It is clear from studies of personal exposure that human activity patterns are crucial in identifying and
determining human exposure to environmental pollutants. Activity pattern data, such as that in the NHAPS
data base, may be used to estimate the prevalence and duration of population exposure, especially for highrisk
groups, to many environmental pollutants (such as tobacco smoke). For example, we can make the
following general observations based on activity pattern data alone:
Americans spend 87% of their time indoors and 6% in an enclosed vehicle (on average)
The percentage of time spent indoors, outdoors, and in vehicles is fairly invariant across people in
different parts of the United States (on average)
Americans and Canadians spend similar amounts of time indoors, outdoors, and in vehicles (on average)
From sociological studies, it appears that the time Americans spend indoors has remained fairly uniform
over the past few decades.
44% of Americans spend time with a smoker each day (ca. 1992-94)
Of any location, Americans spend the largest percentage of time with a smoker in residences (43%,
calculated as an average across individual respondent percentages; ca. 1992-94)
The number of people spending time with smokers in California has decreased between the late 1980’s
and the mid-1990’s (when NHAPS was conducted)
When combined with measurements and/or models of pollutant emission, activity pattern data that possess
high time resolution can be used to provide estimates of actual population exposures caused by a
variety of different pollutant sources. These population exposure models make it possible to estimate, with
increased precision, the frequency distribution of exposure across a population, as well as the likely change
in the distribution when exposure to a particular pollutant source is modified (e.g., by a change in human
behavior).
In the future, investigators may want to consider a number of improvements upon the NHAPS survey
design. For example: (1) The NHAPS survey was limited to a single 24-hour period for each respondent and,
therefore, did not consider any day-to-day variation in the behavior of each respondent. To examine diurnal
cycles in human behavior, future studies should sample individuals on multiple days; (2) The NHAPS
results on the reported presence of a smoker may be biased.4 The diary question on the presence of a
smoker did not require all respondents to specify the portion of time that a smoker was actually present in
each microenvironment. For example, a smoker might have been present for only 10 minutes when the total
time spent in the microenvironment was an hour or more. In such a case, the reported time spent exposed to
a smoker would be one hour, a large positive bias. Future studies should collect more precise information
on the presence of smokers and/or other pollutant sources.