The evidence on airborne PM and public health is consistent in showing adverse health effects at exposures experienced by urban populations in cities throughout the world, in both developed and developing countries. The range of effects is broad, affecting the respiratory and cardiovascular systems and extending to children and adults and to a number of large, susceptible groups within the general population. The risk for various outcomes has been shown to increase with exposure and there is little evidence to suggest a threshold below which no adverse health effects would be anticipated. In fact, the lower range of concentrations at which adverse health effects has been demonstrated is not greatly above the background concentration which has been estimated at 3-5 µg/m3 in the United States and Western Europe for particles smaller than 2.5 micrometer, PM2.5. The epidemiological evidence shows adverse effects of particles after both short-term and long-term exposures.

The choice of indicator for particulate matter also merits consideration. The most recent and extensive epidemiological evidence is largely based on studies using PM10 as the exposure indicator. Further, at present the majority of monitoring data is based on measurement of PM10 as opposed to other particulate matter metrics. As an indicator, PM10 comprises the particle mass that enters the respiratory tract and includes both the coarse (PM10-PM2.5) and fine (PM2.5) particles considered to contribute to the health effects observed in urban environments. In most urban environments, both coarse and fine mode particles are likely to be prominent, the former primarily produced by mechanical processes such as construction activities, road dust resuspension and wind, and the latter primarily from combustion sources.

Based on known health effects, both short-term (24-hour) and long-term (annual) guidelines are needed for both of the PM indicators.

The annual average guideline value of 10 µg/m3 for PM2.5 was chosen to represent the lower end of the range over which significant effects on survival have been observed in the American Cancer Society Study (ACS). Adoption of a guideline at this level places significant weight on the long-term exposure studies using the ACS and Harvard Six-Cities data. In these studies, robust associations were reported between long-term exposure to PM2.5 and mortality. The historical mean PM2.5 concentration was 18 µg/m3 (range of 11.0 to 29.6 µg/m3) in the Six-Cities study and 20 µg/m3 (range of 9.0 to 33.5 µg/m3) in the ACS study. Thresholds were not apparent in either of these studies, although the precise period(s) and pattern(s) of relevant exposure could not be ascertained. In the ACS study, statistical uncertainty in the risk estimates becomes apparent at concentrations of about 13 µg/m3, below which the confidence bounds significantly widen since the concentrations are relatively far from the mean. In the Dockery et al. study, the risks are similar in the cities at the lowest long-term PM2.5 concentrations of 11 and 12.5 µg/m3. Increases in risk are apparent in the city with the next-lowest long-term PM2.5 mean of 14.9 µg/m3, indicating likely effects in the range of 11 to 15 µg/m3. Therefore, an annual concentration of 10 µg/m3 would be below the mean of the most likely effects levels indicated in the available literature. Targeting a long-term mean PM2.5 concentration of 10 µg/m3 would also place some weight on the results of daily exposure time-series studies examining relationships between PM2.5 and acute adverse health outcomes. These studies have long-term (three- to four-year) means in the range of 13 to 18 µg/m3. Although adverse effects on health cannot be entirely ruled out even below that level, the annual average WHO AQG represent levels that have been shown to be achievable in large urban areas in highly developed countries, and attainment is expected to effectively reduce the health risks.

In addition to PM2.5 and PM10, ultra fine particles (UF) have recently attracted significant scientific and medical attention. These are particles smaller than 0.1 micrometer and are measured as number concentration. While there is considerable toxicological evidence of potential detrimental effects of UF particles on human health, the existing body of epidemiological evidence is insufficient to reach a conclusion on the exposure/response relationship to UF particles. Therefore no recommendations can be provided as to guideline concentrations of UF particles at this point.