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EPA-Expo-Box (A Toolbox for Exposure Assessors)

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Lifestages

Overview

Lifestages

Lifestages are "temporal stages of life that have distinct anatomical, physiological, and behavioral or functional characteristics that contribute to potential differences in vulnerability to environmental exposures" (U.S. EPA, 2006). An individual’s lifestage might affect his or her susceptibility to chemicals or pollutants. Examples of lifestages in childhood are conception through fetal development, infancy, toddlerhood, and adolescence. Other lifestages that may be important to consider when assessing human exposure and risk include pregnancy, lactation, and old age.

Members of the population at certain lifestages (e.g., infants, children, pregnant women, older adults) may experience different effects for a given dose of a contaminant than individuals in the general population. Infants also have unique potential exposure as some may be exclusively breastfed up to the first year of life; different chemicals accumulate more or less effectively in human breast milk.

Physiological and behavioral differences between children (including infants) and adults may result in differential exposures. For example, children can be exposed to some chemicals at higher rates or higher concentrations than adults because they consume more of some kinds of foods and water per unit of body weight, have a higher ratio of body surface area to volume, and have different activity patterns. Important behavioral differences can also lead to greater exposures in children. Children may have more opportunities for incidental ingestion of chemicals via hand-to-mouth and object-to-mouth activities. See Chapter 4 of the Exposure Factors Handbook: 2011 Edition (U.S. EPA, 2011) for non-dietary ingestion factors that may be relevant to children. Data on soil and dust ingestion rates are available in Chapter 5 of the Handbook.

Childhood Age Groups

The term "lifestage" is a distinguishable timeframe in an individual’s life characterized by unique and relatively stable behavioral and/or physiological characteristics that are associated with development and growth. Thus, childhood should be viewed as a sequence of lifestages, from conception through fetal development, infancy, and adolescence. EPA (2005) guidance recommends the use of the following childhood age groups:

  • Age groups less than 12 months old include: birth to <1 month, 1 to <3 months, 3 to <6 months, and 6 to <12 months.
  • Age groups greater than 12 months old include: 1 to <2 years, 2 to <3 years, 3 to <6 years, 6 to <11 years, 11 to <16 years, and 16 to <21 years.

(http://yosemite.epa.gov/ochp/ochpweb.nsf/
content/lifestage.htm
)

Additionally, infants and children spend more time on the floor (indoors) or on the ground (outdoors) and explore surfaces and objects such as toys more frequently than adults. Therefore, children are more likely to have increased oral and dermal exposures at those locations or during those activities. For these reasons, children’s behavior and physiology can increase their exposure to environmental contaminants. In addition, for infants and children, defense mechanisms against microbial and chemical contaminants may not be fully developed. It may therefore be important to consider children separately in exposure assessments using the available child-specific exposure factor values. Just as child and adult exposures vary based on behavior and physiology, exposures among children in different age groups also vary. Rapid changes in behavior and physiology can lead to differences in exposure as a child ages. Exposures among infants, toddlers, adolescents, and teenagers can vary widely. To this end, EPA's Office of Children's Health Protection (OCHP) has provided guidance on selecting age groups when assessing childhood exposures to environmental contaminants based on the current understanding of when developmental changes occur that could affect exposure (see text box). Additional information and links related to childhood activity patterns and health hazards are available from the OCHP Home page.

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During pregnancy, changes in the endocrine system and metabolism may influence the body’s response to a toxic substance. Fetuses may be at increased risk if exposure of the pregnant woman to a toxin occurs during critical developmental stages.

Older adults may be more affected by exposures to some types of environmental agents because of physiological differences associated with age (e.g., organ systems become less effective at detoxification). As humans age, the body’s ability to defend against diseases and to heal injuries diminishes. Older adults may be less able to mount an effective defense against contaminants because of a weakened immune system or pre-existing disease (U.S. EPA, 2000).

There are a number of EPA resources available targeted to older adults. EPA has developed a research framework to address and prioritize the exposure and potential adverse health effects concerns for older adults as a susceptible subpopulation (Geller and Zenick, 2005). EPA’s Aging Initiative is an effort to prioritize and study environmental health hazards to older persons as well as to examine the effect that a rapidly growing aging population might have on our environment. In 2007, EPA sponsored a workshop for the development of exposure factors for older adults and prepared a summary report of discussions held at that workshop (U.S. EPA, 2007).

EPA’s Human Exposure and Atmospheric Sciences Division (HEASD) conducts research to better understand risks to various lifestages. Primary research areas include the aging—to better understand aging-related changes in activity, exposure, and pharmacokinetic factors—and children—to understand the exposure, susceptibility, vulnerability, and differential risks of children to pesticides and other chemicals in everyday environments and better characterize activities and exposure factors that contribute to children’s aggregate and cumulative exposures.

The table below provides examples of important considerations specific to assessing exposure to different lifestages; the data supporting these considerations can be found in the tools list.

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Infants and Children
Potential Pathways of Exposure/Other Exposure Considerations Scenario/Setting Considerations Exposure Factors of Interest
For example
  • Ingestion of pesticides in food
  • Dermal exposure to soil
  • Ingestion of soil
  • Ingestion of breast milk by nursing infants
  • Mutagens
  • Endocrine disruptors
For example
  • Schools/daycares
  • Playgrounds
  • Timeframe
For example
  • Activities
  • Time use
  • Mouthing behavior
  • Physiology

Women of Childbearing Age (Including Pregnant and Lactating Women)
Potential Pathways of Exposure/Other Exposure Considerations Scenario/Setting Considerations Exposure Factors of Interest
For example
  • Fish consumption and PCBs
  • Teratogens
  • PBTs
  • Mercury
  • Lead
For example
  • Breastfeeding mothers
  • Occupation
  • Timeframe
For example
  • Fish ingestion rate
  • Percentage of mothers breastfeeding
  • Body weight
  • SES

Older Adults
Potential Pathways of Exposure/Other Exposure Considerations Scenario/Setting Considerations Exposure Factors of Interest
For example
  • Inhalation (particulate pollution)
  • Dermal exposure to pesticides
  • Drinking water
  • Drug interactions
  • Health/disease status
For example
  • Type of residence (e.g., home, hospital, assisted living)
  • Timeframe/cumulative exposure
For example
  • Physiological changes
  • Breathing rate reserve decreases with age
  • Skin thinning
  • Percentage of body fat
  • Activity types
  • Function/activity level
  • Time use
  • SES

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There are a number of tools that can be used to assess differences in exposures for susceptible lifestages (see below).

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