Air Quality Guidelines Global Update 2005 - World Health ...

OZONE
The most thorough study is the Children’s Health Study carried out in multiple
cohorts in 12 communities in southern California (45). The cross-sectional
analyses indicated associations between lung function and annual means of daily
1-hour ozone maxima. An association with small airway function was particularly
pronounced (46). However, the findings were significant only among girls
and in boys spending more time outdoors (FVC –128.6 (SE = 56) per 40 ppb;
FEV1 –136.3 (SE = 51.3)). For the same cohorts studied prospectively, lung function
growth rates showed significant associations with a set of urban pollutants
(PM2.5, nitrogen dioxide and acid vapour), but findings for ozone were not significant
and were inconsistent across age groups and lung function parameters
(47–49). Growth rates in small airway function – primarily expected to be associated
with ozone – were inversely associated with ozone among the youngest
cohort only (48) but not in the eight-year follow-up from age 10 to 18 (49).
Several aspects are of interest. First, the study was limited to 12 communities
with only a two-fold range in ambient ozone levels. Second, within-community
variation in personal exposure (owing to differences in the use of air conditioners,
ventilation patterns and time spent indoors) could not be fully controlled,
and may be a source of noise or bias. Third, if chronic effects of ozone happened
primarily in early life, one may expect discrepancies between cross-sectional results
and those based on growth rates if the latter were observed after the susceptible
period. Thus, null findings in the growth rate analyses do not necessarily
contradict positive findings in the cross-sectional analysis.
Two studies carried out by the University of California at Berkeley (UCB)
chose a powerful cross-sectional design to maximize lifetime exposure to ozone
(50,51). Instead of selecting (a limited number of) communities, UCB freshmen
who had lived all their lives in California were invited to participate. The pilot
study (50) included 130 and the main study (51) 255 non-asthmatic students.
Ozone was interpolated on a monthly basis to each residential location over each
student’s lifetime. The integration of time–activity data into the exposure model
did not affect the results. Both studies observed consistent and significant crosssectional
associations between individual lifetime ozone exposure and, in particular,
small airway function, namely FEF25–75 and FEF75 (but also FEV1) at age
18–20. An increase of 2 μg/m 3 in lifetime 8-hour mean ozone was associated with
2.7% and 2.9% lower FEF75 in males and females, respectively (51). The main
study was large enough to investigate susceptible subgroups, and revealed that
significant effects occurred only among students with small airways (marked by
the ratio FEF25–75/FVC) (51). Effects were robust to adjustment for co-pollutants
(PM and nitrogen dioxide).
Galizia & Kinney (52) employed a similar design, with individual assignment
of long-term exposure to Yale (New Haven) College freshmen who had
geographically diverse residential histories. FEV1 and FEF25–75 were significantly
(and FEF75 borderline) associated with ozone exposure. FEF25–75 was 8.11%
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