3.2 - Exposure Assessment

Exposure Assessment
the science that describes how an individual or population comes in contact with a risk factor, including quantification of the amount of the risk factor across space and time (Lioy, 1990)
  1. Exposure intensity: the agent/risk factor concentration in the medium that is in contact with the body
  2. Exposure frequency: designates how often the exposure occurs
  3. Exposure duration: the length of the time that the exposure occurs
  4. Microenvironment: defined as any location or activity in which a distinct exposure occurs.

When investigating whether an exposure is related to the risk of disease, the epidemiologist must consider many possibilites. Could the effect be related to the concentration? the frequency of exposure? duration? or is there something peculiar to a certain micro-environment?

Example 3-1
Benzo[a]pvrene (BaP) in a hamburger which was cooked in a flame broiling process is a carcinogen. To assess exposure, you may ask: How burned is the burger? How frequently does the person eat burned hamburgers? How long have they been eating burned hamburgers? Is there an additional micro-environmental change that occurs within BaP, the molecule carrying the carcinogen? All these factors could be related to the ability of the exposure to cause a particular outcome.

Environmental tobacco smoke, or second-hand smoke is another example. What questions might you ask?

(Consider: Are children exposed to environmental tobacco smoke in their home environment? How many hours are smokers smoking in the house? How many smokers? How frequently is the child in the house? How long have smokers lived in the house?)

Here's another example.

Example 3-2

Look at Tables 1 and 2 in Paskett et al 2007. What types of tobacco exposure are considered in this evaluation of the relationship of smoking with colorectal cancer? What is the effect of current smoking on the incidence of colorectal cancer?

From Table 1:

Smoking Status (Never, Past, Current)
Age at Smoking Initiation (Never, <20, ≥ 20)
Cigarettes per Day (Never, <25, ≥ 25)
Duration (Never, <20, 20-29, 30-39, ≥ 40)
Passive smoking status (Never, Ever)

From Table 2:

We can see that current smoking increased the risk of invasive colon cancer only 1.03 times (95% confidence interval for HR, 0.77 to 1.38) but that current smoking increased the risk of invasive rectal cancer 1.95 times (95% confidence interval, 1.10 to 3.47).

Other factors may result in differential effects of an exposure. For example, serious adverse health outcomes may be limited to individuals in a specific setting, region, worksite or community. Individual susceptibilities may differ. There may be differences in exposure due to the type of agent (chemical, biological or physical), the medium through which the exposure occurs (air, water, food) or the route of exposure (inhaled, ingested or dermal). In considering a potential association of BaP in grilled burgers with a particular cancer, we might ask how much carcinogen is in the charred meat? how much is in the smoke arising from the meat drippings? etc.

Potentially important aspects of environmental exposure include:

Agent(s) biological, chemical, physical, single agent, multiple agents, mixtures
Source(s) anthropogenic/non-anthropogenic, area/point, stationary/mobile, indoor/outdoor
Transport/carrier medium air, water, soil, dust, food, product/item
Exposure pathway(s) eating contaminated food, breathing contaminated workplace air touching residential surface
Exposure concentration mg/kg (food), mg/litre (water), μg/m3 (air), μg/cm2 contaminated surface), % by weight, fibres/m3 (air)
Exposure route(s) inhalation, dermal contact, ingestion, multiple routes
Exposure duration seconds, minutes, hours, days, weeks, months, years, lifetime
Exposure frequency continuous, intermittent, cyclic, random, rare
Exposure setting(s) occupational/non-occupational, residential/non-residential, indoors/outdoors
Exposed population general population, population subgroups, individuals
Geographic scope site/source specific, local, regional, national, international, global
Time frame past, present, future, trends

As you can see, exposure assessment requires considering many factors that can affect exposure. Let's look further at some of these factors.

Exposure Source Section

What is the point or area of origin for an environmental agent? Sources are classified as primary or secondary .

Primary sources

  • point sources (e.g., incinerator)
  • area sources (e.g., urban runoff),
  • stationary sources (e.g., refinery) versus
  • mobile sources (e.g., automobile) and
  • anthropogenic sources (e.g., landfill)
  • non-anthropogenic sources (e.g., natural vegetation)

Secondary sources / processes

  • condensation of vapors into particles
  • chemical reactions of precursors producing new agents or pollutants

Intake versus Uptake Section

This is an important distinction to make.

Intake involves:

  • Ingestion and inhalation exposure to an agent
  • Intake rate
    • The amount of the agent inhaled or ingested per unit time
  • Intake= exposure concentration * the rate of intake

Uptake

  • Associated with the crossing an absorption barrier (i.e. skin, respiratory tract, gastrointestinal tract)
  • Occurs following intake by inhalation or ingestion.
  • Uptake rate is a function of the exposure concentration, carrier molecules, as well as of the permeability and surface area of the exposed barrier.

Figure 2 (below) illustrates the issue of intake versus uptake. With intake the individual receives a potential dose . If however, the person can excrete the dose, it may not actually concentrate in the body.

diagram

Environmental exposure doesn't always result in adverse effects. The epidemiologist must sort out those exposures which can result in adverse effects from those which do not.

Five Steps in Exposure Assessment Section

  1. Define and characterize the potentially exposed population
  2. Establish a biological/sociological theory to help guide the exposure assessment
  3. Collect the qualitative and/or quantitative data on the population, including the spatial and temporal characteristics
  4. Assess the microenvironment and medium of interest in terms of exposure
  5. Estimate the exposure to the primary agent and other risk factors for the study population

Modeling may be useful to estimate exposure based on the data collected as depicted in Figure 3 below:

assessment flow