In a general sense, exposure can be defined as any of a subject's attributes (association) or any agent (effect) with which the subject may come into contact. These attributes or agents may be relevant to his or her health (Armstrong et al., 1998).
This definition would include smoking, drinking, exposure through an occupation (farmers, pesticide applicators, etc.), or age (e.g., menopause >> endogenous estrogen levels) as exposures.
For an environmental factor, exposure can be more precisely defined as contact with some agent at the boundary between humans and the environment, at a specific concentration, over an interval of time (Wallace, 1995). Exposures can be harmful or beneficial.
Beneficial Factors - Vitamin D intake, Colonoscopy as a preventive measure for colon cancer, …
Exposure Assessment Section
- 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)
- Exposure intensity: the agent/risk factor concentration in the medium that is in contact with the body
- Exposure frequency: designates how often the exposure occurs
- Exposure duration: the length of the time that the exposure occurs
- Microenvironment: defined as any location or activity in which a distinct exposure occurs.
When investigating whether exposure is related to the risk of disease, the epidemiologist must consider many possibilities. Could the effect be related to the concentration? the frequency of exposure? duration? or is there something peculiar to a certain micro-environment?
Example: BaPBenzo[a]pvrene (BaP) in a hamburger that 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.
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 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 certain 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: Section
|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||the 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 Measurements Section
Data can be collected by directly monitoring an individual or indirectly, each method with advantages and disadvantages.
Monitor individuals using some measurement device on their person or by taking biological samples.
- Personal monitoring: personal exposure monitors for particulate matter (PM), patches worn under clothing for pesticide applicators
- Biological monitoring: lead concentrations in blood, biomarkers in urine, blood, breath, hair, nails
- Provides exposure values with minimal assumptions
- Assesses exposure and collects data at the individual level
- May not be practical in a large epidemiological study because of the expense and effort required.
- Heavily dependent upon ½ life of compound (how long it lasts in the body); may have affinity for certain tissues
- Can be affected by inter-individual differences in metabolism, inter-laboratory variation and intra-individual variability due to diurnal variation, diet, season, etc.
- If the chemical does not persist in the body, level will not reflect long-term exposure
- Job exposure matrix (JEM)
- Environmental monitoring/modeling - monitor the environment and assume that people who live in that environment are exposed at the level observed at the monitoring site
- Calculate the concentration of an agent in all locations/activities and multiply by the duration spent in each location/activity, (Examples: Indoor and outdoor exposure to ambient air particles, exposure to on the road)
- Practical and less expensive, the indirect method has been used extensively in epidemiological studies
- Often involves gathering data with a questionnaire.
- Potential problems of a questionnaire: low response rate, non-responsiveness to an individual question, recall bias (not being able to remember), social desirability (false answers), low question validity and reliability.
Environmental Biomarker Research Example Section
The US EPA is working to connect levels of environmental contamination to the risk of adverse effects in the public's health with the ultimate goal of reducing this risk. Check out the Chemical Safety for Sustainability Strategic Research Action Plan 2016-2019 to see how the EPA is working toward this goal. In particular, see Topic 2: Life Cycle Analytics, beginning on page 19.