An introduction to
particulate matter
issues

Particulate matter : sources and presence in
air.

Particulate matter (PM) refers to discrete particles in ambient air
that exist either as solid particles or liquid droplets. The sources of PM
are:

natural, e.g. pollen;

a combination of natural and man-made in variable proportions, e.g.
dust in a park, roadside dust, smoke from vegetation and wood burning;
and

wholly man-made, either: - naturally, e.g. household dust from skin
shedding; or - activity-related, e.g. smoking, cooking and barbecuing,
vehicle use, industrial activities, etc.

The sizes of PMs cover a very wide range: from the tiniest in smokes
and aerosols (e.g. perfumes), through the maximum respirable size 10
microns (µm) up to ‘grit in the eye’ sizes of 100 µm and upwards. Many
particles do not maintain a constant form during their lifetimes in the
atmosphere: many agglomerate to become fewer, larger particles whilst
others react chemically to become something different altogether.

The normal fate of PM is deposition. The rate of deposition depends
upon the size and density of the particle. The deposition rate in still
air approaches 90 to 100 per cent for particles that are larger than 0.5
µm, whilst particles smaller than 0.5 µm will tend increasingly to remain
suspended in the air. Atmospheric turbulence is an important factor and
can have opposite effects. On the one hand, ventilation disperses
concentrations of PM, while on the other hand, turbulence reduces the rate
of deposition. Hence the PMs’ size distributions, their other physical and
chemical properties and their concentrations in ambient air are highly
variable, depending on the particular characteristics of the biosphere at
issue (e.g. which country, urban or rural) and the ecosystem involved
(e.g. indoor, outdoor, roadside, street, canyon, etc.).

The seeds of the PM issues

Atmospheric PM concentrations in ambient air can sometimes provoke
concerns on health. For example, the Grand Canyon’s attractiveness as a
tourist spectacle has been compromised by poor visibility. Another aspect
is that the vitality of vegetation can be compromised by PM. Here it is
the deposition of the particulates which may be instrumental in
interfering with the biological processes, for instance, soot on leaves
can interfere with photosynthesis. However, it is the effect upon human
health that provokes the most concern and has led to calls for legislation
to improve PM urban air quality across whole regions.

These concerns began as people started to understand more about the
diseases that can follow exposures to PM-rich atmospheres, for instance,
those commonly experienced by mine and quarry workers, and from PM
inhalation by cigarette smoking. Health concerns have spread to the very
much lower PM concentrations in ambient air. These have arisen partly by
association—for example, the particulates from diesel vehicle exhausts
carry certain chemicals (called polycyclic aromatic hydrocarbons, PAH)
similar to those found in cigarette tars. However, the emphasis has
shifted to health issues relating to the particle itself and any change in
respiratory response that inhalation of the particle may cause.

The challenges in tackling PM
pollution

PM urban air quality is one of many very complex situations where hasty
actions may be disproportionate and may lead to costly and ineffective
solutions. It has not been possible to prove categorically whether or not
low level ambient PM exposures are a contributory cause of particular
diseases, such as increasingly common complaints like asthma. This allows
free rein to speculation, blaming children’s asthma on the visible exhaust
smoke from traffic, without looking further, even though scientific
findings have not established a ‘cause and effect’ link between exhaust
smoke and asthma.

The EU’s legislative responses to PM issues in the road traffic area
have been focused on diesel exhaust particulate emissions. Measures have
been introduced progressively; the main reductions result from more
stringent emissions standards for new vehicles and for in-service
performance (Euro-2 exhaust limits); sulphur in road diesel has been
reduced to 500 ppm mass from 1st October 1996. The result is that PM air
quality in most places in the EU has already passed its worst and is set
to improve significantly everywhere, as a result of already-enacted and
pending measures, even in the face of growing diesel-engined traffic. Much
research work has been done to identify what really matters in the PM
field and has identified many areas where vital knowledge is still
lacking.

Legislation through the application of precautionary principles to all
the fears that afflict the human condition at every stage of the science
is simply not sustainable—absolute freedom from risk is unattainable.

The way forward

The European Commission is developing a daughter directive on a PM air
quality standard, being one of the specific pollutants covered by the Air
Quality Framework Directive. In parallel, the United Nations Economic
Commission for Europe, under its Convention on Long Range Transboundary
Air Pollution, is developing protocols for heavy metals and for persistent
organic compounds, both of which contribute to PM.

CONCAWE is taking part in helping these processes along a rational path
and will be doing all it can to ensure that necessary environmental
benefits are achieved and that cost-effectiveness is given due weight.
CONCAWE has published report 95/62, Air Quality Standards for
Particulate Matter. This reviews the epidemiological, human and animal
data on exposure to PM and concludes that there is no sound scientific
basis for setting an air quality standard for PM. This conclusion is in
agreement with that of the World Health Organization on the same subject.