Technical Report 126

Background

A Tiered Approach

Exposure assessment is, by necessity, an iterative process. It has been recommended that assessing aggregate exposure should be a tiered approach (Delmaar JE and van Engelen JGM, 2006; Meek et al, 2011), where the lowest tier (0) consists of a rough sum (qualitative or semi-quantitative estimate) of exposure from each product, the mid-tier (1) tends to be a more quantitative estimate, such as a deterministic estimate with conservative assumptions, and the highest tiers (2 & 3) provide more realistic estimations of population exposure, modelled using probabilistic methods and person-orientated approaches, using more detailed exposure input data, such as population distributions or even raw data sets (usually at Tier 3). The rough or low tier estimates can be calculated quickly, often using default assumptions on inputs, yielding conservative exposure values, and if this approach is lower than the “safe” exposure threshold, then it may not be necessary to move to a higher tier. While exposure assessments at the highest tiers (2 & 3) will be data-intensive and often time consuming, they produce more refined and accurate estimates of population exposure, enabling the risk assessor to feel confident that the risk assessment is applicable to the population of interest.

The World Health Organization/International Programme on Chemical Safety (WHO/IPCS) held a workshop on Aggregate/Cumulative Risk Assessment in 2007, that led to development of an iterative framework that adopts the tiered approach for both exposure and hazard assessment (Meek et al, 2011). The exposure portion of this framework is shown in Figure 1. This framework is designed to aid risk assessors in identifying priorities for risk management.

Figure 1: Summary of the exposure component of the framework for tiered exposure and hazard assessment as proposed by Meek et al, 2011.

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Humans are exposed to chemicals that originate from many sources, including indirect exposures via contact with environmental media (e.g. air, water) and direct exposures via food that is consumed, and consumer products used (e.g. household products, electronics, construction materials, cosmetics, textiles). Assessments of human exposure to chemicals may be conducted for different reasons and with different objectives, e.g. to get an estimate of the maximal level of a chemical to which the general population can be safely exposed or to obtain a detailed insight into the distribution of exposure within sub populations. One of the challenges in (regulatory) risk assessment is the estimation of aggregate human exposure originating from a variety of exposure sources often associated with the use of different products and possibly also different exposure routes. Examples of situations where aggregate exposure might be important to consider include the exposure to a chemical that was both a fragrance in cosmetic and household cleaning products and a flavour ingredient consumed in food. Another possible example is the exposure to a phthalate plasticiser via food, building materials and toys.