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Limitations in the 1996 National-Scale Air Toxics Assessment:Variability

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.

The national-scale assessment results show how air concentrations, exposures and risks vary broadly across geographic regions of the country. They do not fully characterize how these three quantities vary between individuals, except to the extent these individuals live in different geographic regions and have the values typical of a census tract in that region. They also do not characterize how emissions and ambient air concentrations vary geographically or in time within a census tract.

To understand this difference, consider the reasons why two places might have different ambient air concentrations, or why two individuals might experience different risks from air toxics:

  • Temporal variation. Sources do not emit air toxic compounds at constant rates. These rates vary in time. Similarly, the meteorological conditions that affect dispersion in the atmosphere vary in time. This means the ambient air concentration at a given location can vary in time.

  • Geographic variation. Different locations are at different distances from a source, and may be in the vicinity of different sources. They also vary with respect to meteorological conditions that affect dispersion in the atmosphere. As a result, the ambient air concentration may vary between different locations.

  • Variation in locations of individuals. Two individuals might live at different locations, perhaps in two different states. Due to geographic variation discussed above, the ambient air concentration might be different for these two individuals.

  • Variation in activity for individuals. Two individuals might live at the same location but engage in different activities (called an "activity pattern") during each day. The concentration indoors often is different from the concentration outdoors. If one person spends more time indoors than another person, there will be a difference in the average air concentration to which the two are exposed, even though the ambient air concentration is the same. Similarly, one person might spend more time in a car, exposed to an air concentration that is typical near roads. The net effect of these factors will be that the concentration of each air toxic in the air actually breathed by these two individuals will be different. The exposure differs for these two individuals.

  • Variation in susceptibility or sensitivity. Two individuals might live at the same location and engage in the same activities, but one person might be more susceptible or sensitive than another. Susceptibility or sensitivity refers to the extent to which an individual (1) takes an air toxic into the body, (2) transports it into an organ or tissue in which a health effect might occur, and/or (3) develops the effect. The more susceptible or sensitive individual will have the highest concentration of the air toxic in their organs or tissues, or the highest chance of severity of a health effect, even when the exposures are the same for both individuals. For example, people breathe at different rates; two individuals placed into exactly the same air may bring different amounts of the air toxic into their bodies. The amount of an air toxic reaching an organ or tissue can also vary between individuals, even if they both bring the same amount into their lungs. The length of time the air toxic remains in the body may differ. And the innate sensitivity to the effect may vary even at equal doses in the tissues. The net effect of these factors is that either the dose of the air toxic delivered to the organs or tissues of the body, or the level or response, or both, will differ between these two individuals, even though the individuals are exposed to exactly the same air.

Each of the above factors can depend on the age, gender, and ethnic group to which an individual belongs. These groups make up different receptor populations or cohorts, and the exposures and risks must be calculated separately for the different groups. There even are variations of all factors within a receptor population. Two Hispanic males, each aged 12 years, can have differences in ambient air concentration, exposure, susceptibility and/or sensitivity.

Which components of variability did the national-scale assessment include?
How was the variability analysis conducted?
What are the results of the variability analysis
How can these results be interpreted?

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