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

NITROGEN DIOXIDE
particles. In some studies, however, the strongest effect was found for nitrogen
dioxide itself, while PM had a weaker or no effect. In particular, stronger indications
of an independent effect of nitrogen dioxide come from studies on hospital
admissions or emergency department visits for respiratory and cardiovascular
diseases, studies on asthma aggravation, and from evaluating indoor effects, especially
among asthmatics and infants at risk of asthma.
Nitrogen dioxide concentrations closely follow vehicle emissions in many situations,
so nitrogen dioxide levels are generally a reasonable marker of exposure
to traffic-related emissions. Health risks from nitrogen oxides may potentially result
from nitrogen dioxide itself, correlated exhaust components such as ultrafine
particles and hydrocarbons, or nitrogen dioxide chemistry products, including
ozone and secondary particles. The issue is complicated by the fact that PM10 or
PM2.5 might be indicators for regionally transported, rather homogeneously distributed
particles as well as locally produced, rather non-homogeneously distributed
combustion particles. Nitrogen dioxide is often recognized as an indicator
of locally produced particles, mainly from mobile sources. This also seems to be
the case in the Southern California Children’s Air Pollution Study, which on the
one hand provides evidence for a strong nitrogen dioxide health effect, but on the
other reports the impossibility of disentangling the effects of nitrogen dioxide
from the effects of elemental carbon. The same issue applies to assessing the role
of nitrogen dioxide in inducing respiratory disease in children.
In conclusion, nitrogen dioxide concentrations are often used to establish
spatial variability in air pollution concentrations from mobile sources and remain
inseparable from the often unmeasured particle emissions from combustion
sources, and in particular from mobile sources. Therefore, it is difficult to
determine whether the effects observed for nitrogen dioxide and PM are the independent
effects of the gaseous pollutant nitrogen dioxide and of PM, or are
independent effects of locally produced fine and ultrafine carbonaceous particles
and of regionally transported accumulation-mode particles.
Guidelines
Evidence from animal toxicological studies indicates that long-term exposure
to nitrogen dioxide at concentrations above current ambient concentrations has
adverse effects. In population studies, nitrogen dioxide has been associated with
adverse health effects even when the annual average nitrogen dioxide concentration
complied with the WHO annual guideline value of 40 μg/m 3 (121). Also,
some indoor studies suggest effects on respiratory symptoms among infants at
concentrations below 40 μg/m 3 . Together, these results support a lowering of the
annual nitrogen dioxide guideline value. However, since nitrogen dioxide is an
important constituent of combustion-generated air pollution and is highly correlated
with other primary and secondary combustion products, it is unclear to
what extent the health effects observed in epidemiological studies are attributable
375