Lead Human Exposure and Health Risk Assessments for Selected ...

1 composed of U.S. Census blocks or block groups, across which media concentrations differ. 11
2 Media concentration estimates (e.g., for outdoor air, soil and indoor dust) are developed for each
3 block or block group, and from these a central tendency estimate is developed of concurrent and
4 lifetime average blood Pb levels for the resident children. Inter-individual variability of blood
5 Pb levels for children within a block or block group is considered through the use of a
6 statistically derived GSD. The specific spatial templates used for each of the point source case
7 studies are presented in Appendices D (Section D.1) and E (Section E.1).
8 2.4.3 Categorization of Policy-relevant Exposure Pathways
9 To inform policy aspects of the Pb NAAQS review, we have attempted to parse the
10 assessment estimates for indoor dust Pb, blood Pb and IQ loss into the fraction associated with
11 policy-relevant background (e.g., diet and drinking water) versus that associated with policy­
12 relevant pathways, which include inhalation, outdoor soil/dust ingestion and indoor dust
13 ingestion (Section 2.1.1). We have further categorized the policy-relevant pathways into one of
14 two categories, “recent air” or “past air”. Conceptually, the recent air category includes those
15 pathways involving Pb that is or has recently been in the air, whether or not it was also in the air
16 in the past, and the past air category includes those pathways involving Pb that was in the air in
17 the past and was not in the air recently.
18 Recent air refers to exposure contributions associated with inhalation of ambient air Pb
19 and ingestion of the fraction of indoor dust Pb derived from recent ambient air Pb. To the extent
20 that ambient air Pb includes contributions from resuspension of previously deposited Pb, that
21 source is represented in the recent air category. Thus, a “recent air” exposure may involve
22 previously deposited Pb that is (1) resuspended into the air and inhaled or (2) resuspended into
23 the air, transported into a building, deposited into the dust, contacted and ingested.
24 Past air includes exposure contributions from ingestion of outdoor soil/dust that is
25 contacted on surfaces outdoors, and ingestion of indoor dust Pb that is derived from past air
26 sources. Although Pb that is currently in outdoor soil/dust may have been in the air recently or
27 some time ago, we have assigned ingestion of outdoor soil/dust Pb contacted outdoors to the past
28 air category in recognition of our inability to maintain a dynamically changing categorization of
11 US Census block groups vary in size from several city blocks in densely populated urban areas to many
square miles in less populated rural areas. Their population count varies from 600 to 3000 people per block group
with the typical block group in the U.S. containing 1,500 people. US Census blocks are more refined than block
groups and typically contain several hundred people or less. Their size can vary from a single city block in urban
areas to multiple square miles in less populated rural locations.
July 2007 2-26 Draft – Do Not Quote or Cite