Page 65 - Fundamentals of Air Pollution 3E

I. Averaging Time 41

the pollution level of the indoor air might be higher than that of the outside
air. However, if none of these sources are inside the building, the pollution
level inside would be expected to be lower than the ambient concentration
outside because of the ability of the surfaces inside the building—walls,
floors, ceilings, furniture, and fixtures—to adsorb or react with gaseous
pollutants and attract and retain particulate pollutants, thereby partially
removing them from the air breathed by occupants of the building. This
adsorption and retention would occur even if doors and windows were
open, but the difference between outdoor and indoor concentrations would
be even greater if they were closed, in which case air could enter the
building only by infiltration through cracks and walls.
Many materials used and dusts generated in buildings and other enclosed
spaces are allergenic to their occupants. Occupants who do not smoke are
exposed to tobacco and its associated gaseous and particulate emissions
from those who do. This occurs to a much greater extent indoors than in
the outdoor air. Many ordinances have been established to limit or prohibit
smoking in public and work places. Attempts have been made to protect
occupants of schoolrooms from infections and communicable diseases by
using ultraviolet light or chemicals to disinfect the air. These attempts have
been unsuccessful because disease transmission occurs instead outdoors
and in unprotected rooms. There is, of course, a well-established technology
for maintaining sterility in hospital operating rooms and for manufacturing
operations in pharmaceutical and similar plants.

I. AVERAGING TIME
The variability inherent in the transport and diffusion process, the time
variability of source strengths, and the scavenging and conversion mecha-
nisms in the atmosphere, which cause pollutants to have an effective half-
life, result in variability in the concentration of a pollutant arriving at a
receptor. Thus, a continuous record of the concentration of a pollutant at
a receptor, as measured by an instrument with rapid response, might look
like Fig. 4-1 (a). If, however, instead of measuring with a rapid-response
instrument, the measurement at the receptor site was made with sampling
and analytical procedures that integrated the concentration arriving at the
receptor over various time periods, e.g., 15 minutes, 1 hour, or 6 hours,
the resulting information would look variously like Figs. 4-l(b), (c), and
(d), respectively. It should be noted that from the information in Fig. 4-
l(a), it is possible to derive mathematically the information in Figs. 4-1 (b),
(c), and (d), and it is possible to derive the information in Figs. 4-l(c) and
(d) from that in Fig. 4-l(b). The converse is not true. With only the informa-
tion from Fig. 4-l(d) available, Figs. 4-l(a), (b), and (c) could never be
constructed, nor could Figs. 4-l(a) and (b) be constructed from Fig. 4-l(c),