Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
* Personal exposures were higher than would be predicted by
measurements at fixed monitoring stations. About 10% of DC
residents appeared to exceed the 8-h standard of 9 µg/g, as
determined by their breath concentrations, although only 1 of the
11 fixed stations exceeded the standard during the monitoring
period.
A study of California homes (Wilson et al., 1993a,b, 1995; Colome
et al., 1994), each monitored for 48 h, indicated that 13 of 277 homes
(about 5%) had indoor 8-h averages exceeding 9 µg/g (the outdoor
standard). Since the outdoor standard is to be exceeded only once per
year, it is clear that the fraction of homes with 8-h indoor averages
exceeding 9 µg/g more than once per year would be larger than the 5%
observed in the single 48-h monitoring period. Homes with gas stoves
and gas furnaces had indoor source levels for carbon monoxide that
were about 3 times higher than homes without such sources. Homes with
wall furnaces had higher levels of carbon monoxide than homes with
forced-air gas furnaces. Homes with smokers ( n = 85) had levels of
carbon monoxide about 0.5 µg/g higher than homes without smokers
( n = 190). Malfunctioning gas furnaces were a major cause of
elevated concentrations of carbon monoxide. However, the homes with
the highest carbon monoxide levels also included some with electric
cooking stoves and electric heat, suggesting that other sources of
carbon monoxide were present in these homes. Such sources could
include cars idling in attached garages or unvented gas or kerosene
space heaters.
12.3.3 Nitrogen dioxide
Nitrogen dioxide is a ubiquitous respiratory irritant for which
air quality standards have been established in many countries (WHO,
1997d). It is emitted by industrial processes and mobile sources, but
also by indoor combustion appliances such as gas cooking stoves and
furnaces. Several studies in the 1970s suggested that children in
homes with gas stoves suffered more infectious disease than children
in homes with electric stoves; a possible connection with nitrogen
dioxide (in lowering resistance) was postulated (Samet & Spengler,
1991). Also, exposure is likely to be higher for those living closer
to roadways.
A study in Helsinki, Finland, explored weekly nitrogen dioxide
exposure of preschool children as well as between- and within-children
variances of repeated personal exposure measurements. The study tested
the hypothesis that exposure to the low levels of nitrogen dioxide in
Helsinki increases the risk of respiratory symptoms in preschool
children (Mukala et al., 1996).
The parents of 246 children, aged 3-6 years, returned a letter of
consent to participate in a personal nitrogen dioxide exposure study.
The children spent their days at one of three daycare centres, two
located in the downtown area and one in a suburban area. All children
carried personal Palmes tubes on outdoor clothing one week at a time
during six consecutive weeks in winter (14 January-4 March 1991) and
seven consecutive weeks in spring (8 April-27 May 1991). Weekly
concentrations of nitrogen dioxide were also measured inside and
outside each daycare centre to assess the usefulness of the stationary
measurements in estimating the variation of exposures. Ambient
concentrations of nitrogen dioxide were monitored at three fixed sites
of the Helsinki Metropolitan Area council network with
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