposure factors. Washington, DC: National Center for Environmental Assessment, Office of Research and Development.]

See also

• Exposure, intake and dose models
• Caltox, risk assessment model
• Fate Assessment Screening Tool Version 2.0 (E-FAST V2.0)
• Dietary Exposure Potential Model (DEPM) from the U.S. EPA [7]