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

PARTICULATE MATTER
range of concentrations (some well above regulatory limits); and that the ambient
PM to which people are exposed is a diverse mixture. There are numerous
sources of ambient PM, comprising both particles released directly from combustion
sources and those formed secondarily through complex chemical reactions.
The complex characteristics of ambient PM have challenged researchers as
they have attempted to characterize PM sources and exposures.
As this chapter discusses, major advances have been made in understanding
the relationship between actual human exposure and ambient concentration
measurements. In developed countries, ambient particles are responsible for
approximately 50% of an individual’s total PM2.5 exposure, with relatively little
variability between different population subgroups (although with substantial
within-group variability). In a developing country scenario with indoor biomass
combustion and consequently much higher indoor exposures, the contribution
of ambient particles, while still substantial, will be smaller. In the absence of indoor
sources, indoor concentrations and consequently exposure to PM of ambient
origin are largely determined by the air exchange characteristics of the indoor
environment. On average, indoor concentrations of PM2.5 of ambient origin are
40–70% of ambient PM2.5 concentrations. With this improved understanding of
the relationships between exposure and ambient concentrations, the extensive
database on ambient concentrations can be reliably used for risk evaluation.
Health risk evaluation
The 2000 review (1) found evidence sufficient to link PM to a variety of adverse
effects on mortality and morbidity, in both the short and the long term. It offered
quantitative estimates of risks for selected outcomes, based on the epidemiological
information. Subsequent evidence reaffirms the multiple associations of PM
with adverse health effects and broadens the range of effects to more conclusively
encompass cardiovascular outcomes. There is substantial new evidence from
time series studies of daily mortality, particularly from multi-city studies that
span Europe and North America. Methodological concerns about time series
studies that surfaced since the previous review have been addressed, and major
re-analyses of key time series studies have been completed. The cohort studies of
mortality have been extended, and the findings of the two most critical studies
validated through an extensive, peer-reviewed re-analysis. Many further studies
of morbidity indicators have also been carried out.
The epidemiological evidence is supported by an increasingly strong foundation
of toxicological research. Various mechanisms have been proposed by which
PM may cause and/or exacerbate acute and chronic diseases. Inflammation due
to the production of reactive oxygen species is emerging as a central mechanism.
However, one of the most vexing research challenges has been identifying physical
and chemical characteristics of PM determining toxicity, so that linkages can
be made back to the sources of the most injurious particles. This review, along
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