National Ambient Air Quality Objectives For Particulate Matter - Executive Summary

Identification Of Reference Levels

The Federal/Provincial Working Group on Air Quality Objectives and Guidelines, as part of its mandate, has identified "Reference Levels", levels above which effects on human health and the environment can be demonstrated. The scientific information provided within the Science Assessment Document forms the basis upon which Reference Levels are established. Given the limited amount of information available, in particular the lack of quantitative dose-response information related to particle effects per se, it was not possible to define Reference Levels for either effects of PM on vegetation or on materials.

Since the effects of particulate matter on visibility are due primarily to the fine particle fraction, defining a Reference Level for PM₁₀ is not appropriate. Defining a Reference Level based on PM_2.5 is extremely difficult for a number of reasons:

1. variability in perceived changes to visual range,
2. the lack of site specific data on fine particle mass in remote areas, and
3. an inability to define a single, natural background concentration for PM_2.5 across Canada due to regional variations in background PM_2.5. Therefore, at this time, no Reference Level for PM_2.5 has been identified to protect visual range.

For particulate matter and human health effects, the Reference Level is derived statistically from several studies and should be interpreted as a level above which there is confidence (statistical significance) in the dose-response relationship and the ability to provide some quantification of adverse endpoints. The Reference Level in this case should not therefore be interpreted as a threshold of effects. On the contrary, there is no clear evidence of a threshold level for the positive associations between particulate matter and both daily mortality and hospitalization rates. That is, any increase in ambient PM is associated with a statistical increase in mortality and hospitalizations, and thus, any Reference Level identified is acknowledged to lie within the "effects range".

Mass concentrations of PM_2.5 and PM₁₀ are recommended as the metrics of choice for PM Reference Levels. According to the weight of evidence presented in the Science Assessment Document, the strength and consistency of the epidemiological evidence for mortality and morbidity effects associated with exposure to both PM₁₀ and PM_2.5 is remarkable, robust, consistent and compelling. PM_2.5 is most clearly associated with adverse health effects in a number of epidemiological studies, and has been shown to have a more robust association with mortality in most studies than other fine particle metrics (such as sulphate or acidity). PM_2.5 is also a more general surrogate for fine particle effects in all regions of the country than other measures (given the strong regional differences in sulphate and acidity levels). A Reference level for PM₁₀ is also recommended given the consistent associations observed in epidemiological studies with mortality and hospitalizations and due to concerns over its link to particular endpoints such as chronic bronchitis and cardiovascular disease.

The recommended Reference Levels for PM₁₀ and PM_2.5 (24 hour averages), statistically derived on the basis of several key epidemiological studies as detailed in the Science Assessment Document, are:

25 µg/m³ for PM₁₀

15 µg/m³ for PM_2.5

While the mortality and hospitalization endpoints have been emphasized in the derivation of the Reference Levels, because of the superior data on these endpoints, they are really only the tip of the iceberg with respect to PM induced human health effects. Other adverse effects such as bronchitis, reduced lung function, restricted activity, absenteeism and increased costs for medication are evident, and are occurring at ambient concentrations currently experienced within Canada.

As more scientific research is conducted, the Reference values will change, either because of better delineation of the adverse effects at lower concentrations, or because of better statistical analysis of the concentration-response relationship at low ambient concentrations.