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

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AIR QUALITY GUIDELINES
personal ozone than might be the case for these other pollutants. Communitybased
single-monitor studies (i.e. with clustered study populations) are more affected
by these sources of error and noise than subject-based designs with individual
assignment of exposure, such as the UCB and ASHMOG studies.
The interaction of outdoor activity, ozone level and asthma observed in the
CHS study (and possibly in men in the AHSMOG study) also indicates that time
spent outdoors needs to be controlled in the exposure assessment. Given prevailing
lifestyles, with >90% of time spent indoors with generally low concentrations,
time spent outdoors (and in outdoor activity) becomes the most important determinant
of exposure to high ozone levels.
The issue of thresholds of no effect has yet to be addressed in studies of chronic
effects. Ozone is a natural constituent of the atmosphere and the lung is equipped
with oxidant defence mechanisms, and one may speculate that some levels of no
effect may exist. An early cross-sectional investigation with NHANES II data observed
inverse associations of ozone, nitrogen dioxide and total suspended particulates
with FVC and FEV1 among people 6–24 years of age (67). The pattern
in these associations with ozone would support speculation about thresholds of
no effect. The results were driven by data from Californian communities in the
upper range of the ozone concentration distribution.
Conclusion of chronic effect studies
Evidence for the chronic effects of ozone has become stronger. Animal data and
some autopsy studies indicate that chronic exposure to ozone induces significant
changes in airways at the level of the terminal and respiratory bronchioli. The reversibility
(or not) of such lesions is a point that deserves clarification. Epidemiological
evidence of chronic effects is less conclusive, owing mostly to an absence
of studies designed specifically to address this question and inherent limitations
in characterizing exposure. The studies with the most efficient approaches and
more individual assignment of exposure provide new evidence for chronic effects
of ozone on small airway function and possibly on asthma. Substantial uncertainties
remain, however, and need to be addressed in future investigations. The
partly inconsistent patterns or lack of associations may originate from limitations
in exposure assessment and/or from the inability to identify those most susceptible
to the chronic effects of ozone. Uncertainties should not be interpreted as
evidence of no adverse chronic effects following repeated daily and seasonal exposure
to ozone.
Evaluation of human health risks
The determination of human health risks associated with ozone exposure has to
take into account some basic principles.
• Data from human controlled exposures indicate that levels of ozone that may
be experienced in several areas of the world induce significant functional and