Of these common urban air pollutants, PM has
been studied in the greatest detail, but there is
increasing awareness of independent and direct
health effects of NO2. PM is a general term that
refers to a complex mixture of solids or liquids
that vary in number, size, shape, surface area,
chemical composition, solubility, and origin.3 The
main components of PM, originating from road
transport, are engine emissions, brake and tyre
wear, and dust from road surfaces. The largest
single source of airborne PM from motor vehicles
is derived from diesel exhausts, and this is an
increasingly important problem in Europe.4 Owing
to the growth in the number of new cars with diesel
engines, diesel exhaust particles (DEPs) account
for the most airborne PM in most European cities.
Biomass combustion particles have a range of
physiochemical properties, depending on the
nature of the biomass and combustion conditions
which, in turn, influences PM toxicity.
The inhalation of toxic particles and gases targets
the natural defences of the lung by increasing
epithelial permeability, decreasing mucociliary
clearance, and depressing macrophage function.
Although individual air pollutants will exert specific
toxic effects on the healthy or diseased respiratory
system, a common chain of molecular events ensue.
Human, animal, and in vitro experimental studies
have demonstrated an increased recruitment and
activation of inflammatory cells, and the generation
of an array of inflammatory mediators, as well as
the activation of intracellular oxidative stress via
the generation of free radicals and depletion of
protective small molecular weight antioxidants,5
and antioxidant enzymes.6 Oxidative stress, in turn,
promotes further inflammation via regulation of
redox-sensitive transcription factors and signalling
via the mitogen-activated protein kinase pathway.
DEPs and O3 are able to increase the production
of the allergic antibody immunoglobulin E (IgE),
thereby increasing sensitisation to allergens.7,8
With widespread urban air quality problems
across Europe, it is perhaps not that surprising,
therefore, that we are experiencing an increased
incidence of airway disease, which in turn provides
continued momentum to the extensive research
effort in this area.