Combustion-derived particles are more harmful than particles from other sources

PM in ambient air has various sources. In targeting control measures, it would be important to know if PM from certain sources or of a certain composition gave rise to special concern from a health perspective, for example owing to its high toxicity. The few epidemiological studies that have addressed this important question specifically suggest that combustion sources are particularly important for health (Laden et al., 2000; Janssen et al., 2002).

Toxicological studies have also pointed to primary combustion-derived particles as having a higher toxic potential (Cassee et al., 2002). These particles are often rich in transition metals and organic compounds, and also have a relatively high surface area (Donaldson et al., 2002).

In contrast, several other single components of the PM mixture (e.g. ammonium salts, chlorides, sulfates, nitrates and wind-blown dust such as silicate clays) have been shown to have a lower toxicity in laboratory studies (Schlesinger & Cassee, 2003).

Yet, the shares of health impacts from different PM sources are difficult to assess

Despite the differences found among constituents studied under laboratory conditions, it is currently not possible to precisely quantify the contributions from different sources and different PM components to the effects on health caused by exposure to ambient PM.

It is therefore prudent to ascertain that proposed control measures do indeed target those components of PM, which have shown relatively toxic, in other words, to check that reductions in PM are not achieved principally by reducing the less toxic fractions).

It is worth noting that some of the components identified as hazardous in toxicological studies can also be found in rural sites in considerable concentrations. These include organic material and transition metals, even though the latter are clearly enriched near sources. However, some of the components with less toxicological activity are also present at considerable levels in aerosols subject to long-range transboundary air pollution, including secondary inorganic aerosols and sea salt.