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

10
Introduction
AIR QUALITY GUIDELINES
Basic definitions
Before discussing in detail the sources of air pollutants it is necessary to establish
a few basic principles that will place the information on sources in context. Air
pollutants may be either emitted into the atmosphere or formed within the atmosphere
itself.
Primary air pollutants
Primary air pollutants are those that are emitted into the atmosphere from a
source such as a factory chimney or exhaust pipe, or through suspension of contaminated
dusts by the wind. In principle, therefore, it is possible to measure the
amounts emitted at the source itself. This is relatively straightforward in terms of
the factory chimney or vehicle exhaust pipe; it becomes very much more difficult
when considering diffuse sources such as wind-blown dusts. When such sources
are added together they comprise an emissions inventory of primary sources, as
described below.
Secondary air pollutants
Secondary air pollutants are those formed within the atmosphere itself. They
arise from chemical reactions of primary pollutants, possibly involving the natural
components of the atmosphere, especially oxygen and water. The most familiar
example is ozone, which arises almost entirely from chemical reactions
that differ with altitude within the atmosphere. Because of this mode of formation,
secondary pollutants cannot readily be included in emissions inventories,
although it is possible to estimate formation rates per unit volume of atmosphere
per unit time (1).
Another important distinction must be made in relation to the physical state
of a pollutant.
Gaseous air pollutants
Gaseous air pollutants are those present as gases or vapours, i.e. as individual
small molecules capable of passing through filters provided they do not adsorb
to or chemically react with the filter medium. Gaseous air pollutants are readily
taken into the human respiratory system, although if water-soluble they may
very quickly be deposited in the upper respiratory tract and not penetrate to the
deep lung.
Particulate air pollutants
Particulate air pollutants comprise material in solid or liquid phase suspended in
the atmosphere. Such particles can be either primary or secondary and cover a
wide range of sizes. Newly formed secondary particles can be as small as 1–2 nm